1f4f864c1SEric Biggers===================================== 2f4f864c1SEric BiggersFilesystem-level encryption (fscrypt) 3f4f864c1SEric Biggers===================================== 4f4f864c1SEric Biggers 5f4f864c1SEric BiggersIntroduction 6f4f864c1SEric Biggers============ 7f4f864c1SEric Biggers 8f4f864c1SEric Biggersfscrypt is a library which filesystems can hook into to support 9f4f864c1SEric Biggerstransparent encryption of files and directories. 10f4f864c1SEric Biggers 11f4f864c1SEric BiggersNote: "fscrypt" in this document refers to the kernel-level portion, 12f4f864c1SEric Biggersimplemented in ``fs/crypto/``, as opposed to the userspace tool 13f4f864c1SEric Biggers`fscrypt <https://github.com/google/fscrypt>`_. This document only 14f4f864c1SEric Biggerscovers the kernel-level portion. For command-line examples of how to 15f4f864c1SEric Biggersuse encryption, see the documentation for the userspace tool `fscrypt 16f4f864c1SEric Biggers<https://github.com/google/fscrypt>`_. Also, it is recommended to use 17f4f864c1SEric Biggersthe fscrypt userspace tool, or other existing userspace tools such as 18f4f864c1SEric Biggers`fscryptctl <https://github.com/google/fscryptctl>`_ or `Android's key 19f4f864c1SEric Biggersmanagement system 20f4f864c1SEric Biggers<https://source.android.com/security/encryption/file-based>`_, over 21f4f864c1SEric Biggersusing the kernel's API directly. Using existing tools reduces the 22f4f864c1SEric Biggerschance of introducing your own security bugs. (Nevertheless, for 23f4f864c1SEric Biggerscompleteness this documentation covers the kernel's API anyway.) 24f4f864c1SEric Biggers 25f4f864c1SEric BiggersUnlike dm-crypt, fscrypt operates at the filesystem level rather than 26f4f864c1SEric Biggersat the block device level. This allows it to encrypt different files 27f4f864c1SEric Biggerswith different keys and to have unencrypted files on the same 28f4f864c1SEric Biggersfilesystem. This is useful for multi-user systems where each user's 29f4f864c1SEric Biggersdata-at-rest needs to be cryptographically isolated from the others. 30f4f864c1SEric BiggersHowever, except for filenames, fscrypt does not encrypt filesystem 31f4f864c1SEric Biggersmetadata. 32f4f864c1SEric Biggers 33f4f864c1SEric BiggersUnlike eCryptfs, which is a stacked filesystem, fscrypt is integrated 34f4f864c1SEric Biggersdirectly into supported filesystems --- currently ext4, F2FS, and 35f4f864c1SEric BiggersUBIFS. This allows encrypted files to be read and written without 36f4f864c1SEric Biggerscaching both the decrypted and encrypted pages in the pagecache, 37f4f864c1SEric Biggersthereby nearly halving the memory used and bringing it in line with 38f4f864c1SEric Biggersunencrypted files. Similarly, half as many dentries and inodes are 39f4f864c1SEric Biggersneeded. eCryptfs also limits encrypted filenames to 143 bytes, 40f4f864c1SEric Biggerscausing application compatibility issues; fscrypt allows the full 255 41f4f864c1SEric Biggersbytes (NAME_MAX). Finally, unlike eCryptfs, the fscrypt API can be 42f4f864c1SEric Biggersused by unprivileged users, with no need to mount anything. 43f4f864c1SEric Biggers 44f4f864c1SEric Biggersfscrypt does not support encrypting files in-place. Instead, it 45f4f864c1SEric Biggerssupports marking an empty directory as encrypted. Then, after 46f4f864c1SEric Biggersuserspace provides the key, all regular files, directories, and 47f4f864c1SEric Biggerssymbolic links created in that directory tree are transparently 48f4f864c1SEric Biggersencrypted. 49f4f864c1SEric Biggers 50f4f864c1SEric BiggersThreat model 51f4f864c1SEric Biggers============ 52f4f864c1SEric Biggers 53f4f864c1SEric BiggersOffline attacks 54f4f864c1SEric Biggers--------------- 55f4f864c1SEric Biggers 56f4f864c1SEric BiggersProvided that userspace chooses a strong encryption key, fscrypt 57f4f864c1SEric Biggersprotects the confidentiality of file contents and filenames in the 58f4f864c1SEric Biggersevent of a single point-in-time permanent offline compromise of the 59f4f864c1SEric Biggersblock device content. fscrypt does not protect the confidentiality of 60f4f864c1SEric Biggersnon-filename metadata, e.g. file sizes, file permissions, file 61f4f864c1SEric Biggerstimestamps, and extended attributes. Also, the existence and location 62f4f864c1SEric Biggersof holes (unallocated blocks which logically contain all zeroes) in 63f4f864c1SEric Biggersfiles is not protected. 64f4f864c1SEric Biggers 65f4f864c1SEric Biggersfscrypt is not guaranteed to protect confidentiality or authenticity 66f4f864c1SEric Biggersif an attacker is able to manipulate the filesystem offline prior to 67f4f864c1SEric Biggersan authorized user later accessing the filesystem. 68f4f864c1SEric Biggers 69f4f864c1SEric BiggersOnline attacks 70f4f864c1SEric Biggers-------------- 71f4f864c1SEric Biggers 72f4f864c1SEric Biggersfscrypt (and storage encryption in general) can only provide limited 73f4f864c1SEric Biggersprotection, if any at all, against online attacks. In detail: 74f4f864c1SEric Biggers 75ba13f2c8SEric BiggersSide-channel attacks 76ba13f2c8SEric Biggers~~~~~~~~~~~~~~~~~~~~ 77ba13f2c8SEric Biggers 78f4f864c1SEric Biggersfscrypt is only resistant to side-channel attacks, such as timing or 79f4f864c1SEric Biggerselectromagnetic attacks, to the extent that the underlying Linux 80abb861faSEric BiggersCryptographic API algorithms or inline encryption hardware are. If a 81abb861faSEric Biggersvulnerable algorithm is used, such as a table-based implementation of 82abb861faSEric BiggersAES, it may be possible for an attacker to mount a side channel attack 83abb861faSEric Biggersagainst the online system. Side channel attacks may also be mounted 84abb861faSEric Biggersagainst applications consuming decrypted data. 85f4f864c1SEric Biggers 86ba13f2c8SEric BiggersUnauthorized file access 87ba13f2c8SEric Biggers~~~~~~~~~~~~~~~~~~~~~~~~ 88f4f864c1SEric Biggers 89ba13f2c8SEric BiggersAfter an encryption key has been added, fscrypt does not hide the 90ba13f2c8SEric Biggersplaintext file contents or filenames from other users on the same 91ba13f2c8SEric Biggerssystem. Instead, existing access control mechanisms such as file mode 92ba13f2c8SEric Biggersbits, POSIX ACLs, LSMs, or namespaces should be used for this purpose. 93f4f864c1SEric Biggers 94ba13f2c8SEric Biggers(For the reasoning behind this, understand that while the key is 95ba13f2c8SEric Biggersadded, the confidentiality of the data, from the perspective of the 96ba13f2c8SEric Biggerssystem itself, is *not* protected by the mathematical properties of 97ba13f2c8SEric Biggersencryption but rather only by the correctness of the kernel. 98ba13f2c8SEric BiggersTherefore, any encryption-specific access control checks would merely 99ba13f2c8SEric Biggersbe enforced by kernel *code* and therefore would be largely redundant 100ba13f2c8SEric Biggerswith the wide variety of access control mechanisms already available.) 101ba13f2c8SEric Biggers 102ba13f2c8SEric BiggersKernel memory compromise 103ba13f2c8SEric Biggers~~~~~~~~~~~~~~~~~~~~~~~~ 104ba13f2c8SEric Biggers 105ba13f2c8SEric BiggersAn attacker who compromises the system enough to read from arbitrary 106ba13f2c8SEric Biggersmemory, e.g. by mounting a physical attack or by exploiting a kernel 107ba13f2c8SEric Biggerssecurity vulnerability, can compromise all encryption keys that are 108ba13f2c8SEric Biggerscurrently in use. 109ba13f2c8SEric Biggers 110ba13f2c8SEric BiggersHowever, fscrypt allows encryption keys to be removed from the kernel, 111ba13f2c8SEric Biggerswhich may protect them from later compromise. 112ba13f2c8SEric Biggers 113ba13f2c8SEric BiggersIn more detail, the FS_IOC_REMOVE_ENCRYPTION_KEY ioctl (or the 114ba13f2c8SEric BiggersFS_IOC_REMOVE_ENCRYPTION_KEY_ALL_USERS ioctl) can wipe a master 115ba13f2c8SEric Biggersencryption key from kernel memory. If it does so, it will also try to 116ba13f2c8SEric Biggersevict all cached inodes which had been "unlocked" using the key, 117ba13f2c8SEric Biggersthereby wiping their per-file keys and making them once again appear 118ba13f2c8SEric Biggers"locked", i.e. in ciphertext or encrypted form. 119ba13f2c8SEric Biggers 120ba13f2c8SEric BiggersHowever, these ioctls have some limitations: 121ba13f2c8SEric Biggers 122ba13f2c8SEric Biggers- Per-file keys for in-use files will *not* be removed or wiped. 123ba13f2c8SEric Biggers Therefore, for maximum effect, userspace should close the relevant 124ba13f2c8SEric Biggers encrypted files and directories before removing a master key, as 125ba13f2c8SEric Biggers well as kill any processes whose working directory is in an affected 126ba13f2c8SEric Biggers encrypted directory. 127ba13f2c8SEric Biggers 128ba13f2c8SEric Biggers- The kernel cannot magically wipe copies of the master key(s) that 129ba13f2c8SEric Biggers userspace might have as well. Therefore, userspace must wipe all 130ba13f2c8SEric Biggers copies of the master key(s) it makes as well; normally this should 131ba13f2c8SEric Biggers be done immediately after FS_IOC_ADD_ENCRYPTION_KEY, without waiting 132ba13f2c8SEric Biggers for FS_IOC_REMOVE_ENCRYPTION_KEY. Naturally, the same also applies 133ba13f2c8SEric Biggers to all higher levels in the key hierarchy. Userspace should also 134ba13f2c8SEric Biggers follow other security precautions such as mlock()ing memory 135ba13f2c8SEric Biggers containing keys to prevent it from being swapped out. 136ba13f2c8SEric Biggers 137ba13f2c8SEric Biggers- In general, decrypted contents and filenames in the kernel VFS 138ba13f2c8SEric Biggers caches are freed but not wiped. Therefore, portions thereof may be 139ba13f2c8SEric Biggers recoverable from freed memory, even after the corresponding key(s) 140ba13f2c8SEric Biggers were wiped. To partially solve this, you can set 141ba13f2c8SEric Biggers CONFIG_PAGE_POISONING=y in your kernel config and add page_poison=1 142ba13f2c8SEric Biggers to your kernel command line. However, this has a performance cost. 143ba13f2c8SEric Biggers 144ba13f2c8SEric Biggers- Secret keys might still exist in CPU registers, in crypto 145ba13f2c8SEric Biggers accelerator hardware (if used by the crypto API to implement any of 146ba13f2c8SEric Biggers the algorithms), or in other places not explicitly considered here. 147ba13f2c8SEric Biggers 148ba13f2c8SEric BiggersLimitations of v1 policies 149ba13f2c8SEric Biggers~~~~~~~~~~~~~~~~~~~~~~~~~~ 150ba13f2c8SEric Biggers 151ba13f2c8SEric Biggersv1 encryption policies have some weaknesses with respect to online 152ba13f2c8SEric Biggersattacks: 153ba13f2c8SEric Biggers 154ba13f2c8SEric Biggers- There is no verification that the provided master key is correct. 155ba13f2c8SEric Biggers Therefore, a malicious user can temporarily associate the wrong key 156ba13f2c8SEric Biggers with another user's encrypted files to which they have read-only 157ba13f2c8SEric Biggers access. Because of filesystem caching, the wrong key will then be 158ba13f2c8SEric Biggers used by the other user's accesses to those files, even if the other 159ba13f2c8SEric Biggers user has the correct key in their own keyring. This violates the 160ba13f2c8SEric Biggers meaning of "read-only access". 161ba13f2c8SEric Biggers 162ba13f2c8SEric Biggers- A compromise of a per-file key also compromises the master key from 163ba13f2c8SEric Biggers which it was derived. 164ba13f2c8SEric Biggers 165ba13f2c8SEric Biggers- Non-root users cannot securely remove encryption keys. 166ba13f2c8SEric Biggers 167ba13f2c8SEric BiggersAll the above problems are fixed with v2 encryption policies. For 168ba13f2c8SEric Biggersthis reason among others, it is recommended to use v2 encryption 169ba13f2c8SEric Biggerspolicies on all new encrypted directories. 