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 80f4f864c1SEric BiggersCryptographic API algorithms are. If a vulnerable algorithm is used, 81f4f864c1SEric Biggerssuch as a table-based implementation of AES, it may be possible for an 82f4f864c1SEric Biggersattacker to mount a side channel attack against the online system. 83f4f864c1SEric BiggersSide channel attacks may also be mounted against applications 84f4f864c1SEric Biggersconsuming 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 179f4f864c1SEric Biggersgreater of the key length needed by the contents and filenames 180f4f864c1SEric Biggersencryption modes being used. For example, if AES-256-XTS is used for 181f4f864c1SEric Biggerscontents encryption, the master key must be 64 bytes (512 bits). Note 182f4f864c1SEric Biggersthat the XTS mode is defined to require a key twice as long as that 183f4f864c1SEric Biggersrequired by the underlying block cipher. 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 295b103fb76SEric Biggerscompliant with the UFS or eMMC standards, which support only 64 IV 296b103fb76SEric Biggersbits per I/O request and may have only a small number of keyslots. 297b103fb76SEric Biggers 298ba13f2c8SEric BiggersKey identifiers 299ba13f2c8SEric Biggers--------------- 300ba13f2c8SEric Biggers 301ba13f2c8SEric BiggersFor master keys used for v2 encryption policies, a unique 16-byte "key 302ba13f2c8SEric Biggersidentifier" is also derived using the KDF. This value is stored in 303ba13f2c8SEric Biggersthe clear, since it is needed to reliably identify the key itself. 304ba13f2c8SEric Biggers 305aa408f83SDaniel RosenbergDirhash keys 306aa408f83SDaniel Rosenberg------------ 307aa408f83SDaniel Rosenberg 308aa408f83SDaniel RosenbergFor directories that are indexed using a secret-keyed dirhash over the 309aa408f83SDaniel Rosenbergplaintext filenames, the KDF is also used to derive a 128-bit 310aa408f83SDaniel RosenbergSipHash-2-4 key per directory in order to hash filenames. This works 311aa408f83SDaniel Rosenbergjust like deriving a per-file encryption key, except that a different 312aa408f83SDaniel RosenbergKDF context is used. Currently, only casefolded ("case-insensitive") 313aa408f83SDaniel Rosenbergencrypted directories use this style of hashing. 314aa408f83SDaniel Rosenberg 315f4f864c1SEric BiggersEncryption modes and usage 316f4f864c1SEric Biggers========================== 317f4f864c1SEric Biggers 318f4f864c1SEric Biggersfscrypt allows one encryption mode to be specified for file contents 319f4f864c1SEric Biggersand one encryption mode to be specified for filenames. Different 320f4f864c1SEric Biggersdirectory trees are permitted to use different encryption modes. 321f4f864c1SEric BiggersCurrently, the following pairs of encryption modes are supported: 322f4f864c1SEric Biggers 323f4f864c1SEric Biggers- AES-256-XTS for contents and AES-256-CTS-CBC for filenames 324f4f864c1SEric Biggers- AES-128-CBC for contents and AES-128-CTS-CBC for filenames 3258094c3ceSEric Biggers- Adiantum for both contents and filenames 326f4f864c1SEric Biggers 3278094c3ceSEric BiggersIf unsure, you should use the (AES-256-XTS, AES-256-CTS-CBC) pair. 3288094c3ceSEric Biggers 329f4f864c1SEric BiggersAES-128-CBC was added only for low-powered embedded devices with 330adbd9b4dSEric Biggerscrypto accelerators such as CAAM or CESA that do not support XTS. To 3314006d799SEric Biggersuse AES-128-CBC, CONFIG_CRYPTO_ESSIV and CONFIG_CRYPTO_SHA256 (or 3324006d799SEric Biggersanother SHA-256 implementation) must be enabled so that ESSIV can be 3334006d799SEric Biggersused. 334f4f864c1SEric Biggers 3358094c3ceSEric BiggersAdiantum is a (primarily) stream cipher-based mode that is fast even 3368094c3ceSEric Biggerson CPUs without dedicated crypto instructions. It's also a true 3378094c3ceSEric Biggerswide-block mode, unlike XTS. It can also eliminate the need to derive 338f592efe7SEric Biggersper-file encryption keys. However, it depends on the security of two 339f592efe7SEric Biggersprimitives, XChaCha12 and AES-256, rather than just one. See the 340f592efe7SEric Biggerspaper "Adiantum: length-preserving encryption for entry-level 341f592efe7SEric Biggersprocessors" (https://eprint.iacr.org/2018/720.pdf) for more details. 342f592efe7SEric BiggersTo use Adiantum, CONFIG_CRYPTO_ADIANTUM must be enabled. Also, fast 3438094c3ceSEric Biggersimplementations of ChaCha and NHPoly1305 should be enabled, e.g. 3448094c3ceSEric BiggersCONFIG_CRYPTO_CHACHA20_NEON and CONFIG_CRYPTO_NHPOLY1305_NEON for ARM. 3458094c3ceSEric Biggers 346f4f864c1SEric BiggersNew encryption modes can be added relatively easily, without changes 347f4f864c1SEric Biggersto individual filesystems. However, authenticated encryption (AE) 348f4f864c1SEric Biggersmodes are not currently supported because of the difficulty of dealing 349f4f864c1SEric Biggerswith ciphertext expansion. 350f4f864c1SEric Biggers 3518094c3ceSEric BiggersContents encryption 3528094c3ceSEric Biggers------------------- 3538094c3ceSEric Biggers 354f4f864c1SEric BiggersFor file contents, each filesystem block is encrypted independently. 355196624e1SChandan RajendraStarting from Linux kernel 5.5, encryption of filesystems with block 356196624e1SChandan Rajendrasize less than system's page size is supported. 357f4f864c1SEric Biggers 3588094c3ceSEric BiggersEach block's IV is set to the logical block number within the file as 3598094c3ceSEric Biggersa little endian number, except that: 360f4f864c1SEric Biggers 3618094c3ceSEric Biggers- With CBC mode encryption, ESSIV is also used. Specifically, each IV 3628094c3ceSEric Biggers is encrypted with AES-256 where the AES-256 key is the SHA-256 hash 3638094c3ceSEric Biggers of the file's data encryption key. 3648094c3ceSEric Biggers 365b103fb76SEric Biggers- With `DIRECT_KEY policies`_, the file's nonce is appended to the IV. 366b103fb76SEric Biggers Currently this is only allowed with the Adiantum encryption mode. 367b103fb76SEric Biggers 368b103fb76SEric Biggers- With `IV_INO_LBLK_64 policies`_, the logical block number is limited 369b103fb76SEric Biggers to 32 bits and is placed in bits 0-31 of the IV. The inode number 370b103fb76SEric Biggers (which is also limited to 32 bits) is placed in bits 32-63. 