170f4f864c1SEric Biggers 171f4f864c1SEric BiggersKey hierarchy 172f4f864c1SEric Biggers============= 173f4f864c1SEric Biggers 174f4f864c1SEric BiggersMaster Keys 175f4f864c1SEric Biggers----------- 176f4f864c1SEric Biggers 177f4f864c1SEric BiggersEach encrypted directory tree is protected by a *master key*. Master 178f4f864c1SEric Biggerskeys can be up to 64 bytes long, and must be at least as long as the 1797f595d6aSEric Biggersgreater of the security strength of the contents and filenames 1807f595d6aSEric Biggersencryption modes being used. For example, if any AES-256 mode is 1817f595d6aSEric Biggersused, the master key must be at least 256 bits, i.e. 32 bytes. A 1827f595d6aSEric Biggersstricter requirement applies if the key is used by a v1 encryption 1837f595d6aSEric Biggerspolicy and AES-256-XTS is used; such keys must be 64 bytes. 184f4f864c1SEric Biggers 185f4f864c1SEric BiggersTo "unlock" an encrypted directory tree, userspace must provide the 186f4f864c1SEric Biggersappropriate master key. There can be any number of master keys, each 187f4f864c1SEric Biggersof which protects any number of directory trees on any number of 188f4f864c1SEric Biggersfilesystems. 189f4f864c1SEric Biggers 190ba13f2c8SEric BiggersMaster keys must be real cryptographic keys, i.e. indistinguishable 191ba13f2c8SEric Biggersfrom random bytestrings of the same length. This implies that users 192ba13f2c8SEric Biggers**must not** directly use a password as a master key, zero-pad a 193ba13f2c8SEric Biggersshorter key, or repeat a shorter key. Security cannot be guaranteed 194ba13f2c8SEric Biggersif userspace makes any such error, as the cryptographic proofs and 195ba13f2c8SEric Biggersanalysis would no longer apply. 196ba13f2c8SEric Biggers 197ba13f2c8SEric BiggersInstead, users should generate master keys either using a 198ba13f2c8SEric Biggerscryptographically secure random number generator, or by using a KDF 199ba13f2c8SEric Biggers(Key Derivation Function). The kernel does not do any key stretching; 200ba13f2c8SEric Biggerstherefore, if userspace derives the key from a low-entropy secret such 201ba13f2c8SEric Biggersas a passphrase, it is critical that a KDF designed for this purpose 202ba13f2c8SEric Biggersbe used, such as scrypt, PBKDF2, or Argon2. 203ba13f2c8SEric Biggers 204ba13f2c8SEric BiggersKey derivation function 205ba13f2c8SEric Biggers----------------------- 206ba13f2c8SEric Biggers 207ba13f2c8SEric BiggersWith one exception, fscrypt never uses the master key(s) for 208ba13f2c8SEric Biggersencryption directly. Instead, they are only used as input to a KDF 209ba13f2c8SEric Biggers(Key Derivation Function) to derive the actual keys. 210ba13f2c8SEric Biggers 211ba13f2c8SEric BiggersThe KDF used for a particular master key differs depending on whether 212ba13f2c8SEric Biggersthe key is used for v1 encryption policies or for v2 encryption 213ba13f2c8SEric Biggerspolicies. Users **must not** use the same key for both v1 and v2 214ba13f2c8SEric Biggersencryption policies. (No real-world attack is currently known on this 215ba13f2c8SEric Biggersspecific case of key reuse, but its security cannot be guaranteed 216ba13f2c8SEric Biggerssince the cryptographic proofs and analysis would no longer apply.) 217ba13f2c8SEric Biggers 218ba13f2c8SEric BiggersFor v1 encryption policies, the KDF only supports deriving per-file 219ba13f2c8SEric Biggersencryption keys. It works by encrypting the master key with 220ba13f2c8SEric BiggersAES-128-ECB, using the file's 16-byte nonce as the AES key. The 221ba13f2c8SEric Biggersresulting ciphertext is used as the derived key. If the ciphertext is 222ba13f2c8SEric Biggerslonger than needed, then it is truncated to the needed length. 223ba13f2c8SEric Biggers 224ba13f2c8SEric BiggersFor v2 encryption policies, the KDF is HKDF-SHA512. The master key is 225ba13f2c8SEric Biggerspassed as the "input keying material", no salt is used, and a distinct 226ba13f2c8SEric Biggers"application-specific information string" is used for each distinct 227ba13f2c8SEric Biggerskey to be derived. For example, when a per-file encryption key is 228ba13f2c8SEric Biggersderived, the application-specific information string is the file's 229ba13f2c8SEric Biggersnonce prefixed with "fscrypt\\0" and a context byte. Different 230ba13f2c8SEric Biggerscontext bytes are used for other types of derived keys. 231ba13f2c8SEric Biggers 232ba13f2c8SEric BiggersHKDF-SHA512 is preferred to the original AES-128-ECB based KDF because 233ba13f2c8SEric BiggersHKDF is more flexible, is nonreversible, and evenly distributes 234ba13f2c8SEric Biggersentropy from the master key. HKDF is also standardized and widely 235ba13f2c8SEric Biggersused by other software, whereas the AES-128-ECB based KDF is ad-hoc. 236f4f864c1SEric Biggers 237f592efe7SEric BiggersPer-file encryption keys 238f592efe7SEric Biggers------------------------ 239f4f864c1SEric Biggers 2408094c3ceSEric BiggersSince each master key can protect many files, it is necessary to 2418094c3ceSEric Biggers"tweak" the encryption of each file so that the same plaintext in two 2428094c3ceSEric Biggersfiles doesn't map to the same ciphertext, or vice versa. In most 2438094c3ceSEric Biggerscases, fscrypt does this by deriving per-file keys. When a new 2448094c3ceSEric Biggersencrypted inode (regular file, directory, or symlink) is created, 2458094c3ceSEric Biggersfscrypt randomly generates a 16-byte nonce and stores it in the 246ba13f2c8SEric Biggersinode's encryption xattr. Then, it uses a KDF (as described in `Key 247ba13f2c8SEric Biggersderivation function`_) to derive the file's key from the master key 248ba13f2c8SEric Biggersand nonce. 249f4f864c1SEric Biggers 2508094c3ceSEric BiggersKey derivation was chosen over key wrapping because wrapped keys would 2518094c3ceSEric Biggersrequire larger xattrs which would be less likely to fit in-line in the 2528094c3ceSEric Biggersfilesystem's inode table, and there didn't appear to be any 2538094c3ceSEric Biggerssignificant advantages to key wrapping. In particular, currently 2548094c3ceSEric Biggersthere is no requirement to support unlocking a file with multiple 2558094c3ceSEric Biggersalternative master keys or to support rotating master keys. Instead, 2568094c3ceSEric Biggersthe master keys may be wrapped in userspace, e.g. as is done by the 2578094c3ceSEric Biggers`fscrypt <https://github.com/google/fscrypt>`_ tool. 2588094c3ceSEric Biggers 259b103fb76SEric BiggersDIRECT_KEY policies 260b103fb76SEric Biggers------------------- 261ba13f2c8SEric Biggers 262ba13f2c8SEric BiggersThe Adiantum encryption mode (see `Encryption modes and usage`_) is 263ba13f2c8SEric Biggerssuitable for both contents and filenames encryption, and it accepts 264ba13f2c8SEric Biggerslong IVs --- long enough to hold both an 8-byte logical block number 265ba13f2c8SEric Biggersand a 16-byte per-file nonce. Also, the overhead of each Adiantum key 266ba13f2c8SEric Biggersis greater than that of an AES-256-XTS key. 267ba13f2c8SEric Biggers 268ba13f2c8SEric BiggersTherefore, to improve performance and save memory, for Adiantum a 269ba13f2c8SEric Biggers"direct key" configuration is supported. When the user has enabled 270ba13f2c8SEric Biggersthis by setting FSCRYPT_POLICY_FLAG_DIRECT_KEY in the fscrypt policy, 271f592efe7SEric Biggersper-file encryption keys are not used. Instead, whenever any data 272f592efe7SEric Biggers(contents or filenames) is encrypted, the file's 16-byte nonce is 273f592efe7SEric Biggersincluded in the IV. Moreover: 274ba13f2c8SEric Biggers 275ba13f2c8SEric Biggers- For v1 encryption policies, the encryption is done directly with the 276ba13f2c8SEric Biggers master key. Because of this, users **must not** use the same master 277ba13f2c8SEric Biggers key for any other purpose, even for other v1 policies. 278ba13f2c8SEric Biggers 279ba13f2c8SEric Biggers- For v2 encryption policies, the encryption is done with a per-mode 280ba13f2c8SEric Biggers key derived using the KDF. Users may use the same master key for 281ba13f2c8SEric Biggers other v2 encryption policies. 282ba13f2c8SEric Biggers 283b103fb76SEric BiggersIV_INO_LBLK_64 policies 284b103fb76SEric Biggers----------------------- 285b103fb76SEric Biggers 286b103fb76SEric BiggersWhen FSCRYPT_POLICY_FLAG_IV_INO_LBLK_64 is set in the fscrypt policy, 287b103fb76SEric Biggersthe encryption keys are derived from the master key, encryption mode 288b103fb76SEric Biggersnumber, and filesystem UUID. This normally results in all files 289b103fb76SEric Biggersprotected by the same master key sharing a single contents encryption 290b103fb76SEric Biggerskey and a single filenames encryption key. To still encrypt different 291b103fb76SEric Biggersfiles' data differently, inode numbers are included in the IVs. 292b103fb76SEric BiggersConsequently, shrinking the filesystem may not be allowed. 293b103fb76SEric Biggers 294b103fb76SEric BiggersThis format is optimized for use with inline encryption hardware 295e3b1078bSEric Biggerscompliant with the UFS standard, which supports only 64 IV bits per 296e3b1078bSEric BiggersI/O request and may have only a small number of keyslots. 297e3b1078bSEric Biggers 298e3b1078bSEric BiggersIV_INO_LBLK_32 policies 299e3b1078bSEric Biggers----------------------- 300e3b1078bSEric Biggers 301e3b1078bSEric BiggersIV_INO_LBLK_32 policies work like IV_INO_LBLK_64, except that for 302e3b1078bSEric BiggersIV_INO_LBLK_32, the inode number is hashed with SipHash-2-4 (where the 303e3b1078bSEric BiggersSipHash key is derived from the master key) and added to the file 304e3b1078bSEric Biggerslogical block number mod 2^32 to produce a 32-bit IV. 305e3b1078bSEric Biggers 306e3b1078bSEric BiggersThis format is optimized for use with inline encryption hardware 307e3b1078bSEric Biggerscompliant with the eMMC v5.2 standard, which supports only 32 IV bits 308e3b1078bSEric Biggersper I/O request and may have only a small number of keyslots. This 309e3b1078bSEric Biggersformat results in some level of IV reuse, so it should only be used 310e3b1078bSEric Biggerswhen necessary due to hardware limitations. 311b103fb76SEric Biggers 312ba13f2c8SEric BiggersKey identifiers 313ba13f2c8SEric Biggers--------------- 314ba13f2c8SEric Biggers 315ba13f2c8SEric BiggersFor master keys used for v2 encryption policies, a unique 16-byte "key 316ba13f2c8SEric Biggersidentifier" is also derived using the KDF. This value is stored in 317ba13f2c8SEric Biggersthe clear, since it is needed to reliably identify the key itself. 318ba13f2c8SEric Biggers 319aa408f83SDaniel RosenbergDirhash keys 320aa408f83SDaniel Rosenberg------------ 321aa408f83SDaniel Rosenberg 322aa408f83SDaniel RosenbergFor directories that are indexed using a secret-keyed dirhash over the 323aa408f83SDaniel Rosenbergplaintext filenames, the KDF is also used to derive a 128-bit 324aa408f83SDaniel RosenbergSipHash-2-4 key per directory in order to hash filenames. This works 325aa408f83SDaniel Rosenbergjust like deriving a per-file encryption key, except that a different 326aa408f83SDaniel RosenbergKDF context is used. Currently, only casefolded ("case-insensitive") 327aa408f83SDaniel Rosenbergencrypted directories use this style of hashing. 328aa408f83SDaniel Rosenberg 329f4f864c1SEric BiggersEncryption modes and usage 330f4f864c1SEric Biggers========================== 331f4f864c1SEric Biggers 332f4f864c1SEric Biggersfscrypt allows one encryption mode to be specified for file contents 333f4f864c1SEric Biggersand one encryption mode to be specified for filenames. Different 334f4f864c1SEric Biggersdirectory trees are permitted to use different encryption modes. 335f4f864c1SEric BiggersCurrently, the following pairs of encryption modes are supported: 336f4f864c1SEric Biggers 337f4f864c1SEric Biggers- AES-256-XTS for contents and AES-256-CTS-CBC for filenames 338f4f864c1SEric Biggers- AES-128-CBC for contents and AES-128-CTS-CBC for filenames 3398094c3ceSEric Biggers- Adiantum for both contents and filenames 3406b2a51ffSNathan Huckleberry- AES-256-XTS for contents and AES-256-HCTR2 for filenames (v2 policies only) 341e0cefadaSTianjia Zhang- SM4-XTS for contents and SM4-CTS-CBC for filenames (v2 policies only) 342f4f864c1SEric Biggers 3438094c3ceSEric BiggersIf unsure, you should use the (AES-256-XTS, AES-256-CTS-CBC) pair. 3448094c3ceSEric Biggers 345f4f864c1SEric BiggersAES-128-CBC was added only for low-powered embedded devices with 346adbd9b4dSEric Biggerscrypto accelerators such as CAAM or CESA that do not support XTS. To 3474006d799SEric Biggersuse AES-128-CBC, CONFIG_CRYPTO_ESSIV and CONFIG_CRYPTO_SHA256 (or 3484006d799SEric Biggersanother SHA-256 implementation) must be enabled so that ESSIV can be 3494006d799SEric Biggersused. 350f4f864c1SEric Biggers 3518094c3ceSEric BiggersAdiantum is a (primarily) stream cipher-based mode that is fast even 3528094c3ceSEric Biggerson CPUs without dedicated crypto instructions. It's also a true 3538094c3ceSEric Biggerswide-block mode, unlike XTS. It can also eliminate the need to derive 354f592efe7SEric Biggersper-file encryption keys. However, it depends on the security of two 355f592efe7SEric Biggersprimitives, XChaCha12 and AES-256, rather than just one. See the 356f592efe7SEric Biggerspaper "Adiantum: length-preserving encryption for entry-level 357f592efe7SEric Biggersprocessors" (https://eprint.iacr.org/2018/720.pdf) for more details. 358f592efe7SEric BiggersTo use Adiantum, CONFIG_CRYPTO_ADIANTUM must be enabled. Also, fast 3598094c3ceSEric Biggersimplementations of ChaCha and NHPoly1305 should be enabled, e.g. 3608094c3ceSEric BiggersCONFIG_CRYPTO_CHACHA20_NEON and CONFIG_CRYPTO_NHPOLY1305_NEON for ARM. 3618094c3ceSEric Biggers 3626b2a51ffSNathan HuckleberryAES-256-HCTR2 is another true wide-block encryption mode that is intended for 3636b2a51ffSNathan Huckleberryuse on CPUs with dedicated crypto instructions. AES-256-HCTR2 has the property 3646b2a51ffSNathan Huckleberrythat a bitflip in the plaintext changes the entire ciphertext. This property 3656b2a51ffSNathan Huckleberrymakes it desirable for filename encryption since initialization vectors are 3666b2a51ffSNathan Huckleberryreused within a directory. For more details on AES-256-HCTR2, see the paper 3676b2a51ffSNathan Huckleberry"Length-preserving encryption with HCTR2" 3686b2a51ffSNathan Huckleberry(https://eprint.iacr.org/2021/1441.pdf). To use AES-256-HCTR2, 3696b2a51ffSNathan HuckleberryCONFIG_CRYPTO_HCTR2 must be enabled. Also, fast implementations of XCTR and 3706b2a51ffSNathan HuckleberryPOLYVAL should be enabled, e.g. CRYPTO_POLYVAL_ARM64_CE and 3716b2a51ffSNathan HuckleberryCRYPTO_AES_ARM64_CE_BLK for ARM64. 3726b2a51ffSNathan Huckleberry 37341952551SEric BiggersSM4 is a Chinese block cipher that is an alternative to AES. It has 37441952551SEric Biggersnot seen as much security review as AES, and it only has a 128-bit key 37541952551SEric Biggerssize. It may be useful in cases where its use is mandated. 37641952551SEric BiggersOtherwise, it should not be used. For SM4 support to be available, it 37741952551SEric Biggersalso needs to be enabled in the kernel crypto API. 37841952551SEric Biggers 379f4f864c1SEric BiggersNew encryption modes can be added relatively easily, without changes 380f4f864c1SEric Biggersto individual filesystems. However, authenticated encryption (AE) 381f4f864c1SEric Biggersmodes are not currently supported because of the difficulty of dealing 382f4f864c1SEric Biggerswith ciphertext expansion. 383f4f864c1SEric Biggers 3848094c3ceSEric BiggersContents encryption 3858094c3ceSEric Biggers------------------- 3868094c3ceSEric Biggers 387f4f864c1SEric BiggersFor file contents, each filesystem block is encrypted independently. 388196624e1SChandan RajendraStarting from Linux kernel 5.5, encryption of filesystems with block 389196624e1SChandan Rajendrasize less than system's page size is supported. 390f4f864c1SEric Biggers 3918094c3ceSEric BiggersEach block's IV is set to the logical block number within the file as 3928094c3ceSEric Biggersa little endian number, except that: 393f4f864c1SEric Biggers 3948094c3ceSEric Biggers- With CBC mode encryption, ESSIV is also used. Specifically, each IV 3958094c3ceSEric Biggers is encrypted with AES-256 where the AES-256 key is the SHA-256 hash 3968094c3ceSEric Biggers of the file's data encryption key. 3978094c3ceSEric Biggers 398b103fb76SEric Biggers- With `DIRECT_KEY policies`_, the file's nonce is appended to the IV. 399b103fb76SEric Biggers Currently this is only allowed with the Adiantum encryption mode. 400b103fb76SEric Biggers 401b103fb76SEric Biggers- With `IV_INO_LBLK_64 policies`_, the logical block number is limited 402b103fb76SEric Biggers to 32 bits and is placed in bits 0-31 of the IV. The inode number 403b103fb76SEric Biggers (which is also limited to 32 bits) is placed in bits 32-63. 404b103fb76SEric Biggers 405e3b1078bSEric Biggers- With `IV_INO_LBLK_32 policies`_, the logical block number is limited 406e3b1078bSEric Biggers to 32 bits and is placed in bits 0-31 of the IV. The inode number 407e3b1078bSEric Biggers is then hashed and added mod 2^32. 408e3b1078bSEric Biggers 409b103fb76SEric BiggersNote that because file logical block numbers are included in the IVs, 410b103fb76SEric Biggersfilesystems must enforce that blocks are never shifted around within 411b103fb76SEric Biggersencrypted files, e.g. via "collapse range" or "insert range". 4128094c3ceSEric Biggers 4138094c3ceSEric BiggersFilenames encryption 4148094c3ceSEric Biggers-------------------- 4158094c3ceSEric Biggers 4168094c3ceSEric BiggersFor filenames, each full filename is encrypted at once. Because of 4178094c3ceSEric Biggersthe requirements to retain support for efficient directory lookups and 4188094c3ceSEric Biggersfilenames of up to 255 bytes, the same IV is used for every filename 4198094c3ceSEric Biggersin a directory. 4208094c3ceSEric Biggers 421b103fb76SEric BiggersHowever, each encrypted directory still uses a unique key, or 422b103fb76SEric Biggersalternatively has the file's nonce (for `DIRECT_KEY policies`_) or 423b103fb76SEric Biggersinode number (for `IV_INO_LBLK_64 policies`_) included in the IVs. 424b103fb76SEric BiggersThus, IV reuse is limited to within a single directory. 4258094c3ceSEric Biggers 4266b2a51ffSNathan HuckleberryWith CTS-CBC, the IV reuse means that when the plaintext filenames share a 4276b2a51ffSNathan Huckleberrycommon prefix at least as long as the cipher block size (16 bytes for AES), the 4286b2a51ffSNathan Huckleberrycorresponding encrypted filenames will also share a common prefix. This is 4296b2a51ffSNathan Huckleberryundesirable. Adiantum and HCTR2 do not have this weakness, as they are 4306b2a51ffSNathan Huckleberrywide-block encryption modes. 4318094c3ceSEric Biggers 4328094c3ceSEric BiggersAll supported filenames encryption modes accept any plaintext length 4338094c3ceSEric Biggers>= 16 bytes; cipher block alignment is not required. However, 4348094c3ceSEric Biggersfilenames shorter than 16 bytes are NUL-padded to 16 bytes before 4358094c3ceSEric Biggersbeing encrypted. In addition, to reduce leakage of filename lengths 4368094c3ceSEric Biggersvia their ciphertexts, all filenames are NUL-padded to the next 4, 8, 4378094c3ceSEric Biggers16, or 32-byte boundary (configurable). 32 is recommended since this 4388094c3ceSEric Biggersprovides the best confidentiality, at the cost of making directory 4398094c3ceSEric Biggersentries consume slightly more space. Note that since NUL (``\0``) is 4408094c3ceSEric Biggersnot otherwise a valid character in filenames, the padding will never 4418094c3ceSEric Biggersproduce duplicate plaintexts. 442f4f864c1SEric Biggers 443f4f864c1SEric BiggersSymbolic link targets are considered a type of filename and are 4448094c3ceSEric Biggersencrypted in the same way as filenames in directory entries, except 4458094c3ceSEric Biggersthat IV reuse is not a problem as each symlink has its own inode. 446f4f864c1SEric Biggers 447f4f864c1SEric BiggersUser API 448f4f864c1SEric Biggers======== 449f4f864c1SEric Biggers 450f4f864c1SEric BiggersSetting an encryption policy 451f4f864c1SEric Biggers---------------------------- 452f4f864c1SEric Biggers 453ba13f2c8SEric BiggersFS_IOC_SET_ENCRYPTION_POLICY 454ba13f2c8SEric Biggers~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 455ba13f2c8SEric Biggers 456f4f864c1SEric BiggersThe FS_IOC_SET_ENCRYPTION_POLICY ioctl sets an encryption policy on an 457f4f864c1SEric Biggersempty directory or verifies that a directory or regular file already 45874e2f8d3SMauro Carvalho Chehabhas the specified encryption policy. It takes in a pointer to 45974e2f8d3SMauro Carvalho Chehabstruct fscrypt_policy_v1 or struct fscrypt_policy_v2, defined as 46074e2f8d3SMauro Carvalho Chehabfollows:: 461f4f864c1SEric Biggers 462ba13f2c8SEric Biggers #define FSCRYPT_POLICY_V1 0 4632336d0deSEric Biggers #define FSCRYPT_KEY_DESCRIPTOR_SIZE 8 464ba13f2c8SEric Biggers struct fscrypt_policy_v1 { 465f4f864c1SEric Biggers __u8 version; 466f4f864c1SEric Biggers __u8 contents_encryption_mode; 467f4f864c1SEric Biggers __u8 filenames_encryption_mode; 468f4f864c1SEric Biggers __u8 flags; 4692336d0deSEric Biggers __u8 master_key_descriptor[FSCRYPT_KEY_DESCRIPTOR_SIZE]; 470f4f864c1SEric Biggers }; 471ba13f2c8SEric Biggers #define fscrypt_policy fscrypt_policy_v1 472ba13f2c8SEric Biggers 473ba13f2c8SEric Biggers #define FSCRYPT_POLICY_V2 2 474ba13f2c8SEric Biggers #define FSCRYPT_KEY_IDENTIFIER_SIZE 16 475ba13f2c8SEric Biggers struct fscrypt_policy_v2 { 476ba13f2c8SEric Biggers __u8 version; 477ba13f2c8SEric Biggers __u8 contents_encryption_mode; 478ba13f2c8SEric Biggers __u8 filenames_encryption_mode; 479ba13f2c8SEric Biggers __u8 flags; 480ba13f2c8SEric Biggers __u8 __reserved[4]; 481ba13f2c8SEric Biggers __u8 master_key_identifier[FSCRYPT_KEY_IDENTIFIER_SIZE]; 482ba13f2c8SEric Biggers }; 483f4f864c1SEric Biggers 484f4f864c1SEric BiggersThis structure must be initialized as follows: 485f4f864c1SEric Biggers 48674e2f8d3SMauro Carvalho Chehab- ``version`` must be FSCRYPT_POLICY_V1 (0) if 48774e2f8d3SMauro Carvalho Chehab struct fscrypt_policy_v1 is used or FSCRYPT_POLICY_V2 (2) if 48874e2f8d3SMauro Carvalho Chehab struct fscrypt_policy_v2 is used. (Note: we refer to the original 48974e2f8d3SMauro Carvalho Chehab policy version as "v1", though its version code is really 0.) 49074e2f8d3SMauro Carvalho Chehab For new encrypted directories, use v2 policies. 491f4f864c1SEric Biggers 492f4f864c1SEric Biggers- ``contents_encryption_mode`` and ``filenames_encryption_mode`` must 4932336d0deSEric Biggers be set to constants from ``<linux/fscrypt.h>`` which identify the 4942336d0deSEric Biggers encryption modes to use. If unsure, use FSCRYPT_MODE_AES_256_XTS 4952336d0deSEric Biggers (1) for ``contents_encryption_mode`` and FSCRYPT_MODE_AES_256_CTS 4962336d0deSEric Biggers (4) for ``filenames_encryption_mode``. 497f4f864c1SEric Biggers 498b103fb76SEric Biggers- ``flags`` contains optional flags from ``<linux/fscrypt.h>``: 499b103fb76SEric Biggers 500b103fb76SEric Biggers - FSCRYPT_POLICY_FLAGS_PAD_*: The amount of NUL padding to use when 501b103fb76SEric Biggers encrypting filenames. If unsure, use FSCRYPT_POLICY_FLAGS_PAD_32 502b103fb76SEric Biggers (0x3). 503b103fb76SEric Biggers - FSCRYPT_POLICY_FLAG_DIRECT_KEY: See `DIRECT_KEY policies`_. 504b103fb76SEric Biggers - FSCRYPT_POLICY_FLAG_IV_INO_LBLK_64: See `IV_INO_LBLK_64 505e3b1078bSEric Biggers policies`_. 506e3b1078bSEric Biggers - FSCRYPT_POLICY_FLAG_IV_INO_LBLK_32: See `IV_INO_LBLK_32 507e3b1078bSEric Biggers policies`_. 508e3b1078bSEric Biggers 509e3b1078bSEric Biggers v1 encryption policies only support the PAD_* and DIRECT_KEY flags. 510e3b1078bSEric Biggers The other flags are only supported by v2 encryption policies. 511e3b1078bSEric Biggers 512e3b1078bSEric Biggers The DIRECT_KEY, IV_INO_LBLK_64, and IV_INO_LBLK_32 flags are 513e3b1078bSEric Biggers mutually exclusive. 