371b103fb76SEric Biggers 372b103fb76SEric BiggersNote that because file logical block numbers are included in the IVs, 373b103fb76SEric Biggersfilesystems must enforce that blocks are never shifted around within 374b103fb76SEric Biggersencrypted files, e.g. via "collapse range" or "insert range". 3758094c3ceSEric Biggers 3768094c3ceSEric BiggersFilenames encryption 3778094c3ceSEric Biggers-------------------- 3788094c3ceSEric Biggers 3798094c3ceSEric BiggersFor filenames, each full filename is encrypted at once. Because of 3808094c3ceSEric Biggersthe requirements to retain support for efficient directory lookups and 3818094c3ceSEric Biggersfilenames of up to 255 bytes, the same IV is used for every filename 3828094c3ceSEric Biggersin a directory. 3838094c3ceSEric Biggers 384b103fb76SEric BiggersHowever, each encrypted directory still uses a unique key, or 385b103fb76SEric Biggersalternatively has the file's nonce (for `DIRECT_KEY policies`_) or 386b103fb76SEric Biggersinode number (for `IV_INO_LBLK_64 policies`_) included in the IVs. 387b103fb76SEric BiggersThus, IV reuse is limited to within a single directory. 3888094c3ceSEric Biggers 3898094c3ceSEric BiggersWith CTS-CBC, the IV reuse means that when the plaintext filenames 3908094c3ceSEric Biggersshare a common prefix at least as long as the cipher block size (16 3918094c3ceSEric Biggersbytes for AES), the corresponding encrypted filenames will also share 3928094c3ceSEric Biggersa common prefix. This is undesirable. Adiantum does not have this 3938094c3ceSEric Biggersweakness, as it is a wide-block encryption mode. 3948094c3ceSEric Biggers 3958094c3ceSEric BiggersAll supported filenames encryption modes accept any plaintext length 3968094c3ceSEric Biggers>= 16 bytes; cipher block alignment is not required. However, 3978094c3ceSEric Biggersfilenames shorter than 16 bytes are NUL-padded to 16 bytes before 3988094c3ceSEric Biggersbeing encrypted. In addition, to reduce leakage of filename lengths 3998094c3ceSEric Biggersvia their ciphertexts, all filenames are NUL-padded to the next 4, 8, 4008094c3ceSEric Biggers16, or 32-byte boundary (configurable). 32 is recommended since this 4018094c3ceSEric Biggersprovides the best confidentiality, at the cost of making directory 4028094c3ceSEric Biggersentries consume slightly more space. Note that since NUL (``\0``) is 4038094c3ceSEric Biggersnot otherwise a valid character in filenames, the padding will never 4048094c3ceSEric Biggersproduce duplicate plaintexts. 405f4f864c1SEric Biggers 406f4f864c1SEric BiggersSymbolic link targets are considered a type of filename and are 4078094c3ceSEric Biggersencrypted in the same way as filenames in directory entries, except 4088094c3ceSEric Biggersthat IV reuse is not a problem as each symlink has its own inode. 409f4f864c1SEric Biggers 410f4f864c1SEric BiggersUser API 411f4f864c1SEric Biggers======== 412f4f864c1SEric Biggers 413f4f864c1SEric BiggersSetting an encryption policy 414f4f864c1SEric Biggers---------------------------- 415f4f864c1SEric Biggers 416ba13f2c8SEric BiggersFS_IOC_SET_ENCRYPTION_POLICY 417ba13f2c8SEric Biggers~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 418ba13f2c8SEric Biggers 419f4f864c1SEric BiggersThe FS_IOC_SET_ENCRYPTION_POLICY ioctl sets an encryption policy on an 420f4f864c1SEric Biggersempty directory or verifies that a directory or regular file already 421f4f864c1SEric Biggershas the specified encryption policy. It takes in a pointer to a 422ba13f2c8SEric Biggers:c:type:`struct fscrypt_policy_v1` or a :c:type:`struct 423ba13f2c8SEric Biggersfscrypt_policy_v2`, defined as follows:: 424f4f864c1SEric Biggers 425ba13f2c8SEric Biggers #define FSCRYPT_POLICY_V1 0 4262336d0deSEric Biggers #define FSCRYPT_KEY_DESCRIPTOR_SIZE 8 427ba13f2c8SEric Biggers struct fscrypt_policy_v1 { 428f4f864c1SEric Biggers __u8 version; 429f4f864c1SEric Biggers __u8 contents_encryption_mode; 430f4f864c1SEric Biggers __u8 filenames_encryption_mode; 431f4f864c1SEric Biggers __u8 flags; 4322336d0deSEric Biggers __u8 master_key_descriptor[FSCRYPT_KEY_DESCRIPTOR_SIZE]; 433f4f864c1SEric Biggers }; 434ba13f2c8SEric Biggers #define fscrypt_policy fscrypt_policy_v1 435ba13f2c8SEric Biggers 436ba13f2c8SEric Biggers #define FSCRYPT_POLICY_V2 2 437ba13f2c8SEric Biggers #define FSCRYPT_KEY_IDENTIFIER_SIZE 16 438ba13f2c8SEric Biggers struct fscrypt_policy_v2 { 439ba13f2c8SEric Biggers __u8 version; 440ba13f2c8SEric Biggers __u8 contents_encryption_mode; 441ba13f2c8SEric Biggers __u8 filenames_encryption_mode; 442ba13f2c8SEric Biggers __u8 flags; 443ba13f2c8SEric Biggers __u8 __reserved[4]; 444ba13f2c8SEric Biggers __u8 master_key_identifier[FSCRYPT_KEY_IDENTIFIER_SIZE]; 445ba13f2c8SEric Biggers }; 446f4f864c1SEric Biggers 447f4f864c1SEric BiggersThis structure must be initialized as follows: 448f4f864c1SEric Biggers 449ba13f2c8SEric Biggers- ``version`` must be FSCRYPT_POLICY_V1 (0) if the struct is 450ba13f2c8SEric Biggers :c:type:`fscrypt_policy_v1` or FSCRYPT_POLICY_V2 (2) if the struct 451ba13f2c8SEric Biggers is :c:type:`fscrypt_policy_v2`. (Note: we refer to the original 452ba13f2c8SEric Biggers policy version as "v1", though its version code is really 0.) For 453ba13f2c8SEric Biggers new encrypted directories, use v2 policies. 454f4f864c1SEric Biggers 455f4f864c1SEric Biggers- ``contents_encryption_mode`` and ``filenames_encryption_mode`` must 4562336d0deSEric Biggers be set to constants from ``<linux/fscrypt.h>`` which identify the 4572336d0deSEric Biggers encryption modes to use. If unsure, use FSCRYPT_MODE_AES_256_XTS 4582336d0deSEric Biggers (1) for ``contents_encryption_mode`` and FSCRYPT_MODE_AES_256_CTS 4592336d0deSEric Biggers (4) for ``filenames_encryption_mode``. 460f4f864c1SEric Biggers 461b103fb76SEric Biggers- ``flags`` contains optional flags from ``<linux/fscrypt.h>``: 462b103fb76SEric Biggers 463b103fb76SEric Biggers - FSCRYPT_POLICY_FLAGS_PAD_*: The amount of NUL padding to use when 464b103fb76SEric Biggers encrypting filenames. If unsure, use FSCRYPT_POLICY_FLAGS_PAD_32 465b103fb76SEric Biggers (0x3). 