514f4f864c1SEric Biggers 515ba13f2c8SEric Biggers- For v2 encryption policies, ``__reserved`` must be zeroed. 516ba13f2c8SEric Biggers 517ba13f2c8SEric Biggers- For v1 encryption policies, ``master_key_descriptor`` specifies how 518ba13f2c8SEric Biggers to find the master key in a keyring; see `Adding keys`_. It is up 519ba13f2c8SEric Biggers to userspace to choose a unique ``master_key_descriptor`` for each 520ba13f2c8SEric Biggers master key. The e4crypt and fscrypt tools use the first 8 bytes of 521f4f864c1SEric Biggers ``SHA-512(SHA-512(master_key))``, but this particular scheme is not 522f4f864c1SEric Biggers required. Also, the master key need not be in the keyring yet when 523f4f864c1SEric Biggers FS_IOC_SET_ENCRYPTION_POLICY is executed. However, it must be added 524f4f864c1SEric Biggers before any files can be created in the encrypted directory. 525f4f864c1SEric Biggers 526ba13f2c8SEric Biggers For v2 encryption policies, ``master_key_descriptor`` has been 527ba13f2c8SEric Biggers replaced with ``master_key_identifier``, which is longer and cannot 528ba13f2c8SEric Biggers be arbitrarily chosen. Instead, the key must first be added using 529ba13f2c8SEric Biggers `FS_IOC_ADD_ENCRYPTION_KEY`_. Then, the ``key_spec.u.identifier`` 53074e2f8d3SMauro Carvalho Chehab the kernel returned in the struct fscrypt_add_key_arg must 53174e2f8d3SMauro Carvalho Chehab be used as the ``master_key_identifier`` in 53274e2f8d3SMauro Carvalho Chehab struct fscrypt_policy_v2. 533ba13f2c8SEric Biggers 534f4f864c1SEric BiggersIf the file is not yet encrypted, then FS_IOC_SET_ENCRYPTION_POLICY 535f4f864c1SEric Biggersverifies that the file is an empty directory. If so, the specified 536f4f864c1SEric Biggersencryption policy is assigned to the directory, turning it into an 537f4f864c1SEric Biggersencrypted directory. After that, and after providing the 538f4f864c1SEric Biggerscorresponding master key as described in `Adding keys`_, all regular 539f4f864c1SEric Biggersfiles, directories (recursively), and symlinks created in the 540f4f864c1SEric Biggersdirectory will be encrypted, inheriting the same encryption policy. 541f4f864c1SEric BiggersThe filenames in the directory's entries will be encrypted as well. 542f4f864c1SEric Biggers 543f4f864c1SEric BiggersAlternatively, if the file is already encrypted, then 544f4f864c1SEric BiggersFS_IOC_SET_ENCRYPTION_POLICY validates that the specified encryption 545f4f864c1SEric Biggerspolicy exactly matches the actual one. If they match, then the ioctl 546f4f864c1SEric Biggersreturns 0. Otherwise, it fails with EEXIST. This works on both 547f4f864c1SEric Biggersregular files and directories, including nonempty directories. 548f4f864c1SEric Biggers 549ba13f2c8SEric BiggersWhen a v2 encryption policy is assigned to a directory, it is also 550ba13f2c8SEric Biggersrequired that either the specified key has been added by the current 551ba13f2c8SEric Biggersuser or that the caller has CAP_FOWNER in the initial user namespace. 552ba13f2c8SEric Biggers(This is needed to prevent a user from encrypting their data with 553ba13f2c8SEric Biggersanother user's key.) The key must remain added while 554ba13f2c8SEric BiggersFS_IOC_SET_ENCRYPTION_POLICY is executing. However, if the new 555ba13f2c8SEric Biggersencrypted directory does not need to be accessed immediately, then the 556ba13f2c8SEric Biggerskey can be removed right away afterwards. 557ba13f2c8SEric Biggers 558f4f864c1SEric BiggersNote that the ext4 filesystem does not allow the root directory to be 559f4f864c1SEric Biggersencrypted, even if it is empty. Users who want to encrypt an entire 560f4f864c1SEric Biggersfilesystem with one key should consider using dm-crypt instead. 561f4f864c1SEric Biggers 562f4f864c1SEric BiggersFS_IOC_SET_ENCRYPTION_POLICY can fail with the following errors: 563f4f864c1SEric Biggers 564f4f864c1SEric Biggers- ``EACCES``: the file is not owned by the process's uid, nor does the 565f4f864c1SEric Biggers process have the CAP_FOWNER capability in a namespace with the file 566f4f864c1SEric Biggers owner's uid mapped 567f4f864c1SEric Biggers- ``EEXIST``: the file is already encrypted with an encryption policy 568f4f864c1SEric Biggers different from the one specified 569f4f864c1SEric Biggers- ``EINVAL``: an invalid encryption policy was specified (invalid 5706e1918cfSDaniel Rosenberg version, mode(s), or flags; or reserved bits were set); or a v1 5716e1918cfSDaniel Rosenberg encryption policy was specified but the directory has the casefold 5726e1918cfSDaniel Rosenberg flag enabled (casefolding is incompatible with v1 policies). 573ba13f2c8SEric Biggers- ``ENOKEY``: a v2 encryption policy was specified, but the key with 574ba13f2c8SEric Biggers the specified ``master_key_identifier`` has not been added, nor does 575ba13f2c8SEric Biggers the process have the CAP_FOWNER capability in the initial user 576ba13f2c8SEric Biggers namespace 577f4f864c1SEric Biggers- ``ENOTDIR``: the file is unencrypted and is a regular file, not a 578f4f864c1SEric Biggers directory 579f4f864c1SEric Biggers- ``ENOTEMPTY``: the file is unencrypted and is a nonempty directory 580f4f864c1SEric Biggers- ``ENOTTY``: this type of filesystem does not implement encryption 581f4f864c1SEric Biggers- ``EOPNOTSUPP``: the kernel was not configured with encryption 582643fa961SChandan Rajendra support for filesystems, or the filesystem superblock has not 583f4f864c1SEric Biggers had encryption enabled on it. (For example, to use encryption on an 584643fa961SChandan Rajendra ext4 filesystem, CONFIG_FS_ENCRYPTION must be enabled in the 585f4f864c1SEric Biggers kernel config, and the superblock must have had the "encrypt" 586f4f864c1SEric Biggers feature flag enabled using ``tune2fs -O encrypt`` or ``mkfs.ext4 -O 587f4f864c1SEric Biggers encrypt``.) 588f4f864c1SEric Biggers- ``EPERM``: this directory may not be encrypted, e.g. because it is 589f4f864c1SEric Biggers the root directory of an ext4 filesystem 590f4f864c1SEric Biggers- ``EROFS``: the filesystem is readonly 591f4f864c1SEric Biggers 592f4f864c1SEric BiggersGetting an encryption policy 593f4f864c1SEric Biggers---------------------------- 594f4f864c1SEric Biggers 595ba13f2c8SEric BiggersTwo ioctls are available to get a file's encryption policy: 596f4f864c1SEric Biggers 597ba13f2c8SEric Biggers- `FS_IOC_GET_ENCRYPTION_POLICY_EX`_ 598ba13f2c8SEric Biggers- `FS_IOC_GET_ENCRYPTION_POLICY`_ 599ba13f2c8SEric Biggers 600ba13f2c8SEric BiggersThe extended (_EX) version of the ioctl is more general and is 601ba13f2c8SEric Biggersrecommended to use when possible. However, on older kernels only the 602ba13f2c8SEric Biggersoriginal ioctl is available. Applications should try the extended 603ba13f2c8SEric Biggersversion, and if it fails with ENOTTY fall back to the original 604ba13f2c8SEric Biggersversion. 605ba13f2c8SEric Biggers 606ba13f2c8SEric BiggersFS_IOC_GET_ENCRYPTION_POLICY_EX 607ba13f2c8SEric Biggers~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 608ba13f2c8SEric Biggers 609ba13f2c8SEric BiggersThe FS_IOC_GET_ENCRYPTION_POLICY_EX ioctl retrieves the encryption 610ba13f2c8SEric Biggerspolicy, if any, for a directory or regular file. No additional 611ba13f2c8SEric Biggerspermissions are required beyond the ability to open the file. It 61274e2f8d3SMauro Carvalho Chehabtakes in a pointer to struct fscrypt_get_policy_ex_arg, 613ba13f2c8SEric Biggersdefined as follows:: 614ba13f2c8SEric Biggers 615ba13f2c8SEric Biggers struct fscrypt_get_policy_ex_arg { 616ba13f2c8SEric Biggers __u64 policy_size; /* input/output */ 617ba13f2c8SEric Biggers union { 618ba13f2c8SEric Biggers __u8 version; 619ba13f2c8SEric Biggers struct fscrypt_policy_v1 v1; 620ba13f2c8SEric Biggers struct fscrypt_policy_v2 v2; 621ba13f2c8SEric Biggers } policy; /* output */ 622ba13f2c8SEric Biggers }; 623ba13f2c8SEric Biggers 624ba13f2c8SEric BiggersThe caller must initialize ``policy_size`` to the size available for 625ba13f2c8SEric Biggersthe policy struct, i.e. ``sizeof(arg.policy)``. 626ba13f2c8SEric Biggers 627ba13f2c8SEric BiggersOn success, the policy struct is returned in ``policy``, and its 628ba13f2c8SEric Biggersactual size is returned in ``policy_size``. ``policy.version`` should 629ba13f2c8SEric Biggersbe checked to determine the version of policy returned. Note that the 630ba13f2c8SEric Biggersversion code for the "v1" policy is actually 0 (FSCRYPT_POLICY_V1). 631ba13f2c8SEric Biggers 632ba13f2c8SEric BiggersFS_IOC_GET_ENCRYPTION_POLICY_EX can fail with the following errors: 633f4f864c1SEric Biggers 634f4f864c1SEric Biggers- ``EINVAL``: the file is encrypted, but it uses an unrecognized 635ba13f2c8SEric Biggers encryption policy version 636f4f864c1SEric Biggers- ``ENODATA``: the file is not encrypted 637ba13f2c8SEric Biggers- ``ENOTTY``: this type of filesystem does not implement encryption, 638ba13f2c8SEric Biggers or this kernel is too old to support FS_IOC_GET_ENCRYPTION_POLICY_EX 639ba13f2c8SEric Biggers (try FS_IOC_GET_ENCRYPTION_POLICY instead) 640f4f864c1SEric Biggers- ``EOPNOTSUPP``: the kernel was not configured with encryption 6410642ea24SChao Yu support for this filesystem, or the filesystem superblock has not 6420642ea24SChao Yu had encryption enabled on it 643ba13f2c8SEric Biggers- ``EOVERFLOW``: the file is encrypted and uses a recognized 644ba13f2c8SEric Biggers encryption policy version, but the policy struct does not fit into 645ba13f2c8SEric Biggers the provided buffer 646f4f864c1SEric Biggers 647f4f864c1SEric BiggersNote: if you only need to know whether a file is encrypted or not, on 648f4f864c1SEric Biggersmost filesystems it is also possible to use the FS_IOC_GETFLAGS ioctl 649f4f864c1SEric Biggersand check for FS_ENCRYPT_FL, or to use the statx() system call and 650f4f864c1SEric Biggerscheck for STATX_ATTR_ENCRYPTED in stx_attributes. 651f4f864c1SEric Biggers 652ba13f2c8SEric BiggersFS_IOC_GET_ENCRYPTION_POLICY 653ba13f2c8SEric Biggers~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 654ba13f2c8SEric Biggers 655ba13f2c8SEric BiggersThe FS_IOC_GET_ENCRYPTION_POLICY ioctl can also retrieve the 656ba13f2c8SEric Biggersencryption policy, if any, for a directory or regular file. However, 657ba13f2c8SEric Biggersunlike `FS_IOC_GET_ENCRYPTION_POLICY_EX`_, 658ba13f2c8SEric BiggersFS_IOC_GET_ENCRYPTION_POLICY only supports the original policy 65974e2f8d3SMauro Carvalho Chehabversion. It takes in a pointer directly to struct fscrypt_policy_v1 66074e2f8d3SMauro Carvalho Chehabrather than struct fscrypt_get_policy_ex_arg. 661ba13f2c8SEric Biggers 662ba13f2c8SEric BiggersThe error codes for FS_IOC_GET_ENCRYPTION_POLICY are the same as those 663ba13f2c8SEric Biggersfor FS_IOC_GET_ENCRYPTION_POLICY_EX, except that 664ba13f2c8SEric BiggersFS_IOC_GET_ENCRYPTION_POLICY also returns ``EINVAL`` if the file is 665ba13f2c8SEric Biggersencrypted using a newer encryption policy version. 666ba13f2c8SEric Biggers 667f4f864c1SEric BiggersGetting the per-filesystem salt 668f4f864c1SEric Biggers------------------------------- 669f4f864c1SEric Biggers 670f4f864c1SEric BiggersSome filesystems, such as ext4 and F2FS, also support the deprecated 671f4f864c1SEric Biggersioctl FS_IOC_GET_ENCRYPTION_PWSALT. This ioctl retrieves a randomly 672f4f864c1SEric Biggersgenerated 16-byte value stored in the filesystem superblock. This 673f4f864c1SEric Biggersvalue is intended to used as a salt when deriving an encryption key 674f4f864c1SEric Biggersfrom a passphrase or other low-entropy user credential. 675f4f864c1SEric Biggers 676f4f864c1SEric BiggersFS_IOC_GET_ENCRYPTION_PWSALT is deprecated. Instead, prefer to 677f4f864c1SEric Biggersgenerate and manage any needed salt(s) in userspace. 678f4f864c1SEric Biggers 679e98ad464SEric BiggersGetting a file's encryption nonce 680e98ad464SEric Biggers--------------------------------- 681e98ad464SEric Biggers 682e98ad464SEric BiggersSince Linux v5.7, the ioctl FS_IOC_GET_ENCRYPTION_NONCE is supported. 683e98ad464SEric BiggersOn encrypted files and directories it gets the inode's 16-byte nonce. 684e98ad464SEric BiggersOn unencrypted files and directories, it fails with ENODATA. 685e98ad464SEric Biggers 686e98ad464SEric BiggersThis ioctl can be useful for automated tests which verify that the 687e98ad464SEric Biggersencryption is being done correctly. It is not needed for normal use 688e98ad464SEric Biggersof fscrypt. 689e98ad464SEric Biggers 690f4f864c1SEric BiggersAdding keys 691f4f864c1SEric Biggers----------- 692f4f864c1SEric Biggers 693ba13f2c8SEric BiggersFS_IOC_ADD_ENCRYPTION_KEY 694ba13f2c8SEric Biggers~~~~~~~~~~~~~~~~~~~~~~~~~ 695ba13f2c8SEric Biggers 696ba13f2c8SEric BiggersThe FS_IOC_ADD_ENCRYPTION_KEY ioctl adds a master encryption key to 697ba13f2c8SEric Biggersthe filesystem, making all files on the filesystem which were 698ba13f2c8SEric Biggersencrypted using that key appear "unlocked", i.e. in plaintext form. 699ba13f2c8SEric BiggersIt can be executed on any file or directory on the target filesystem, 700ba13f2c8SEric Biggersbut using the filesystem's root directory is recommended. It takes in 70174e2f8d3SMauro Carvalho Chehaba pointer to struct fscrypt_add_key_arg, defined as follows:: 702ba13f2c8SEric Biggers 703ba13f2c8SEric Biggers struct fscrypt_add_key_arg { 704ba13f2c8SEric Biggers struct fscrypt_key_specifier key_spec; 705ba13f2c8SEric Biggers __u32 raw_size; 70693edd392SEric Biggers __u32 key_id; 70793edd392SEric Biggers __u32 __reserved[8]; 708ba13f2c8SEric Biggers __u8 raw[]; 709ba13f2c8SEric Biggers }; 710ba13f2c8SEric Biggers 711ba13f2c8SEric Biggers #define FSCRYPT_KEY_SPEC_TYPE_DESCRIPTOR 1 712ba13f2c8SEric Biggers #define FSCRYPT_KEY_SPEC_TYPE_IDENTIFIER 2 713ba13f2c8SEric Biggers 714ba13f2c8SEric Biggers struct fscrypt_key_specifier { 715ba13f2c8SEric Biggers __u32 type; /* one of FSCRYPT_KEY_SPEC_TYPE_* */ 716ba13f2c8SEric Biggers __u32 __reserved; 717ba13f2c8SEric Biggers union { 718ba13f2c8SEric Biggers __u8 __reserved[32]; /* reserve some extra space */ 719ba13f2c8SEric Biggers __u8 descriptor[FSCRYPT_KEY_DESCRIPTOR_SIZE]; 720ba13f2c8SEric Biggers __u8 identifier[FSCRYPT_KEY_IDENTIFIER_SIZE]; 721ba13f2c8SEric Biggers } u; 722ba13f2c8SEric Biggers }; 723ba13f2c8SEric Biggers 72493edd392SEric Biggers struct fscrypt_provisioning_key_payload { 72593edd392SEric Biggers __u32 type; 72693edd392SEric Biggers __u32 __reserved; 72793edd392SEric Biggers __u8 raw[]; 72893edd392SEric Biggers }; 72993edd392SEric Biggers 73074e2f8d3SMauro Carvalho Chehabstruct fscrypt_add_key_arg must be zeroed, then initialized 731ba13f2c8SEric Biggersas follows: 732ba13f2c8SEric Biggers 733ba13f2c8SEric Biggers- If the key is being added for use by v1 encryption policies, then 734ba13f2c8SEric Biggers ``key_spec.type`` must contain FSCRYPT_KEY_SPEC_TYPE_DESCRIPTOR, and 735ba13f2c8SEric Biggers ``key_spec.u.descriptor`` must contain the descriptor of the key 736ba13f2c8SEric Biggers being added, corresponding to the value in the 73774e2f8d3SMauro Carvalho Chehab ``master_key_descriptor`` field of struct fscrypt_policy_v1. 73874e2f8d3SMauro Carvalho Chehab To add this type of key, the calling process must have the 73974e2f8d3SMauro Carvalho Chehab CAP_SYS_ADMIN capability in the initial user namespace. 740ba13f2c8SEric Biggers 741ba13f2c8SEric Biggers Alternatively, if the key is being added for use by v2 encryption 742ba13f2c8SEric Biggers policies, then ``key_spec.type`` must contain 743ba13f2c8SEric Biggers FSCRYPT_KEY_SPEC_TYPE_IDENTIFIER, and ``key_spec.u.identifier`` is 744ba13f2c8SEric Biggers an *output* field which the kernel fills in with a cryptographic 745ba13f2c8SEric Biggers hash of the key. To add this type of key, the calling process does 746ba13f2c8SEric Biggers not need any privileges. However, the number of keys that can be 747ba13f2c8SEric Biggers added is limited by the user's quota for the keyrings service (see 748ba13f2c8SEric Biggers ``Documentation/security/keys/core.rst``). 749ba13f2c8SEric Biggers 750ba13f2c8SEric Biggers- ``raw_size`` must be the size of the ``raw`` key provided, in bytes. 75193edd392SEric Biggers Alternatively, if ``key_id`` is nonzero, this field must be 0, since 75293edd392SEric Biggers in that case the size is implied by the specified Linux keyring key. 75393edd392SEric Biggers 75493edd392SEric Biggers- ``key_id`` is 0 if the raw key is given directly in the ``raw`` 75593edd392SEric Biggers field. Otherwise ``key_id`` is the ID of a Linux keyring key of 75674e2f8d3SMauro Carvalho Chehab type "fscrypt-provisioning" whose payload is 75774e2f8d3SMauro Carvalho Chehab struct fscrypt_provisioning_key_payload whose ``raw`` field contains 75874e2f8d3SMauro Carvalho Chehab the raw key and whose ``type`` field matches ``key_spec.type``. 75974e2f8d3SMauro Carvalho Chehab Since ``raw`` is variable-length, the total size of this key's 76074e2f8d3SMauro Carvalho Chehab payload must be ``sizeof(struct fscrypt_provisioning_key_payload)`` 76174e2f8d3SMauro Carvalho Chehab plus the raw key size. The process must have Search permission on 76274e2f8d3SMauro Carvalho Chehab this key. 76393edd392SEric Biggers 76493edd392SEric Biggers Most users should leave this 0 and specify the raw key directly. 76593edd392SEric Biggers The support for specifying a Linux keyring key is intended mainly to 76693edd392SEric Biggers allow re-adding keys after a filesystem is unmounted and re-mounted, 76793edd392SEric Biggers without having to store the raw keys in userspace memory. 768ba13f2c8SEric Biggers 769ba13f2c8SEric Biggers- ``raw`` is a variable-length field which must contain the actual 77093edd392SEric Biggers key, ``raw_size`` bytes long. Alternatively, if ``key_id`` is 77193edd392SEric Biggers nonzero, then this field is unused. 772ba13f2c8SEric Biggers 773ba13f2c8SEric BiggersFor v2 policy keys, the kernel keeps track of which user (identified 774ba13f2c8SEric Biggersby effective user ID) added the key, and only allows the key to be 775ba13f2c8SEric Biggersremoved by that user --- or by "root", if they use 776ba13f2c8SEric Biggers`FS_IOC_REMOVE_ENCRYPTION_KEY_ALL_USERS`_. 777ba13f2c8SEric Biggers 778ba13f2c8SEric BiggersHowever, if another user has added the key, it may be desirable to 779ba13f2c8SEric Biggersprevent that other user from unexpectedly removing it. Therefore, 780ba13f2c8SEric BiggersFS_IOC_ADD_ENCRYPTION_KEY may also be used to add a v2 policy key 781ba13f2c8SEric Biggers*again*, even if it's already added by other user(s). In this case, 782ba13f2c8SEric BiggersFS_IOC_ADD_ENCRYPTION_KEY will just install a claim to the key for the 783ba13f2c8SEric Biggerscurrent user, rather than actually add the key again (but the raw key 784ba13f2c8SEric Biggersmust still be provided, as a proof of knowledge). 785ba13f2c8SEric Biggers 786ba13f2c8SEric BiggersFS_IOC_ADD_ENCRYPTION_KEY returns 0 if either the key or a claim to 787ba13f2c8SEric Biggersthe key was either added or already exists. 788ba13f2c8SEric Biggers 789ba13f2c8SEric BiggersFS_IOC_ADD_ENCRYPTION_KEY can fail with the following errors: 790ba13f2c8SEric Biggers 791ba13f2c8SEric Biggers- ``EACCES``: FSCRYPT_KEY_SPEC_TYPE_DESCRIPTOR was specified, but the 792ba13f2c8SEric Biggers caller does not have the CAP_SYS_ADMIN capability in the initial 79393edd392SEric Biggers user namespace; or the raw key was specified by Linux key ID but the 79493edd392SEric Biggers process lacks Search permission on the key. 795ba13f2c8SEric Biggers- ``EDQUOT``: the key quota for this user would be exceeded by adding 796ba13f2c8SEric Biggers the key 797ba13f2c8SEric Biggers- ``EINVAL``: invalid key size or key specifier type, or reserved bits 798ba13f2c8SEric Biggers were set 79993edd392SEric Biggers- ``EKEYREJECTED``: the raw key was specified by Linux key ID, but the 80093edd392SEric Biggers key has the wrong type 80193edd392SEric Biggers- ``ENOKEY``: the raw key was specified by Linux key ID, but no key 80293edd392SEric Biggers exists with that ID 803ba13f2c8SEric Biggers- ``ENOTTY``: this type of filesystem does not implement encryption 804ba13f2c8SEric Biggers- ``EOPNOTSUPP``: the kernel was not configured with encryption 805ba13f2c8SEric Biggers support for this filesystem, or the filesystem superblock has not 806ba13f2c8SEric Biggers had encryption enabled on it 807ba13f2c8SEric Biggers 808ba13f2c8SEric BiggersLegacy method 809ba13f2c8SEric Biggers~~~~~~~~~~~~~ 810ba13f2c8SEric Biggers 811ba13f2c8SEric BiggersFor v1 encryption policies, a master encryption key can also be 812ba13f2c8SEric Biggersprovided by adding it to a process-subscribed keyring, e.g. to a 813ba13f2c8SEric Biggerssession keyring, or to a user keyring if the user keyring is linked 814ba13f2c8SEric Biggersinto the session keyring. 815ba13f2c8SEric Biggers 816ba13f2c8SEric BiggersThis method is deprecated (and not supported for v2 encryption 817ba13f2c8SEric Biggerspolicies) for several reasons. First, it cannot be used in 818ba13f2c8SEric Biggerscombination with FS_IOC_REMOVE_ENCRYPTION_KEY (see `Removing keys`_), 819ba13f2c8SEric Biggersso for removing a key a workaround such as keyctl_unlink() in 820ba13f2c8SEric Biggerscombination with ``sync; echo 2 > /proc/sys/vm/drop_caches`` would 821ba13f2c8SEric Biggershave to be used. Second, it doesn't match the fact that the 822ba13f2c8SEric Biggerslocked/unlocked status of encrypted files (i.e. whether they appear to 823ba13f2c8SEric Biggersbe in plaintext form or in ciphertext form) is global. This mismatch 824ba13f2c8SEric Biggershas caused much confusion as well as real problems when processes 825ba13f2c8SEric Biggersrunning under different UIDs, such as a ``sudo`` command, need to 826ba13f2c8SEric Biggersaccess encrypted files. 827ba13f2c8SEric Biggers 828ba13f2c8SEric BiggersNevertheless, to add a key to one of the process-subscribed keyrings, 829ba13f2c8SEric Biggersthe add_key() system call can be used (see: 830f4f864c1SEric Biggers``Documentation/security/keys/core.rst``). The key type must be 831f4f864c1SEric Biggers"logon"; keys of this type are kept in kernel memory and cannot be 832f4f864c1SEric Biggersread back by userspace. The key description must be "fscrypt:" 833f4f864c1SEric Biggersfollowed by the 16-character lower case hex representation of the 834f4f864c1SEric Biggers``master_key_descriptor`` that was set in the encryption policy. The 835f4f864c1SEric Biggerskey payload must conform to the following structure:: 836f4f864c1SEric Biggers 8372336d0deSEric Biggers #define FSCRYPT_MAX_KEY_SIZE 64 838f4f864c1SEric Biggers 839f4f864c1SEric Biggers struct fscrypt_key { 840ba13f2c8SEric Biggers __u32 mode; 841ba13f2c8SEric Biggers __u8 raw[FSCRYPT_MAX_KEY_SIZE]; 842ba13f2c8SEric Biggers __u32 size; 843f4f864c1SEric Biggers }; 844f4f864c1SEric Biggers 845f4f864c1SEric Biggers``mode`` is ignored; just set it to 0. The actual key is provided in 846f4f864c1SEric Biggers``raw`` with ``size`` indicating its size in bytes. That is, the 847f4f864c1SEric Biggersbytes ``raw[0..size-1]`` (inclusive) are the actual key. 848f4f864c1SEric Biggers 849f4f864c1SEric BiggersThe key description prefix "fscrypt:" may alternatively be replaced 850f4f864c1SEric Biggerswith a filesystem-specific prefix such as "ext4:". However, the 851f4f864c1SEric Biggersfilesystem-specific prefixes are deprecated and should not be used in 852f4f864c1SEric Biggersnew programs. 853f4f864c1SEric Biggers 854ba13f2c8SEric BiggersRemoving keys 855ba13f2c8SEric Biggers------------- 856f4f864c1SEric Biggers 857ba13f2c8SEric BiggersTwo ioctls are available for removing a key that was added by 858ba13f2c8SEric Biggers`FS_IOC_ADD_ENCRYPTION_KEY`_: 859ba13f2c8SEric Biggers 860ba13f2c8SEric Biggers- `FS_IOC_REMOVE_ENCRYPTION_KEY`_ 861ba13f2c8SEric Biggers- `FS_IOC_REMOVE_ENCRYPTION_KEY_ALL_USERS`_ 862ba13f2c8SEric Biggers 863ba13f2c8SEric BiggersThese two ioctls differ only in cases where v2 policy keys are added 864ba13f2c8SEric Biggersor removed by non-root users. 