466b103fb76SEric Biggers - FSCRYPT_POLICY_FLAG_DIRECT_KEY: See `DIRECT_KEY policies`_. 467b103fb76SEric Biggers - FSCRYPT_POLICY_FLAG_IV_INO_LBLK_64: See `IV_INO_LBLK_64 468b103fb76SEric Biggers policies`_. This is mutually exclusive with DIRECT_KEY and is not 469b103fb76SEric Biggers supported on v1 policies. 470f4f864c1SEric Biggers 471ba13f2c8SEric Biggers- For v2 encryption policies, ``__reserved`` must be zeroed. 472ba13f2c8SEric Biggers 473ba13f2c8SEric Biggers- For v1 encryption policies, ``master_key_descriptor`` specifies how 474ba13f2c8SEric Biggers to find the master key in a keyring; see `Adding keys`_. It is up 475ba13f2c8SEric Biggers to userspace to choose a unique ``master_key_descriptor`` for each 476ba13f2c8SEric Biggers master key. The e4crypt and fscrypt tools use the first 8 bytes of 477f4f864c1SEric Biggers ``SHA-512(SHA-512(master_key))``, but this particular scheme is not 478f4f864c1SEric Biggers required. Also, the master key need not be in the keyring yet when 479f4f864c1SEric Biggers FS_IOC_SET_ENCRYPTION_POLICY is executed. However, it must be added 480f4f864c1SEric Biggers before any files can be created in the encrypted directory. 481f4f864c1SEric Biggers 482ba13f2c8SEric Biggers For v2 encryption policies, ``master_key_descriptor`` has been 483ba13f2c8SEric Biggers replaced with ``master_key_identifier``, which is longer and cannot 484ba13f2c8SEric Biggers be arbitrarily chosen. Instead, the key must first be added using 485ba13f2c8SEric Biggers `FS_IOC_ADD_ENCRYPTION_KEY`_. Then, the ``key_spec.u.identifier`` 486ba13f2c8SEric Biggers the kernel returned in the :c:type:`struct fscrypt_add_key_arg` must 487ba13f2c8SEric Biggers be used as the ``master_key_identifier`` in the :c:type:`struct 488ba13f2c8SEric Biggers fscrypt_policy_v2`. 489ba13f2c8SEric Biggers 490f4f864c1SEric BiggersIf the file is not yet encrypted, then FS_IOC_SET_ENCRYPTION_POLICY 491f4f864c1SEric Biggersverifies that the file is an empty directory. If so, the specified 492f4f864c1SEric Biggersencryption policy is assigned to the directory, turning it into an 493f4f864c1SEric Biggersencrypted directory. After that, and after providing the 494f4f864c1SEric Biggerscorresponding master key as described in `Adding keys`_, all regular 495f4f864c1SEric Biggersfiles, directories (recursively), and symlinks created in the 496f4f864c1SEric Biggersdirectory will be encrypted, inheriting the same encryption policy. 497f4f864c1SEric BiggersThe filenames in the directory's entries will be encrypted as well. 498f4f864c1SEric Biggers 499f4f864c1SEric BiggersAlternatively, if the file is already encrypted, then 500f4f864c1SEric BiggersFS_IOC_SET_ENCRYPTION_POLICY validates that the specified encryption 501f4f864c1SEric Biggerspolicy exactly matches the actual one. If they match, then the ioctl 502f4f864c1SEric Biggersreturns 0. Otherwise, it fails with EEXIST. This works on both 503f4f864c1SEric Biggersregular files and directories, including nonempty directories. 504f4f864c1SEric Biggers 505ba13f2c8SEric BiggersWhen a v2 encryption policy is assigned to a directory, it is also 506ba13f2c8SEric Biggersrequired that either the specified key has been added by the current 507ba13f2c8SEric Biggersuser or that the caller has CAP_FOWNER in the initial user namespace. 508ba13f2c8SEric Biggers(This is needed to prevent a user from encrypting their data with 509ba13f2c8SEric Biggersanother user's key.) The key must remain added while 510ba13f2c8SEric BiggersFS_IOC_SET_ENCRYPTION_POLICY is executing. However, if the new 511ba13f2c8SEric Biggersencrypted directory does not need to be accessed immediately, then the 512ba13f2c8SEric Biggerskey can be removed right away afterwards. 513ba13f2c8SEric Biggers 514f4f864c1SEric BiggersNote that the ext4 filesystem does not allow the root directory to be 515f4f864c1SEric Biggersencrypted, even if it is empty. Users who want to encrypt an entire 516f4f864c1SEric Biggersfilesystem with one key should consider using dm-crypt instead. 517f4f864c1SEric Biggers 518f4f864c1SEric BiggersFS_IOC_SET_ENCRYPTION_POLICY can fail with the following errors: 519f4f864c1SEric Biggers 520f4f864c1SEric Biggers- ``EACCES``: the file is not owned by the process's uid, nor does the 521f4f864c1SEric Biggers process have the CAP_FOWNER capability in a namespace with the file 522f4f864c1SEric Biggers owner's uid mapped 523f4f864c1SEric Biggers- ``EEXIST``: the file is already encrypted with an encryption policy 524f4f864c1SEric Biggers different from the one specified 525f4f864c1SEric Biggers- ``EINVAL``: an invalid encryption policy was specified (invalid 5266e1918cfSDaniel Rosenberg version, mode(s), or flags; or reserved bits were set); or a v1 5276e1918cfSDaniel Rosenberg encryption policy was specified but the directory has the casefold 5286e1918cfSDaniel Rosenberg flag enabled (casefolding is incompatible with v1 policies). 529ba13f2c8SEric Biggers- ``ENOKEY``: a v2 encryption policy was specified, but the key with 530ba13f2c8SEric Biggers the specified ``master_key_identifier`` has not been added, nor does 531ba13f2c8SEric Biggers the process have the CAP_FOWNER capability in the initial user 532ba13f2c8SEric Biggers namespace 533f4f864c1SEric Biggers- ``ENOTDIR``: the file is unencrypted and is a regular file, not a 534f4f864c1SEric Biggers directory 535f4f864c1SEric Biggers- ``ENOTEMPTY``: the file is unencrypted and is a nonempty directory 536f4f864c1SEric Biggers- ``ENOTTY``: this type of filesystem does not implement encryption 537f4f864c1SEric Biggers- ``EOPNOTSUPP``: the kernel was not configured with encryption 538643fa961SChandan Rajendra support for filesystems, or the filesystem superblock has not 539f4f864c1SEric Biggers had encryption enabled on it. (For example, to use encryption on an 540643fa961SChandan Rajendra ext4 filesystem, CONFIG_FS_ENCRYPTION must be enabled in the 541f4f864c1SEric Biggers kernel config, and the superblock must have had the "encrypt" 542f4f864c1SEric Biggers feature flag enabled using ``tune2fs -O encrypt`` or ``mkfs.ext4 -O 543f4f864c1SEric Biggers encrypt``.) 