865ba13f2c8SEric Biggers 866ba13f2c8SEric BiggersThese ioctls don't work on keys that were added via the legacy 867ba13f2c8SEric Biggersprocess-subscribed keyrings mechanism. 868ba13f2c8SEric Biggers 869ba13f2c8SEric BiggersBefore using these ioctls, read the `Kernel memory compromise`_ 870ba13f2c8SEric Biggerssection for a discussion of the security goals and limitations of 871ba13f2c8SEric Biggersthese ioctls. 872ba13f2c8SEric Biggers 873ba13f2c8SEric BiggersFS_IOC_REMOVE_ENCRYPTION_KEY 874ba13f2c8SEric Biggers~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 875ba13f2c8SEric Biggers 876ba13f2c8SEric BiggersThe FS_IOC_REMOVE_ENCRYPTION_KEY ioctl removes a claim to a master 877ba13f2c8SEric Biggersencryption key from the filesystem, and possibly removes the key 878ba13f2c8SEric Biggersitself. It can be executed on any file or directory on the target 879ba13f2c8SEric Biggersfilesystem, but using the filesystem's root directory is recommended. 88074e2f8d3SMauro Carvalho ChehabIt takes in a pointer to struct fscrypt_remove_key_arg, defined 88174e2f8d3SMauro Carvalho Chehabas follows:: 882ba13f2c8SEric Biggers 883ba13f2c8SEric Biggers struct fscrypt_remove_key_arg { 884ba13f2c8SEric Biggers struct fscrypt_key_specifier key_spec; 885ba13f2c8SEric Biggers #define FSCRYPT_KEY_REMOVAL_STATUS_FLAG_FILES_BUSY 0x00000001 886ba13f2c8SEric Biggers #define FSCRYPT_KEY_REMOVAL_STATUS_FLAG_OTHER_USERS 0x00000002 887ba13f2c8SEric Biggers __u32 removal_status_flags; /* output */ 888ba13f2c8SEric Biggers __u32 __reserved[5]; 889ba13f2c8SEric Biggers }; 890ba13f2c8SEric Biggers 891ba13f2c8SEric BiggersThis structure must be zeroed, then initialized as follows: 892ba13f2c8SEric Biggers 893ba13f2c8SEric Biggers- The key to remove is specified by ``key_spec``: 894ba13f2c8SEric Biggers 895ba13f2c8SEric Biggers - To remove a key used by v1 encryption policies, set 896ba13f2c8SEric Biggers ``key_spec.type`` to FSCRYPT_KEY_SPEC_TYPE_DESCRIPTOR and fill 897ba13f2c8SEric Biggers in ``key_spec.u.descriptor``. To remove this type of key, the 898ba13f2c8SEric Biggers calling process must have the CAP_SYS_ADMIN capability in the 899ba13f2c8SEric Biggers initial user namespace. 900ba13f2c8SEric Biggers 901ba13f2c8SEric Biggers - To remove a key used by v2 encryption policies, set 902ba13f2c8SEric Biggers ``key_spec.type`` to FSCRYPT_KEY_SPEC_TYPE_IDENTIFIER and fill 903ba13f2c8SEric Biggers in ``key_spec.u.identifier``. 904ba13f2c8SEric Biggers 905ba13f2c8SEric BiggersFor v2 policy keys, this ioctl is usable by non-root users. However, 906ba13f2c8SEric Biggersto make this possible, it actually just removes the current user's 907ba13f2c8SEric Biggersclaim to the key, undoing a single call to FS_IOC_ADD_ENCRYPTION_KEY. 908ba13f2c8SEric BiggersOnly after all claims are removed is the key really removed. 909ba13f2c8SEric Biggers 910ba13f2c8SEric BiggersFor example, if FS_IOC_ADD_ENCRYPTION_KEY was called with uid 1000, 911ba13f2c8SEric Biggersthen the key will be "claimed" by uid 1000, and 912ba13f2c8SEric BiggersFS_IOC_REMOVE_ENCRYPTION_KEY will only succeed as uid 1000. Or, if 913ba13f2c8SEric Biggersboth uids 1000 and 2000 added the key, then for each uid 914ba13f2c8SEric BiggersFS_IOC_REMOVE_ENCRYPTION_KEY will only remove their own claim. Only 915ba13f2c8SEric Biggersonce *both* are removed is the key really removed. (Think of it like 916ba13f2c8SEric Biggersunlinking a file that may have hard links.) 917ba13f2c8SEric Biggers 918ba13f2c8SEric BiggersIf FS_IOC_REMOVE_ENCRYPTION_KEY really removes the key, it will also 919ba13f2c8SEric Biggerstry to "lock" all files that had been unlocked with the key. It won't 920ba13f2c8SEric Biggerslock files that are still in-use, so this ioctl is expected to be used 921ba13f2c8SEric Biggersin cooperation with userspace ensuring that none of the files are 922ba13f2c8SEric Biggersstill open. However, if necessary, this ioctl can be executed again 923ba13f2c8SEric Biggerslater to retry locking any remaining files. 924ba13f2c8SEric Biggers 925ba13f2c8SEric BiggersFS_IOC_REMOVE_ENCRYPTION_KEY returns 0 if either the key was removed 926ba13f2c8SEric Biggers(but may still have files remaining to be locked), the user's claim to 927ba13f2c8SEric Biggersthe key was removed, or the key was already removed but had files 928ba13f2c8SEric Biggersremaining to be the locked so the ioctl retried locking them. In any 929ba13f2c8SEric Biggersof these cases, ``removal_status_flags`` is filled in with the 930ba13f2c8SEric Biggersfollowing informational status flags: 931ba13f2c8SEric Biggers 932ba13f2c8SEric Biggers- ``FSCRYPT_KEY_REMOVAL_STATUS_FLAG_FILES_BUSY``: set if some file(s) 933ba13f2c8SEric Biggers are still in-use. Not guaranteed to be set in the case where only 934ba13f2c8SEric Biggers the user's claim to the key was removed. 935ba13f2c8SEric Biggers- ``FSCRYPT_KEY_REMOVAL_STATUS_FLAG_OTHER_USERS``: set if only the 936ba13f2c8SEric Biggers user's claim to the key was removed, not the key itself 937ba13f2c8SEric Biggers 938ba13f2c8SEric BiggersFS_IOC_REMOVE_ENCRYPTION_KEY can fail with the following errors: 939ba13f2c8SEric Biggers 940ba13f2c8SEric Biggers- ``EACCES``: The FSCRYPT_KEY_SPEC_TYPE_DESCRIPTOR key specifier type 941ba13f2c8SEric Biggers was specified, but the caller does not have the CAP_SYS_ADMIN 942ba13f2c8SEric Biggers capability in the initial user namespace 943ba13f2c8SEric Biggers- ``EINVAL``: invalid key specifier type, or reserved bits were set 944ba13f2c8SEric Biggers- ``ENOKEY``: the key object was not found at all, i.e. it was never 945ba13f2c8SEric Biggers added in the first place or was already fully removed including all 946ba13f2c8SEric Biggers files locked; or, the user does not have a claim to the key (but 947ba13f2c8SEric Biggers someone else does). 948ba13f2c8SEric Biggers- ``ENOTTY``: this type of filesystem does not implement encryption 949ba13f2c8SEric Biggers- ``EOPNOTSUPP``: the kernel was not configured with encryption 950ba13f2c8SEric Biggers support for this filesystem, or the filesystem superblock has not 951ba13f2c8SEric Biggers had encryption enabled on it 952ba13f2c8SEric Biggers 953ba13f2c8SEric BiggersFS_IOC_REMOVE_ENCRYPTION_KEY_ALL_USERS 954ba13f2c8SEric Biggers~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 955ba13f2c8SEric Biggers 956ba13f2c8SEric BiggersFS_IOC_REMOVE_ENCRYPTION_KEY_ALL_USERS is exactly the same as 957ba13f2c8SEric Biggers`FS_IOC_REMOVE_ENCRYPTION_KEY`_, except that for v2 policy keys, the 958ba13f2c8SEric BiggersALL_USERS version of the ioctl will remove all users' claims to the 959ba13f2c8SEric Biggerskey, not just the current user's. I.e., the key itself will always be 960ba13f2c8SEric Biggersremoved, no matter how many users have added it. This difference is 961ba13f2c8SEric Biggersonly meaningful if non-root users are adding and removing keys. 962ba13f2c8SEric Biggers 963ba13f2c8SEric BiggersBecause of this, FS_IOC_REMOVE_ENCRYPTION_KEY_ALL_USERS also requires 964ba13f2c8SEric Biggers"root", namely the CAP_SYS_ADMIN capability in the initial user 965ba13f2c8SEric Biggersnamespace. Otherwise it will fail with EACCES. 966ba13f2c8SEric Biggers 967ba13f2c8SEric BiggersGetting key status 968ba13f2c8SEric Biggers------------------ 969ba13f2c8SEric Biggers 970ba13f2c8SEric BiggersFS_IOC_GET_ENCRYPTION_KEY_STATUS 971ba13f2c8SEric Biggers~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 972ba13f2c8SEric Biggers 973ba13f2c8SEric BiggersThe FS_IOC_GET_ENCRYPTION_KEY_STATUS ioctl retrieves the status of a 974ba13f2c8SEric Biggersmaster encryption key. It can be executed on any file or directory on 975ba13f2c8SEric Biggersthe target filesystem, but using the filesystem's root directory is 97674e2f8d3SMauro Carvalho Chehabrecommended. It takes in a pointer to 97774e2f8d3SMauro Carvalho Chehabstruct fscrypt_get_key_status_arg, defined as follows:: 978ba13f2c8SEric Biggers 979ba13f2c8SEric Biggers struct fscrypt_get_key_status_arg { 980ba13f2c8SEric Biggers /* input */ 981ba13f2c8SEric Biggers struct fscrypt_key_specifier key_spec; 982ba13f2c8SEric Biggers __u32 __reserved[6]; 983ba13f2c8SEric Biggers 984ba13f2c8SEric Biggers /* output */ 985ba13f2c8SEric Biggers #define FSCRYPT_KEY_STATUS_ABSENT 1 986ba13f2c8SEric Biggers #define FSCRYPT_KEY_STATUS_PRESENT 2 987ba13f2c8SEric Biggers #define FSCRYPT_KEY_STATUS_INCOMPLETELY_REMOVED 3 988ba13f2c8SEric Biggers __u32 status; 989ba13f2c8SEric Biggers #define FSCRYPT_KEY_STATUS_FLAG_ADDED_BY_SELF 0x00000001 990ba13f2c8SEric Biggers __u32 status_flags; 991ba13f2c8SEric Biggers __u32 user_count; 992ba13f2c8SEric Biggers __u32 __out_reserved[13]; 993ba13f2c8SEric Biggers }; 994ba13f2c8SEric Biggers 995ba13f2c8SEric BiggersThe caller must zero all input fields, then fill in ``key_spec``: 996ba13f2c8SEric Biggers 997ba13f2c8SEric Biggers - To get the status of a key for v1 encryption policies, set 998ba13f2c8SEric Biggers ``key_spec.type`` to FSCRYPT_KEY_SPEC_TYPE_DESCRIPTOR and fill 999ba13f2c8SEric Biggers in ``key_spec.u.descriptor``. 1000ba13f2c8SEric Biggers 1001ba13f2c8SEric Biggers - To get the status of a key for v2 encryption policies, set 1002ba13f2c8SEric Biggers ``key_spec.type`` to FSCRYPT_KEY_SPEC_TYPE_IDENTIFIER and fill 1003ba13f2c8SEric Biggers in ``key_spec.u.identifier``. 1004ba13f2c8SEric Biggers 1005ba13f2c8SEric BiggersOn success, 0 is returned and the kernel fills in the output fields: 1006ba13f2c8SEric Biggers 1007ba13f2c8SEric Biggers- ``status`` indicates whether the key is absent, present, or 1008ba13f2c8SEric Biggers incompletely removed. Incompletely removed means that the master 1009ba13f2c8SEric Biggers secret has been removed, but some files are still in use; i.e., 1010ba13f2c8SEric Biggers `FS_IOC_REMOVE_ENCRYPTION_KEY`_ returned 0 but set the informational 1011ba13f2c8SEric Biggers status flag FSCRYPT_KEY_REMOVAL_STATUS_FLAG_FILES_BUSY. 1012ba13f2c8SEric Biggers 1013ba13f2c8SEric Biggers- ``status_flags`` can contain the following flags: 1014ba13f2c8SEric Biggers 1015ba13f2c8SEric Biggers - ``FSCRYPT_KEY_STATUS_FLAG_ADDED_BY_SELF`` indicates that the key 1016ba13f2c8SEric Biggers has added by the current user. This is only set for keys 1017ba13f2c8SEric Biggers identified by ``identifier`` rather than by ``descriptor``. 1018ba13f2c8SEric Biggers 1019ba13f2c8SEric Biggers- ``user_count`` specifies the number of users who have added the key. 1020ba13f2c8SEric Biggers This is only set for keys identified by ``identifier`` rather than 1021ba13f2c8SEric Biggers by ``descriptor``. 1022ba13f2c8SEric Biggers 1023ba13f2c8SEric BiggersFS_IOC_GET_ENCRYPTION_KEY_STATUS can fail with the following errors: 1024ba13f2c8SEric Biggers 1025ba13f2c8SEric Biggers- ``EINVAL``: invalid key specifier type, or reserved bits were set 1026ba13f2c8SEric Biggers- ``ENOTTY``: this type of filesystem does not implement encryption 1027ba13f2c8SEric Biggers- ``EOPNOTSUPP``: the kernel was not configured with encryption 1028ba13f2c8SEric Biggers support for this filesystem, or the filesystem superblock has not 1029ba13f2c8SEric Biggers had encryption enabled on it 1030ba13f2c8SEric Biggers 1031ba13f2c8SEric BiggersAmong other use cases, FS_IOC_GET_ENCRYPTION_KEY_STATUS can be useful 1032ba13f2c8SEric Biggersfor determining whether the key for a given encrypted directory needs 1033ba13f2c8SEric Biggersto be added before prompting the user for the passphrase needed to 1034ba13f2c8SEric Biggersderive the key. 1035ba13f2c8SEric Biggers 1036ba13f2c8SEric BiggersFS_IOC_GET_ENCRYPTION_KEY_STATUS can only get the status of keys in 1037ba13f2c8SEric Biggersthe filesystem-level keyring, i.e. the keyring managed by 1038ba13f2c8SEric Biggers`FS_IOC_ADD_ENCRYPTION_KEY`_ and `FS_IOC_REMOVE_ENCRYPTION_KEY`_. It 1039ba13f2c8SEric Biggerscannot get the status of a key that has only been added for use by v1 1040ba13f2c8SEric Biggersencryption policies using the legacy mechanism involving 1041ba13f2c8SEric Biggersprocess-subscribed keyrings. 