544f4f864c1SEric Biggers- ``EPERM``: this directory may not be encrypted, e.g. because it is 545f4f864c1SEric Biggers the root directory of an ext4 filesystem 546f4f864c1SEric Biggers- ``EROFS``: the filesystem is readonly 547f4f864c1SEric Biggers 548f4f864c1SEric BiggersGetting an encryption policy 549f4f864c1SEric Biggers---------------------------- 550f4f864c1SEric Biggers 551ba13f2c8SEric BiggersTwo ioctls are available to get a file's encryption policy: 552f4f864c1SEric Biggers 553ba13f2c8SEric Biggers- `FS_IOC_GET_ENCRYPTION_POLICY_EX`_ 554ba13f2c8SEric Biggers- `FS_IOC_GET_ENCRYPTION_POLICY`_ 555ba13f2c8SEric Biggers 556ba13f2c8SEric BiggersThe extended (_EX) version of the ioctl is more general and is 557ba13f2c8SEric Biggersrecommended to use when possible. However, on older kernels only the 558ba13f2c8SEric Biggersoriginal ioctl is available. Applications should try the extended 559ba13f2c8SEric Biggersversion, and if it fails with ENOTTY fall back to the original 560ba13f2c8SEric Biggersversion. 561ba13f2c8SEric Biggers 562ba13f2c8SEric BiggersFS_IOC_GET_ENCRYPTION_POLICY_EX 563ba13f2c8SEric Biggers~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 564ba13f2c8SEric Biggers 565ba13f2c8SEric BiggersThe FS_IOC_GET_ENCRYPTION_POLICY_EX ioctl retrieves the encryption 566ba13f2c8SEric Biggerspolicy, if any, for a directory or regular file. No additional 567ba13f2c8SEric Biggerspermissions are required beyond the ability to open the file. It 568ba13f2c8SEric Biggerstakes in a pointer to a :c:type:`struct fscrypt_get_policy_ex_arg`, 569ba13f2c8SEric Biggersdefined as follows:: 570ba13f2c8SEric Biggers 571ba13f2c8SEric Biggers struct fscrypt_get_policy_ex_arg { 572ba13f2c8SEric Biggers __u64 policy_size; /* input/output */ 573ba13f2c8SEric Biggers union { 574ba13f2c8SEric Biggers __u8 version; 575ba13f2c8SEric Biggers struct fscrypt_policy_v1 v1; 576ba13f2c8SEric Biggers struct fscrypt_policy_v2 v2; 577ba13f2c8SEric Biggers } policy; /* output */ 578ba13f2c8SEric Biggers }; 579ba13f2c8SEric Biggers 580ba13f2c8SEric BiggersThe caller must initialize ``policy_size`` to the size available for 581ba13f2c8SEric Biggersthe policy struct, i.e. ``sizeof(arg.policy)``. 582ba13f2c8SEric Biggers 583ba13f2c8SEric BiggersOn success, the policy struct is returned in ``policy``, and its 584ba13f2c8SEric Biggersactual size is returned in ``policy_size``. ``policy.version`` should 585ba13f2c8SEric Biggersbe checked to determine the version of policy returned. Note that the 586ba13f2c8SEric Biggersversion code for the "v1" policy is actually 0 (FSCRYPT_POLICY_V1). 587ba13f2c8SEric Biggers 588ba13f2c8SEric BiggersFS_IOC_GET_ENCRYPTION_POLICY_EX can fail with the following errors: 589f4f864c1SEric Biggers 590f4f864c1SEric Biggers- ``EINVAL``: the file is encrypted, but it uses an unrecognized 591ba13f2c8SEric Biggers encryption policy version 592f4f864c1SEric Biggers- ``ENODATA``: the file is not encrypted 593ba13f2c8SEric Biggers- ``ENOTTY``: this type of filesystem does not implement encryption, 594ba13f2c8SEric Biggers or this kernel is too old to support FS_IOC_GET_ENCRYPTION_POLICY_EX 595ba13f2c8SEric Biggers (try FS_IOC_GET_ENCRYPTION_POLICY instead) 596f4f864c1SEric Biggers- ``EOPNOTSUPP``: the kernel was not configured with encryption 5970642ea24SChao Yu support for this filesystem, or the filesystem superblock has not 5980642ea24SChao Yu had encryption enabled on it 599ba13f2c8SEric Biggers- ``EOVERFLOW``: the file is encrypted and uses a recognized 600ba13f2c8SEric Biggers encryption policy version, but the policy struct does not fit into 601ba13f2c8SEric Biggers the provided buffer 602f4f864c1SEric Biggers 603f4f864c1SEric BiggersNote: if you only need to know whether a file is encrypted or not, on 604f4f864c1SEric Biggersmost filesystems it is also possible to use the FS_IOC_GETFLAGS ioctl 605f4f864c1SEric Biggersand check for FS_ENCRYPT_FL, or to use the statx() system call and 606f4f864c1SEric Biggerscheck for STATX_ATTR_ENCRYPTED in stx_attributes. 607f4f864c1SEric Biggers 608ba13f2c8SEric BiggersFS_IOC_GET_ENCRYPTION_POLICY 609ba13f2c8SEric Biggers~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 610ba13f2c8SEric Biggers 611ba13f2c8SEric BiggersThe FS_IOC_GET_ENCRYPTION_POLICY ioctl can also retrieve the 612ba13f2c8SEric Biggersencryption policy, if any, for a directory or regular file. However, 613ba13f2c8SEric Biggersunlike `FS_IOC_GET_ENCRYPTION_POLICY_EX`_, 614ba13f2c8SEric BiggersFS_IOC_GET_ENCRYPTION_POLICY only supports the original policy 615ba13f2c8SEric Biggersversion. It takes in a pointer directly to a :c:type:`struct 616ba13f2c8SEric Biggersfscrypt_policy_v1` rather than a :c:type:`struct 617ba13f2c8SEric Biggersfscrypt_get_policy_ex_arg`. 618ba13f2c8SEric Biggers 619ba13f2c8SEric BiggersThe error codes for FS_IOC_GET_ENCRYPTION_POLICY are the same as those 620ba13f2c8SEric Biggersfor FS_IOC_GET_ENCRYPTION_POLICY_EX, except that 621ba13f2c8SEric BiggersFS_IOC_GET_ENCRYPTION_POLICY also returns ``EINVAL`` if the file is 622ba13f2c8SEric Biggersencrypted using a newer encryption policy version. 623ba13f2c8SEric Biggers 624f4f864c1SEric BiggersGetting the per-filesystem salt 625f4f864c1SEric Biggers------------------------------- 626f4f864c1SEric Biggers 627f4f864c1SEric BiggersSome filesystems, such as ext4 and F2FS, also support the deprecated 628f4f864c1SEric Biggersioctl FS_IOC_GET_ENCRYPTION_PWSALT. This ioctl retrieves a randomly 629f4f864c1SEric Biggersgenerated 16-byte value stored in the filesystem superblock. This 630f4f864c1SEric Biggersvalue is intended to used as a salt when deriving an encryption key 631f4f864c1SEric Biggersfrom a passphrase or other low-entropy user credential. 632f4f864c1SEric Biggers 633f4f864c1SEric BiggersFS_IOC_GET_ENCRYPTION_PWSALT is deprecated. Instead, prefer to 634f4f864c1SEric Biggersgenerate and manage any needed salt(s) in userspace. 635f4f864c1SEric Biggers 636*e98ad464SEric BiggersGetting a file's encryption nonce 637*e98ad464SEric Biggers--------------------------------- 638*e98ad464SEric Biggers 639*e98ad464SEric BiggersSince Linux v5.7, the ioctl FS_IOC_GET_ENCRYPTION_NONCE is supported. 640*e98ad464SEric BiggersOn encrypted files and directories it gets the inode's 16-byte nonce. 