1042f4f864c1SEric Biggers 1043f4f864c1SEric BiggersAccess semantics 1044f4f864c1SEric Biggers================ 1045f4f864c1SEric Biggers 1046f4f864c1SEric BiggersWith the key 1047f4f864c1SEric Biggers------------ 1048f4f864c1SEric Biggers 1049f4f864c1SEric BiggersWith the encryption key, encrypted regular files, directories, and 1050f4f864c1SEric Biggerssymlinks behave very similarly to their unencrypted counterparts --- 1051f4f864c1SEric Biggersafter all, the encryption is intended to be transparent. However, 1052f4f864c1SEric Biggersastute users may notice some differences in behavior: 1053f4f864c1SEric Biggers 1054f4f864c1SEric Biggers- Unencrypted files, or files encrypted with a different encryption 1055f4f864c1SEric Biggers policy (i.e. different key, modes, or flags), cannot be renamed or 1056f4f864c1SEric Biggers linked into an encrypted directory; see `Encryption policy 1057f5e55e77SEric Biggers enforcement`_. Attempts to do so will fail with EXDEV. However, 1058f4f864c1SEric Biggers encrypted files can be renamed within an encrypted directory, or 1059f4f864c1SEric Biggers into an unencrypted directory. 1060f4f864c1SEric Biggers 1061f5e55e77SEric Biggers Note: "moving" an unencrypted file into an encrypted directory, e.g. 1062f5e55e77SEric Biggers with the `mv` program, is implemented in userspace by a copy 1063f5e55e77SEric Biggers followed by a delete. Be aware that the original unencrypted data 1064f5e55e77SEric Biggers may remain recoverable from free space on the disk; prefer to keep 1065f5e55e77SEric Biggers all files encrypted from the very beginning. The `shred` program 1066f5e55e77SEric Biggers may be used to overwrite the source files but isn't guaranteed to be 1067f5e55e77SEric Biggers effective on all filesystems and storage devices. 1068f5e55e77SEric Biggers 1069cdaa1b19SEric Biggers- Direct I/O is supported on encrypted files only under some 1070cdaa1b19SEric Biggers circumstances. For details, see `Direct I/O support`_. 1071f4f864c1SEric Biggers 1072457b1e35SEric Biggers- The fallocate operations FALLOC_FL_COLLAPSE_RANGE and 1073457b1e35SEric Biggers FALLOC_FL_INSERT_RANGE are not supported on encrypted files and will 1074457b1e35SEric Biggers fail with EOPNOTSUPP. 1075f4f864c1SEric Biggers 1076f4f864c1SEric Biggers- Online defragmentation of encrypted files is not supported. The 1077f4f864c1SEric Biggers EXT4_IOC_MOVE_EXT and F2FS_IOC_MOVE_RANGE ioctls will fail with 1078f4f864c1SEric Biggers EOPNOTSUPP. 1079f4f864c1SEric Biggers 1080f4f864c1SEric Biggers- The ext4 filesystem does not support data journaling with encrypted 1081f4f864c1SEric Biggers regular files. It will fall back to ordered data mode instead. 1082f4f864c1SEric Biggers 1083f4f864c1SEric Biggers- DAX (Direct Access) is not supported on encrypted files. 1084f4f864c1SEric Biggers 10852f46a2bcSEric Biggers- The maximum length of an encrypted symlink is 2 bytes shorter than 10862f46a2bcSEric Biggers the maximum length of an unencrypted symlink. For example, on an 10872f46a2bcSEric Biggers EXT4 filesystem with a 4K block size, unencrypted symlinks can be up 10882f46a2bcSEric Biggers to 4095 bytes long, while encrypted symlinks can only be up to 4093 10892f46a2bcSEric Biggers bytes long (both lengths excluding the terminating null). 1090f4f864c1SEric Biggers 1091f4f864c1SEric BiggersNote that mmap *is* supported. This is possible because the pagecache 1092f4f864c1SEric Biggersfor an encrypted file contains the plaintext, not the ciphertext. 1093f4f864c1SEric Biggers 1094f4f864c1SEric BiggersWithout the key 1095f4f864c1SEric Biggers--------------- 1096f4f864c1SEric Biggers 1097f4f864c1SEric BiggersSome filesystem operations may be performed on encrypted regular 1098f4f864c1SEric Biggersfiles, directories, and symlinks even before their encryption key has 1099ba13f2c8SEric Biggersbeen added, or after their encryption key has been removed: 1100f4f864c1SEric Biggers 1101f4f864c1SEric Biggers- File metadata may be read, e.g. using stat(). 1102f4f864c1SEric Biggers 1103f4f864c1SEric Biggers- Directories may be listed, in which case the filenames will be 1104f4f864c1SEric Biggers listed in an encoded form derived from their ciphertext. The 1105f4f864c1SEric Biggers current encoding algorithm is described in `Filename hashing and 1106f4f864c1SEric Biggers encoding`_. The algorithm is subject to change, but it is 1107f4f864c1SEric Biggers guaranteed that the presented filenames will be no longer than 1108f4f864c1SEric Biggers NAME_MAX bytes, will not contain the ``/`` or ``\0`` characters, and 1109f4f864c1SEric Biggers will uniquely identify directory entries. 1110f4f864c1SEric Biggers 1111f4f864c1SEric Biggers The ``.`` and ``..`` directory entries are special. They are always 1112f4f864c1SEric Biggers present and are not encrypted or encoded. 1113f4f864c1SEric Biggers 1114f4f864c1SEric Biggers- Files may be deleted. That is, nondirectory files may be deleted 1115f4f864c1SEric Biggers with unlink() as usual, and empty directories may be deleted with 1116f4f864c1SEric Biggers rmdir() as usual. Therefore, ``rm`` and ``rm -r`` will work as 1117f4f864c1SEric Biggers expected. 1118f4f864c1SEric Biggers 1119f4f864c1SEric Biggers- Symlink targets may be read and followed, but they will be presented 1120f4f864c1SEric Biggers in encrypted form, similar to filenames in directories. Hence, they 1121f4f864c1SEric Biggers are unlikely to point to anywhere useful. 1122f4f864c1SEric Biggers 1123f4f864c1SEric BiggersWithout the key, regular files cannot be opened or truncated. 1124f4f864c1SEric BiggersAttempts to do so will fail with ENOKEY. This implies that any 1125f4f864c1SEric Biggersregular file operations that require a file descriptor, such as 1126f4f864c1SEric Biggersread(), write(), mmap(), fallocate(), and ioctl(), are also forbidden. 1127f4f864c1SEric Biggers 1128f4f864c1SEric BiggersAlso without the key, files of any type (including directories) cannot 1129f4f864c1SEric Biggersbe created or linked into an encrypted directory, nor can a name in an 1130f4f864c1SEric Biggersencrypted directory be the source or target of a rename, nor can an 1131f4f864c1SEric BiggersO_TMPFILE temporary file be created in an encrypted directory. All 1132f4f864c1SEric Biggerssuch operations will fail with ENOKEY. 1133f4f864c1SEric Biggers 1134f4f864c1SEric BiggersIt is not currently possible to backup and restore encrypted files 1135f4f864c1SEric Biggerswithout the encryption key. This would require special APIs which 1136f4f864c1SEric Biggershave not yet been implemented. 1137f4f864c1SEric Biggers 1138f4f864c1SEric BiggersEncryption policy enforcement 1139f4f864c1SEric Biggers============================= 1140f4f864c1SEric Biggers 1141f4f864c1SEric BiggersAfter an encryption policy has been set on a directory, all regular 1142f4f864c1SEric Biggersfiles, directories, and symbolic links created in that directory 1143f4f864c1SEric Biggers(recursively) will inherit that encryption policy. Special files --- 1144f4f864c1SEric Biggersthat is, named pipes, device nodes, and UNIX domain sockets --- will 1145f4f864c1SEric Biggersnot be encrypted. 1146f4f864c1SEric Biggers 1147f4f864c1SEric BiggersExcept for those special files, it is forbidden to have unencrypted 1148f4f864c1SEric Biggersfiles, or files encrypted with a different encryption policy, in an 1149f4f864c1SEric Biggersencrypted directory tree. Attempts to link or rename such a file into 1150f5e55e77SEric Biggersan encrypted directory will fail with EXDEV. This is also enforced 1151f4f864c1SEric Biggersduring ->lookup() to provide limited protection against offline 1152f4f864c1SEric Biggersattacks that try to disable or downgrade encryption in known locations 1153f4f864c1SEric Biggerswhere applications may later write sensitive data. It is recommended 1154f4f864c1SEric Biggersthat systems implementing a form of "verified boot" take advantage of 1155f4f864c1SEric Biggersthis by validating all top-level encryption policies prior to access. 1156f4f864c1SEric Biggers 1157abb861faSEric BiggersInline encryption support 1158abb861faSEric Biggers========================= 1159abb861faSEric Biggers 1160abb861faSEric BiggersBy default, fscrypt uses the kernel crypto API for all cryptographic 1161abb861faSEric Biggersoperations (other than HKDF, which fscrypt partially implements 1162abb861faSEric Biggersitself). The kernel crypto API supports hardware crypto accelerators, 1163abb861faSEric Biggersbut only ones that work in the traditional way where all inputs and 1164abb861faSEric Biggersoutputs (e.g. plaintexts and ciphertexts) are in memory. fscrypt can 1165abb861faSEric Biggerstake advantage of such hardware, but the traditional acceleration 1166abb861faSEric Biggersmodel isn't particularly efficient and fscrypt hasn't been optimized 1167abb861faSEric Biggersfor it. 1168abb861faSEric Biggers 1169abb861faSEric BiggersInstead, many newer systems (especially mobile SoCs) have *inline 1170abb861faSEric Biggersencryption hardware* that can encrypt/decrypt data while it is on its 1171abb861faSEric Biggersway to/from the storage device. Linux supports inline encryption 1172abb861faSEric Biggersthrough a set of extensions to the block layer called *blk-crypto*. 1173abb861faSEric Biggersblk-crypto allows filesystems to attach encryption contexts to bios 1174abb861faSEric Biggers(I/O requests) to specify how the data will be encrypted or decrypted 1175abb861faSEric Biggersin-line. For more information about blk-crypto, see 1176abb861faSEric Biggers:ref:`Documentation/block/inline-encryption.rst <inline_encryption>`. 1177abb861faSEric Biggers 1178abb861faSEric BiggersOn supported filesystems (currently ext4 and f2fs), fscrypt can use 1179abb861faSEric Biggersblk-crypto instead of the kernel crypto API to encrypt/decrypt file 1180abb861faSEric Biggerscontents. To enable this, set CONFIG_FS_ENCRYPTION_INLINE_CRYPT=y in 1181abb861faSEric Biggersthe kernel configuration, and specify the "inlinecrypt" mount option 1182abb861faSEric Biggerswhen mounting the filesystem. 1183abb861faSEric Biggers 1184abb861faSEric BiggersNote that the "inlinecrypt" mount option just specifies to use inline 1185abb861faSEric Biggersencryption when possible; it doesn't force its use. fscrypt will 1186abb861faSEric Biggersstill fall back to using the kernel crypto API on files where the 1187abb861faSEric Biggersinline encryption hardware doesn't have the needed crypto capabilities 1188abb861faSEric Biggers(e.g. support for the needed encryption algorithm and data unit size) 1189abb861faSEric Biggersand where blk-crypto-fallback is unusable. (For blk-crypto-fallback 1190abb861faSEric Biggersto be usable, it must be enabled in the kernel configuration with 1191abb861faSEric BiggersCONFIG_BLK_INLINE_ENCRYPTION_FALLBACK=y.) 1192abb861faSEric Biggers 1193abb861faSEric BiggersCurrently fscrypt always uses the filesystem block size (which is 1194abb861faSEric Biggersusually 4096 bytes) as the data unit size. Therefore, it can only use 1195abb861faSEric Biggersinline encryption hardware that supports that data unit size. 1196abb861faSEric Biggers 1197abb861faSEric BiggersInline encryption doesn't affect the ciphertext or other aspects of 1198abb861faSEric Biggersthe on-disk format, so users may freely switch back and forth between 1199abb861faSEric Biggersusing "inlinecrypt" and not using "inlinecrypt". 