641*e98ad464SEric BiggersOn unencrypted files and directories, it fails with ENODATA. 642*e98ad464SEric Biggers 643*e98ad464SEric BiggersThis ioctl can be useful for automated tests which verify that the 644*e98ad464SEric Biggersencryption is being done correctly. It is not needed for normal use 645*e98ad464SEric Biggersof fscrypt. 646*e98ad464SEric Biggers 647f4f864c1SEric BiggersAdding keys 648f4f864c1SEric Biggers----------- 649f4f864c1SEric Biggers 650ba13f2c8SEric BiggersFS_IOC_ADD_ENCRYPTION_KEY 651ba13f2c8SEric Biggers~~~~~~~~~~~~~~~~~~~~~~~~~ 652ba13f2c8SEric Biggers 653ba13f2c8SEric BiggersThe FS_IOC_ADD_ENCRYPTION_KEY ioctl adds a master encryption key to 654ba13f2c8SEric Biggersthe filesystem, making all files on the filesystem which were 655ba13f2c8SEric Biggersencrypted using that key appear "unlocked", i.e. in plaintext form. 656ba13f2c8SEric BiggersIt can be executed on any file or directory on the target filesystem, 657ba13f2c8SEric Biggersbut using the filesystem's root directory is recommended. It takes in 658ba13f2c8SEric Biggersa pointer to a :c:type:`struct fscrypt_add_key_arg`, defined as 659ba13f2c8SEric Biggersfollows:: 660ba13f2c8SEric Biggers 661ba13f2c8SEric Biggers struct fscrypt_add_key_arg { 662ba13f2c8SEric Biggers struct fscrypt_key_specifier key_spec; 663ba13f2c8SEric Biggers __u32 raw_size; 66493edd392SEric Biggers __u32 key_id; 66593edd392SEric Biggers __u32 __reserved[8]; 666ba13f2c8SEric Biggers __u8 raw[]; 667ba13f2c8SEric Biggers }; 668ba13f2c8SEric Biggers 669ba13f2c8SEric Biggers #define FSCRYPT_KEY_SPEC_TYPE_DESCRIPTOR 1 670ba13f2c8SEric Biggers #define FSCRYPT_KEY_SPEC_TYPE_IDENTIFIER 2 671ba13f2c8SEric Biggers 672ba13f2c8SEric Biggers struct fscrypt_key_specifier { 673ba13f2c8SEric Biggers __u32 type; /* one of FSCRYPT_KEY_SPEC_TYPE_* */ 674ba13f2c8SEric Biggers __u32 __reserved; 675ba13f2c8SEric Biggers union { 676ba13f2c8SEric Biggers __u8 __reserved[32]; /* reserve some extra space */ 677ba13f2c8SEric Biggers __u8 descriptor[FSCRYPT_KEY_DESCRIPTOR_SIZE]; 678ba13f2c8SEric Biggers __u8 identifier[FSCRYPT_KEY_IDENTIFIER_SIZE]; 679ba13f2c8SEric Biggers } u; 680ba13f2c8SEric Biggers }; 681ba13f2c8SEric Biggers 68293edd392SEric Biggers struct fscrypt_provisioning_key_payload { 68393edd392SEric Biggers __u32 type; 68493edd392SEric Biggers __u32 __reserved; 68593edd392SEric Biggers __u8 raw[]; 68693edd392SEric Biggers }; 68793edd392SEric Biggers 688ba13f2c8SEric Biggers:c:type:`struct fscrypt_add_key_arg` must be zeroed, then initialized 689ba13f2c8SEric Biggersas follows: 690ba13f2c8SEric Biggers 691ba13f2c8SEric Biggers- If the key is being added for use by v1 encryption policies, then 692ba13f2c8SEric Biggers ``key_spec.type`` must contain FSCRYPT_KEY_SPEC_TYPE_DESCRIPTOR, and 693ba13f2c8SEric Biggers ``key_spec.u.descriptor`` must contain the descriptor of the key 694ba13f2c8SEric Biggers being added, corresponding to the value in the 695ba13f2c8SEric Biggers ``master_key_descriptor`` field of :c:type:`struct 696ba13f2c8SEric Biggers fscrypt_policy_v1`. To add this type of key, the calling process 697ba13f2c8SEric Biggers must have the CAP_SYS_ADMIN capability in the initial user 698ba13f2c8SEric Biggers namespace. 699ba13f2c8SEric Biggers 700ba13f2c8SEric Biggers Alternatively, if the key is being added for use by v2 encryption 701ba13f2c8SEric Biggers policies, then ``key_spec.type`` must contain 702ba13f2c8SEric Biggers FSCRYPT_KEY_SPEC_TYPE_IDENTIFIER, and ``key_spec.u.identifier`` is 703ba13f2c8SEric Biggers an *output* field which the kernel fills in with a cryptographic 704ba13f2c8SEric Biggers hash of the key. To add this type of key, the calling process does 705ba13f2c8SEric Biggers not need any privileges. However, the number of keys that can be 706ba13f2c8SEric Biggers added is limited by the user's quota for the keyrings service (see 707ba13f2c8SEric Biggers ``Documentation/security/keys/core.rst``). 708ba13f2c8SEric Biggers 709ba13f2c8SEric Biggers- ``raw_size`` must be the size of the ``raw`` key provided, in bytes. 71093edd392SEric Biggers Alternatively, if ``key_id`` is nonzero, this field must be 0, since 71193edd392SEric Biggers in that case the size is implied by the specified Linux keyring key. 71293edd392SEric Biggers 71393edd392SEric Biggers- ``key_id`` is 0 if the raw key is given directly in the ``raw`` 71493edd392SEric Biggers field. Otherwise ``key_id`` is the ID of a Linux keyring key of 71593edd392SEric Biggers type "fscrypt-provisioning" whose payload is a :c:type:`struct 71693edd392SEric Biggers fscrypt_provisioning_key_payload` whose ``raw`` field contains the 71793edd392SEric Biggers raw key and whose ``type`` field matches ``key_spec.type``. Since 71893edd392SEric Biggers ``raw`` is variable-length, the total size of this key's payload 71993edd392SEric Biggers must be ``sizeof(struct fscrypt_provisioning_key_payload)`` plus the 72093edd392SEric Biggers raw key size. The process must have Search permission on this key. 72193edd392SEric Biggers 72293edd392SEric Biggers Most users should leave this 0 and specify the raw key directly. 72393edd392SEric Biggers The support for specifying a Linux keyring key is intended mainly to 72493edd392SEric Biggers allow re-adding keys after a filesystem is unmounted and re-mounted, 72593edd392SEric Biggers without having to store the raw keys in userspace memory. 726ba13f2c8SEric Biggers 727ba13f2c8SEric Biggers- ``raw`` is a variable-length field which must contain the actual 72893edd392SEric Biggers key, ``raw_size`` bytes long. Alternatively, if ``key_id`` is 72993edd392SEric Biggers nonzero, then this field is unused. 730ba13f2c8SEric Biggers 731ba13f2c8SEric BiggersFor v2 policy keys, the kernel keeps track of which user (identified 732ba13f2c8SEric Biggersby effective user ID) added the key, and only allows the key to be 733ba13f2c8SEric Biggersremoved by that user --- or by "root", if they use 734ba13f2c8SEric Biggers`FS_IOC_REMOVE_ENCRYPTION_KEY_ALL_USERS`_. 735ba13f2c8SEric Biggers 736ba13f2c8SEric BiggersHowever, if another user has added the key, it may be desirable to 737ba13f2c8SEric Biggersprevent that other user from unexpectedly removing it. Therefore, 738ba13f2c8SEric BiggersFS_IOC_ADD_ENCRYPTION_KEY may also be used to add a v2 policy key 739ba13f2c8SEric Biggers*again*, even if it's already added by other user(s). In this case, 740ba13f2c8SEric BiggersFS_IOC_ADD_ENCRYPTION_KEY will just install a claim to the key for the 741ba13f2c8SEric Biggerscurrent user, rather than actually add the key again (but the raw key 742ba13f2c8SEric Biggersmust still be provided, as a proof of knowledge). 743ba13f2c8SEric Biggers 744ba13f2c8SEric BiggersFS_IOC_ADD_ENCRYPTION_KEY returns 0 if either the key or a claim to 745ba13f2c8SEric Biggersthe key was either added or already exists. 746ba13f2c8SEric Biggers 747ba13f2c8SEric BiggersFS_IOC_ADD_ENCRYPTION_KEY can fail with the following errors: 748ba13f2c8SEric Biggers 749ba13f2c8SEric Biggers- ``EACCES``: FSCRYPT_KEY_SPEC_TYPE_DESCRIPTOR was specified, but the 750ba13f2c8SEric Biggers caller does not have the CAP_SYS_ADMIN capability in the initial 75193edd392SEric Biggers user namespace; or the raw key was specified by Linux key ID but the 75293edd392SEric Biggers process lacks Search permission on the key. 753ba13f2c8SEric Biggers- ``EDQUOT``: the key quota for this user would be exceeded by adding 754ba13f2c8SEric Biggers the key 755ba13f2c8SEric Biggers- ``EINVAL``: invalid key size or key specifier type, or reserved bits 756ba13f2c8SEric Biggers were set 75793edd392SEric Biggers- ``EKEYREJECTED``: the raw key was specified by Linux key ID, but the 75893edd392SEric Biggers key has the wrong type 75993edd392SEric Biggers- ``ENOKEY``: the raw key was specified by Linux key ID, but no key 76093edd392SEric Biggers exists with that ID 761ba13f2c8SEric Biggers- ``ENOTTY``: this type of filesystem does not implement encryption 762ba13f2c8SEric Biggers- ``EOPNOTSUPP``: the kernel was not configured with encryption 763ba13f2c8SEric Biggers support for this filesystem, or the filesystem superblock has not 764ba13f2c8SEric Biggers had encryption enabled on it 765ba13f2c8SEric Biggers 766ba13f2c8SEric BiggersLegacy method 767ba13f2c8SEric Biggers~~~~~~~~~~~~~ 768ba13f2c8SEric Biggers 769ba13f2c8SEric BiggersFor v1 encryption policies, a master encryption key can also be 770ba13f2c8SEric Biggersprovided by adding it to a process-subscribed keyring, e.g. to a 771ba13f2c8SEric Biggerssession keyring, or to a user keyring if the user keyring is linked 772ba13f2c8SEric Biggersinto the session keyring. 773ba13f2c8SEric Biggers 774ba13f2c8SEric BiggersThis method is deprecated (and not supported for v2 encryption 775ba13f2c8SEric Biggerspolicies) for several reasons. First, it cannot be used in 776ba13f2c8SEric Biggerscombination with FS_IOC_REMOVE_ENCRYPTION_KEY (see `Removing keys`_), 777ba13f2c8SEric Biggersso for removing a key a workaround such as keyctl_unlink() in 778ba13f2c8SEric Biggerscombination with ``sync; echo 2 > /proc/sys/vm/drop_caches`` would 779ba13f2c8SEric Biggershave to be used. Second, it doesn't match the fact that the 780ba13f2c8SEric Biggerslocked/unlocked status of encrypted files (i.e. whether they appear to 781ba13f2c8SEric Biggersbe in plaintext form or in ciphertext form) is global. This mismatch 782ba13f2c8SEric Biggershas caused much confusion as well as real problems when processes 783ba13f2c8SEric Biggersrunning under different UIDs, such as a ``sudo`` command, need to 784ba13f2c8SEric Biggersaccess encrypted files. 785ba13f2c8SEric Biggers 786ba13f2c8SEric BiggersNevertheless, to add a key to one of the process-subscribed keyrings, 787ba13f2c8SEric Biggersthe add_key() system call can be used (see: 788f4f864c1SEric Biggers``Documentation/security/keys/core.rst``). The key type must be 789f4f864c1SEric Biggers"logon"; keys of this type are kept in kernel memory and cannot be 790f4f864c1SEric Biggersread back by userspace. The key description must be "fscrypt:" 791f4f864c1SEric Biggersfollowed by the 16-character lower case hex representation of the 792f4f864c1SEric Biggers``master_key_descriptor`` that was set in the encryption policy. The 793f4f864c1SEric Biggerskey payload must conform to the following structure:: 794f4f864c1SEric Biggers 7952336d0deSEric Biggers #define FSCRYPT_MAX_KEY_SIZE 64 796f4f864c1SEric Biggers 797f4f864c1SEric Biggers struct fscrypt_key { 798ba13f2c8SEric Biggers __u32 mode; 799ba13f2c8SEric Biggers __u8 raw[FSCRYPT_MAX_KEY_SIZE]; 800ba13f2c8SEric Biggers __u32 size; 801f4f864c1SEric Biggers }; 802f4f864c1SEric Biggers 803f4f864c1SEric Biggers``mode`` is ignored; just set it to 0. The actual key is provided in 804f4f864c1SEric Biggers``raw`` with ``size`` indicating its size in bytes. That is, the 805f4f864c1SEric Biggersbytes ``raw[0..size-1]`` (inclusive) are the actual key. 806f4f864c1SEric Biggers 807f4f864c1SEric BiggersThe key description prefix "fscrypt:" may alternatively be replaced 808f4f864c1SEric Biggerswith a filesystem-specific prefix such as "ext4:". However, the 809f4f864c1SEric Biggersfilesystem-specific prefixes are deprecated and should not be used in 810f4f864c1SEric Biggersnew programs. 811f4f864c1SEric Biggers 812ba13f2c8SEric BiggersRemoving keys 813ba13f2c8SEric Biggers------------- 814f4f864c1SEric Biggers 815ba13f2c8SEric BiggersTwo ioctls are available for removing a key that was added by 816ba13f2c8SEric Biggers`FS_IOC_ADD_ENCRYPTION_KEY`_: 817ba13f2c8SEric Biggers 818ba13f2c8SEric Biggers- `FS_IOC_REMOVE_ENCRYPTION_KEY`_ 819ba13f2c8SEric Biggers- `FS_IOC_REMOVE_ENCRYPTION_KEY_ALL_USERS`_ 820ba13f2c8SEric Biggers 821ba13f2c8SEric BiggersThese two ioctls differ only in cases where v2 policy keys are added 822ba13f2c8SEric Biggersor removed by non-root users. 823ba13f2c8SEric Biggers 824ba13f2c8SEric BiggersThese ioctls don't work on keys that were added via the legacy 825ba13f2c8SEric Biggersprocess-subscribed keyrings mechanism. 826ba13f2c8SEric Biggers 827ba13f2c8SEric BiggersBefore using these ioctls, read the `Kernel memory compromise`_ 828ba13f2c8SEric Biggerssection for a discussion of the security goals and limitations of 829ba13f2c8SEric Biggersthese ioctls. 