1200abb861faSEric Biggers 1201cdaa1b19SEric BiggersDirect I/O support 1202cdaa1b19SEric Biggers================== 1203cdaa1b19SEric Biggers 1204cdaa1b19SEric BiggersFor direct I/O on an encrypted file to work, the following conditions 1205cdaa1b19SEric Biggersmust be met (in addition to the conditions for direct I/O on an 1206cdaa1b19SEric Biggersunencrypted file): 1207cdaa1b19SEric Biggers 1208cdaa1b19SEric Biggers* The file must be using inline encryption. Usually this means that 1209cdaa1b19SEric Biggers the filesystem must be mounted with ``-o inlinecrypt`` and inline 1210cdaa1b19SEric Biggers encryption hardware must be present. However, a software fallback 1211cdaa1b19SEric Biggers is also available. For details, see `Inline encryption support`_. 1212cdaa1b19SEric Biggers 1213cdaa1b19SEric Biggers* The I/O request must be fully aligned to the filesystem block size. 1214cdaa1b19SEric Biggers This means that the file position the I/O is targeting, the lengths 1215cdaa1b19SEric Biggers of all I/O segments, and the memory addresses of all I/O buffers 1216cdaa1b19SEric Biggers must be multiples of this value. Note that the filesystem block 1217cdaa1b19SEric Biggers size may be greater than the logical block size of the block device. 1218cdaa1b19SEric Biggers 1219cdaa1b19SEric BiggersIf either of the above conditions is not met, then direct I/O on the 1220cdaa1b19SEric Biggersencrypted file will fall back to buffered I/O. 1221cdaa1b19SEric Biggers 1222f4f864c1SEric BiggersImplementation details 1223f4f864c1SEric Biggers====================== 1224f4f864c1SEric Biggers 1225f4f864c1SEric BiggersEncryption context 1226f4f864c1SEric Biggers------------------ 1227f4f864c1SEric Biggers 122874e2f8d3SMauro Carvalho ChehabAn encryption policy is represented on-disk by 122974e2f8d3SMauro Carvalho Chehabstruct fscrypt_context_v1 or struct fscrypt_context_v2. It is up to 123074e2f8d3SMauro Carvalho Chehabindividual filesystems to decide where to store it, but normally it 123174e2f8d3SMauro Carvalho Chehabwould be stored in a hidden extended attribute. It should *not* be 1232ba13f2c8SEric Biggersexposed by the xattr-related system calls such as getxattr() and 1233ba13f2c8SEric Biggerssetxattr() because of the special semantics of the encryption xattr. 1234ba13f2c8SEric Biggers(In particular, there would be much confusion if an encryption policy 1235ba13f2c8SEric Biggerswere to be added to or removed from anything other than an empty 1236ba13f2c8SEric Biggersdirectory.) These structs are defined as follows:: 1237f4f864c1SEric Biggers 12381d6217a4SEric Biggers #define FSCRYPT_FILE_NONCE_SIZE 16 1239f4f864c1SEric Biggers 1240ba13f2c8SEric Biggers #define FSCRYPT_KEY_DESCRIPTOR_SIZE 8 1241ba13f2c8SEric Biggers struct fscrypt_context_v1 { 1242ba13f2c8SEric Biggers u8 version; 1243f4f864c1SEric Biggers u8 contents_encryption_mode; 1244f4f864c1SEric Biggers u8 filenames_encryption_mode; 1245f4f864c1SEric Biggers u8 flags; 12462336d0deSEric Biggers u8 master_key_descriptor[FSCRYPT_KEY_DESCRIPTOR_SIZE]; 12471d6217a4SEric Biggers u8 nonce[FSCRYPT_FILE_NONCE_SIZE]; 1248f4f864c1SEric Biggers }; 1249f4f864c1SEric Biggers 1250ba13f2c8SEric Biggers #define FSCRYPT_KEY_IDENTIFIER_SIZE 16 1251ba13f2c8SEric Biggers struct fscrypt_context_v2 { 1252ba13f2c8SEric Biggers u8 version; 1253ba13f2c8SEric Biggers u8 contents_encryption_mode; 1254ba13f2c8SEric Biggers u8 filenames_encryption_mode; 1255ba13f2c8SEric Biggers u8 flags; 1256ba13f2c8SEric Biggers u8 __reserved[4]; 1257ba13f2c8SEric Biggers u8 master_key_identifier[FSCRYPT_KEY_IDENTIFIER_SIZE]; 12581d6217a4SEric Biggers u8 nonce[FSCRYPT_FILE_NONCE_SIZE]; 1259ba13f2c8SEric Biggers }; 1260ba13f2c8SEric Biggers 1261ba13f2c8SEric BiggersThe context structs contain the same information as the corresponding 1262ba13f2c8SEric Biggerspolicy structs (see `Setting an encryption policy`_), except that the 1263ba13f2c8SEric Biggerscontext structs also contain a nonce. The nonce is randomly generated 1264ba13f2c8SEric Biggersby the kernel and is used as KDF input or as a tweak to cause 1265f592efe7SEric Biggersdifferent files to be encrypted differently; see `Per-file encryption 1266f592efe7SEric Biggerskeys`_ and `DIRECT_KEY policies`_. 1267f4f864c1SEric Biggers 1268f4f864c1SEric BiggersData path changes 1269f4f864c1SEric Biggers----------------- 1270f4f864c1SEric Biggers 1271abb861faSEric BiggersWhen inline encryption is used, filesystems just need to associate 1272abb861faSEric Biggersencryption contexts with bios to specify how the block layer or the 1273abb861faSEric Biggersinline encryption hardware will encrypt/decrypt the file contents. 1274abb861faSEric Biggers 1275abb861faSEric BiggersWhen inline encryption isn't used, filesystems must encrypt/decrypt 1276abb861faSEric Biggersthe file contents themselves, as described below: 1277abb861faSEric Biggers 127808830c8bSMatthew Wilcox (Oracle)For the read path (->read_folio()) of regular files, filesystems can 1279f4f864c1SEric Biggersread the ciphertext into the page cache and decrypt it in-place. The 1280*51e4e315SEric Biggersfolio lock must be held until decryption has finished, to prevent the 1281*51e4e315SEric Biggersfolio from becoming visible to userspace prematurely. 1282f4f864c1SEric Biggers 1283f4f864c1SEric BiggersFor the write path (->writepage()) of regular files, filesystems 1284f4f864c1SEric Biggerscannot encrypt data in-place in the page cache, since the cached 1285f4f864c1SEric Biggersplaintext must be preserved. Instead, filesystems must encrypt into a 1286f4f864c1SEric Biggerstemporary buffer or "bounce page", then write out the temporary 1287f4f864c1SEric Biggersbuffer. Some filesystems, such as UBIFS, already use temporary 1288f4f864c1SEric Biggersbuffers regardless of encryption. Other filesystems, such as ext4 and 1289f4f864c1SEric BiggersF2FS, have to allocate bounce pages specially for encryption. 1290f4f864c1SEric Biggers 1291f4f864c1SEric BiggersFilename hashing and encoding 1292f4f864c1SEric Biggers----------------------------- 1293f4f864c1SEric Biggers 1294f4f864c1SEric BiggersModern filesystems accelerate directory lookups by using indexed 1295f4f864c1SEric Biggersdirectories. An indexed directory is organized as a tree keyed by 1296f4f864c1SEric Biggersfilename hashes. When a ->lookup() is requested, the filesystem 1297f4f864c1SEric Biggersnormally hashes the filename being looked up so that it can quickly 1298f4f864c1SEric Biggersfind the corresponding directory entry, if any. 1299f4f864c1SEric Biggers 1300f4f864c1SEric BiggersWith encryption, lookups must be supported and efficient both with and 1301f4f864c1SEric Biggerswithout the encryption key. Clearly, it would not work to hash the 1302f4f864c1SEric Biggersplaintext filenames, since the plaintext filenames are unavailable 1303f4f864c1SEric Biggerswithout the key. (Hashing the plaintext filenames would also make it 1304f4f864c1SEric Biggersimpossible for the filesystem's fsck tool to optimize encrypted 1305f4f864c1SEric Biggersdirectories.) Instead, filesystems hash the ciphertext filenames, 1306f4f864c1SEric Biggersi.e. the bytes actually stored on-disk in the directory entries. When 1307f4f864c1SEric Biggersasked to do a ->lookup() with the key, the filesystem just encrypts 1308f4f864c1SEric Biggersthe user-supplied name to get the ciphertext. 1309f4f864c1SEric Biggers 1310f4f864c1SEric BiggersLookups without the key are more complicated. The raw ciphertext may 1311f4f864c1SEric Biggerscontain the ``\0`` and ``/`` characters, which are illegal in 1312ba47b515SEric Biggersfilenames. Therefore, readdir() must base64url-encode the ciphertext 1313ba47b515SEric Biggersfor presentation. For most filenames, this works fine; on ->lookup(), 1314ba47b515SEric Biggersthe filesystem just base64url-decodes the user-supplied name to get 1315ba47b515SEric Biggersback to the raw ciphertext. 1316f4f864c1SEric Biggers 1317ba47b515SEric BiggersHowever, for very long filenames, base64url encoding would cause the 1318f4f864c1SEric Biggersfilename length to exceed NAME_MAX. To prevent this, readdir() 1319f4f864c1SEric Biggersactually presents long filenames in an abbreviated form which encodes 1320f4f864c1SEric Biggersa strong "hash" of the ciphertext filename, along with the optional 1321f4f864c1SEric Biggersfilesystem-specific hash(es) needed for directory lookups. This 1322f4f864c1SEric Biggersallows the filesystem to still, with a high degree of confidence, map 1323f4f864c1SEric Biggersthe filename given in ->lookup() back to a particular directory entry 132474e2f8d3SMauro Carvalho Chehabthat was previously listed by readdir(). See 132574e2f8d3SMauro Carvalho Chehabstruct fscrypt_nokey_name in the source for more details. 1326f4f864c1SEric Biggers 1327f4f864c1SEric BiggersNote that the precise way that filenames are presented to userspace 1328f4f864c1SEric Biggerswithout the key is subject to change in the future. It is only meant 1329f4f864c1SEric Biggersas a way to temporarily present valid filenames so that commands like 1330f4f864c1SEric Biggers``rm -r`` work as expected on encrypted directories. 133105643363SEric Biggers 133205643363SEric BiggersTests 133305643363SEric Biggers===== 133405643363SEric Biggers 133505643363SEric BiggersTo test fscrypt, use xfstests, which is Linux's de facto standard 133605643363SEric Biggersfilesystem test suite. First, run all the tests in the "encrypt" 1337880253eaSSatya Tangiralagroup on the relevant filesystem(s). One can also run the tests 1338880253eaSSatya Tangiralawith the 'inlinecrypt' mount option to test the implementation for 1339880253eaSSatya Tangiralainline encryption support. For example, to test ext4 and 134005643363SEric Biggersf2fs encryption using `kvm-xfstests 134105643363SEric Biggers<https://github.com/tytso/xfstests-bld/blob/master/Documentation/kvm-quickstart.md>`_:: 134205643363SEric Biggers 134305643363SEric Biggers kvm-xfstests -c ext4,f2fs -g encrypt 13445fee3609SSatya Tangirala kvm-xfstests -c ext4,f2fs -g encrypt -m inlinecrypt 134505643363SEric Biggers 134605643363SEric BiggersUBIFS encryption can also be tested this way, but it should be done in 134705643363SEric Biggersa separate command, and it takes some time for kvm-xfstests to set up 134805643363SEric Biggersemulated UBI volumes:: 134905643363SEric Biggers 135005643363SEric Biggers kvm-xfstests -c ubifs -g encrypt 135105643363SEric Biggers 135205643363SEric BiggersNo tests should fail. However, tests that use non-default encryption 135305643363SEric Biggersmodes (e.g. generic/549 and generic/550) will be skipped if the needed 135405643363SEric Biggersalgorithms were not built into the kernel's crypto API. Also, tests 135505643363SEric Biggersthat access the raw block device (e.g. generic/399, generic/548, 135605643363SEric Biggersgeneric/549, generic/550) will be skipped on UBIFS. 135705643363SEric Biggers 135805643363SEric BiggersBesides running the "encrypt" group tests, for ext4 and f2fs it's also 135905643363SEric Biggerspossible to run most xfstests with the "test_dummy_encryption" mount 136005643363SEric Biggersoption. This option causes all new files to be automatically 136105643363SEric Biggersencrypted with a dummy key, without having to make any API calls. 136205643363SEric BiggersThis tests the encrypted I/O paths more thoroughly. To do this with 136305643363SEric Biggerskvm-xfstests, use the "encrypt" filesystem configuration:: 136405643363SEric Biggers 136505643363SEric Biggers kvm-xfstests -c ext4/encrypt,f2fs/encrypt -g auto 13665fee3609SSatya Tangirala kvm-xfstests -c ext4/encrypt,f2fs/encrypt -g auto -m inlinecrypt 136705643363SEric Biggers 136805643363SEric BiggersBecause this runs many more tests than "-g encrypt" does, it takes 136905643363SEric Biggersmuch longer to run; so also consider using `gce-xfstests 137005643363SEric Biggers<https://github.com/tytso/xfstests-bld/blob/master/Documentation/gce-xfstests.md>`_ 137105643363SEric Biggersinstead of kvm-xfstests:: 137205643363SEric Biggers 137305643363SEric Biggers gce-xfstests -c ext4/encrypt,f2fs/encrypt -g auto 13745fee3609SSatya Tangirala gce-xfstests -c ext4/encrypt,f2fs/encrypt -g auto -m inlinecrypt 1375