830ba13f2c8SEric Biggers 831ba13f2c8SEric BiggersFS_IOC_REMOVE_ENCRYPTION_KEY 832ba13f2c8SEric Biggers~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 833ba13f2c8SEric Biggers 834ba13f2c8SEric BiggersThe FS_IOC_REMOVE_ENCRYPTION_KEY ioctl removes a claim to a master 835ba13f2c8SEric Biggersencryption key from the filesystem, and possibly removes the key 836ba13f2c8SEric Biggersitself. It can be executed on any file or directory on the target 837ba13f2c8SEric Biggersfilesystem, but using the filesystem's root directory is recommended. 838ba13f2c8SEric BiggersIt takes in a pointer to a :c:type:`struct fscrypt_remove_key_arg`, 839ba13f2c8SEric Biggersdefined as follows:: 840ba13f2c8SEric Biggers 841ba13f2c8SEric Biggers struct fscrypt_remove_key_arg { 842ba13f2c8SEric Biggers struct fscrypt_key_specifier key_spec; 843ba13f2c8SEric Biggers #define FSCRYPT_KEY_REMOVAL_STATUS_FLAG_FILES_BUSY 0x00000001 844ba13f2c8SEric Biggers #define FSCRYPT_KEY_REMOVAL_STATUS_FLAG_OTHER_USERS 0x00000002 845ba13f2c8SEric Biggers __u32 removal_status_flags; /* output */ 846ba13f2c8SEric Biggers __u32 __reserved[5]; 847ba13f2c8SEric Biggers }; 848ba13f2c8SEric Biggers 849ba13f2c8SEric BiggersThis structure must be zeroed, then initialized as follows: 850ba13f2c8SEric Biggers 851ba13f2c8SEric Biggers- The key to remove is specified by ``key_spec``: 852ba13f2c8SEric Biggers 853ba13f2c8SEric Biggers - To remove a key used by v1 encryption policies, set 854ba13f2c8SEric Biggers ``key_spec.type`` to FSCRYPT_KEY_SPEC_TYPE_DESCRIPTOR and fill 855ba13f2c8SEric Biggers in ``key_spec.u.descriptor``. To remove this type of key, the 856ba13f2c8SEric Biggers calling process must have the CAP_SYS_ADMIN capability in the 857ba13f2c8SEric Biggers initial user namespace. 858ba13f2c8SEric Biggers 859ba13f2c8SEric Biggers - To remove a key used by v2 encryption policies, set 860ba13f2c8SEric Biggers ``key_spec.type`` to FSCRYPT_KEY_SPEC_TYPE_IDENTIFIER and fill 861ba13f2c8SEric Biggers in ``key_spec.u.identifier``. 862ba13f2c8SEric Biggers 863ba13f2c8SEric BiggersFor v2 policy keys, this ioctl is usable by non-root users. However, 864ba13f2c8SEric Biggersto make this possible, it actually just removes the current user's 865ba13f2c8SEric Biggersclaim to the key, undoing a single call to FS_IOC_ADD_ENCRYPTION_KEY. 866ba13f2c8SEric BiggersOnly after all claims are removed is the key really removed. 867ba13f2c8SEric Biggers 868ba13f2c8SEric BiggersFor example, if FS_IOC_ADD_ENCRYPTION_KEY was called with uid 1000, 869ba13f2c8SEric Biggersthen the key will be "claimed" by uid 1000, and 870ba13f2c8SEric BiggersFS_IOC_REMOVE_ENCRYPTION_KEY will only succeed as uid 1000. Or, if 871ba13f2c8SEric Biggersboth uids 1000 and 2000 added the key, then for each uid 872ba13f2c8SEric BiggersFS_IOC_REMOVE_ENCRYPTION_KEY will only remove their own claim. Only 873ba13f2c8SEric Biggersonce *both* are removed is the key really removed. (Think of it like 874ba13f2c8SEric Biggersunlinking a file that may have hard links.) 875ba13f2c8SEric Biggers 876ba13f2c8SEric BiggersIf FS_IOC_REMOVE_ENCRYPTION_KEY really removes the key, it will also 877ba13f2c8SEric Biggerstry to "lock" all files that had been unlocked with the key. It won't 878ba13f2c8SEric Biggerslock files that are still in-use, so this ioctl is expected to be used 879ba13f2c8SEric Biggersin cooperation with userspace ensuring that none of the files are 880ba13f2c8SEric Biggersstill open. However, if necessary, this ioctl can be executed again 881ba13f2c8SEric Biggerslater to retry locking any remaining files. 882ba13f2c8SEric Biggers 883ba13f2c8SEric BiggersFS_IOC_REMOVE_ENCRYPTION_KEY returns 0 if either the key was removed 884ba13f2c8SEric Biggers(but may still have files remaining to be locked), the user's claim to 885ba13f2c8SEric Biggersthe key was removed, or the key was already removed but had files 886ba13f2c8SEric Biggersremaining to be the locked so the ioctl retried locking them. In any 887ba13f2c8SEric Biggersof these cases, ``removal_status_flags`` is filled in with the 888ba13f2c8SEric Biggersfollowing informational status flags: 889ba13f2c8SEric Biggers 890ba13f2c8SEric Biggers- ``FSCRYPT_KEY_REMOVAL_STATUS_FLAG_FILES_BUSY``: set if some file(s) 891ba13f2c8SEric Biggers are still in-use. Not guaranteed to be set in the case where only 892ba13f2c8SEric Biggers the user's claim to the key was removed. 893ba13f2c8SEric Biggers- ``FSCRYPT_KEY_REMOVAL_STATUS_FLAG_OTHER_USERS``: set if only the 894ba13f2c8SEric Biggers user's claim to the key was removed, not the key itself 895ba13f2c8SEric Biggers 896ba13f2c8SEric BiggersFS_IOC_REMOVE_ENCRYPTION_KEY can fail with the following errors: 897ba13f2c8SEric Biggers 898ba13f2c8SEric Biggers- ``EACCES``: The FSCRYPT_KEY_SPEC_TYPE_DESCRIPTOR key specifier type 899ba13f2c8SEric Biggers was specified, but the caller does not have the CAP_SYS_ADMIN 900ba13f2c8SEric Biggers capability in the initial user namespace 901ba13f2c8SEric Biggers- ``EINVAL``: invalid key specifier type, or reserved bits were set 902ba13f2c8SEric Biggers- ``ENOKEY``: the key object was not found at all, i.e. it was never 903ba13f2c8SEric Biggers added in the first place or was already fully removed including all 904ba13f2c8SEric Biggers files locked; or, the user does not have a claim to the key (but 905ba13f2c8SEric Biggers someone else does). 906ba13f2c8SEric Biggers- ``ENOTTY``: this type of filesystem does not implement encryption 907ba13f2c8SEric Biggers- ``EOPNOTSUPP``: the kernel was not configured with encryption 908ba13f2c8SEric Biggers support for this filesystem, or the filesystem superblock has not 909ba13f2c8SEric Biggers had encryption enabled on it 910ba13f2c8SEric Biggers 911ba13f2c8SEric BiggersFS_IOC_REMOVE_ENCRYPTION_KEY_ALL_USERS 912ba13f2c8SEric Biggers~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 913ba13f2c8SEric Biggers 914ba13f2c8SEric BiggersFS_IOC_REMOVE_ENCRYPTION_KEY_ALL_USERS is exactly the same as 915ba13f2c8SEric Biggers`FS_IOC_REMOVE_ENCRYPTION_KEY`_, except that for v2 policy keys, the 916ba13f2c8SEric BiggersALL_USERS version of the ioctl will remove all users' claims to the 917ba13f2c8SEric Biggerskey, not just the current user's. I.e., the key itself will always be 918ba13f2c8SEric Biggersremoved, no matter how many users have added it. This difference is 919ba13f2c8SEric Biggersonly meaningful if non-root users are adding and removing keys. 920ba13f2c8SEric Biggers 921ba13f2c8SEric BiggersBecause of this, FS_IOC_REMOVE_ENCRYPTION_KEY_ALL_USERS also requires 922ba13f2c8SEric Biggers"root", namely the CAP_SYS_ADMIN capability in the initial user 923ba13f2c8SEric Biggersnamespace. Otherwise it will fail with EACCES. 924ba13f2c8SEric Biggers 925ba13f2c8SEric BiggersGetting key status 926ba13f2c8SEric Biggers------------------ 927ba13f2c8SEric Biggers 928ba13f2c8SEric BiggersFS_IOC_GET_ENCRYPTION_KEY_STATUS 929ba13f2c8SEric Biggers~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 930ba13f2c8SEric Biggers 931ba13f2c8SEric BiggersThe FS_IOC_GET_ENCRYPTION_KEY_STATUS ioctl retrieves the status of a 932ba13f2c8SEric Biggersmaster encryption key. It can be executed on any file or directory on 933ba13f2c8SEric Biggersthe target filesystem, but using the filesystem's root directory is 934ba13f2c8SEric Biggersrecommended. It takes in a pointer to a :c:type:`struct 935ba13f2c8SEric Biggersfscrypt_get_key_status_arg`, defined as follows:: 936ba13f2c8SEric Biggers 937ba13f2c8SEric Biggers struct fscrypt_get_key_status_arg { 938ba13f2c8SEric Biggers /* input */ 939ba13f2c8SEric Biggers struct fscrypt_key_specifier key_spec; 940ba13f2c8SEric Biggers __u32 __reserved[6]; 941ba13f2c8SEric Biggers 942ba13f2c8SEric Biggers /* output */ 943ba13f2c8SEric Biggers #define FSCRYPT_KEY_STATUS_ABSENT 1 944ba13f2c8SEric Biggers #define FSCRYPT_KEY_STATUS_PRESENT 2 945ba13f2c8SEric Biggers #define FSCRYPT_KEY_STATUS_INCOMPLETELY_REMOVED 3 946ba13f2c8SEric Biggers __u32 status; 947ba13f2c8SEric Biggers #define FSCRYPT_KEY_STATUS_FLAG_ADDED_BY_SELF 0x00000001 948ba13f2c8SEric Biggers __u32 status_flags; 949ba13f2c8SEric Biggers __u32 user_count; 950ba13f2c8SEric Biggers __u32 __out_reserved[13]; 951ba13f2c8SEric Biggers }; 952ba13f2c8SEric Biggers 953ba13f2c8SEric BiggersThe caller must zero all input fields, then fill in ``key_spec``: 954ba13f2c8SEric Biggers 955ba13f2c8SEric Biggers - To get the status of a key for v1 encryption policies, set 956ba13f2c8SEric Biggers ``key_spec.type`` to FSCRYPT_KEY_SPEC_TYPE_DESCRIPTOR and fill 957ba13f2c8SEric Biggers in ``key_spec.u.descriptor``. 958ba13f2c8SEric Biggers 959ba13f2c8SEric Biggers - To get the status of a key for v2 encryption policies, set 960ba13f2c8SEric Biggers ``key_spec.type`` to FSCRYPT_KEY_SPEC_TYPE_IDENTIFIER and fill 961ba13f2c8SEric Biggers in ``key_spec.u.identifier``. 962ba13f2c8SEric Biggers 963ba13f2c8SEric BiggersOn success, 0 is returned and the kernel fills in the output fields: 964ba13f2c8SEric Biggers 965ba13f2c8SEric Biggers- ``status`` indicates whether the key is absent, present, or 966ba13f2c8SEric Biggers incompletely removed. Incompletely removed means that the master 967ba13f2c8SEric Biggers secret has been removed, but some files are still in use; i.e., 968ba13f2c8SEric Biggers `FS_IOC_REMOVE_ENCRYPTION_KEY`_ returned 0 but set the informational 969ba13f2c8SEric Biggers status flag FSCRYPT_KEY_REMOVAL_STATUS_FLAG_FILES_BUSY. 970ba13f2c8SEric Biggers 971ba13f2c8SEric Biggers- ``status_flags`` can contain the following flags: 972ba13f2c8SEric Biggers 973ba13f2c8SEric Biggers - ``FSCRYPT_KEY_STATUS_FLAG_ADDED_BY_SELF`` indicates that the key 974ba13f2c8SEric Biggers has added by the current user. This is only set for keys 975ba13f2c8SEric Biggers identified by ``identifier`` rather than by ``descriptor``. 976ba13f2c8SEric Biggers 977ba13f2c8SEric Biggers- ``user_count`` specifies the number of users who have added the key. 978ba13f2c8SEric Biggers This is only set for keys identified by ``identifier`` rather than 979ba13f2c8SEric Biggers by ``descriptor``. 980ba13f2c8SEric Biggers 981ba13f2c8SEric BiggersFS_IOC_GET_ENCRYPTION_KEY_STATUS can fail with the following errors: 982ba13f2c8SEric Biggers 983ba13f2c8SEric Biggers- ``EINVAL``: invalid key specifier type, or reserved bits were set 984ba13f2c8SEric Biggers- ``ENOTTY``: this type of filesystem does not implement encryption 985ba13f2c8SEric Biggers- ``EOPNOTSUPP``: the kernel was not configured with encryption 986ba13f2c8SEric Biggers support for this filesystem, or the filesystem superblock has not 987ba13f2c8SEric Biggers had encryption enabled on it 988ba13f2c8SEric Biggers 989ba13f2c8SEric BiggersAmong other use cases, FS_IOC_GET_ENCRYPTION_KEY_STATUS can be useful 990ba13f2c8SEric Biggersfor determining whether the key for a given encrypted directory needs 991ba13f2c8SEric Biggersto be added before prompting the user for the passphrase needed to 992ba13f2c8SEric Biggersderive the key. 993ba13f2c8SEric Biggers 994ba13f2c8SEric BiggersFS_IOC_GET_ENCRYPTION_KEY_STATUS can only get the status of keys in 995ba13f2c8SEric Biggersthe filesystem-level keyring, i.e. the keyring managed by 996ba13f2c8SEric Biggers`FS_IOC_ADD_ENCRYPTION_KEY`_ and `FS_IOC_REMOVE_ENCRYPTION_KEY`_. It 997ba13f2c8SEric Biggerscannot get the status of a key that has only been added for use by v1 998ba13f2c8SEric Biggersencryption policies using the legacy mechanism involving 999ba13f2c8SEric Biggersprocess-subscribed keyrings. 1000f4f864c1SEric Biggers 1001f4f864c1SEric BiggersAccess semantics 1002f4f864c1SEric Biggers================ 1003f4f864c1SEric Biggers 1004f4f864c1SEric BiggersWith the key 1005f4f864c1SEric Biggers------------ 1006f4f864c1SEric Biggers 1007f4f864c1SEric BiggersWith the encryption key, encrypted regular files, directories, and 1008f4f864c1SEric Biggerssymlinks behave very similarly to their unencrypted counterparts --- 1009f4f864c1SEric Biggersafter all, the encryption is intended to be transparent. However, 1010f4f864c1SEric Biggersastute users may notice some differences in behavior: 1011f4f864c1SEric Biggers 1012f4f864c1SEric Biggers- Unencrypted files, or files encrypted with a different encryption 1013f4f864c1SEric Biggers policy (i.e. different key, modes, or flags), cannot be renamed or 1014f4f864c1SEric Biggers linked into an encrypted directory; see `Encryption policy 1015f5e55e77SEric Biggers enforcement`_. Attempts to do so will fail with EXDEV. However, 1016f4f864c1SEric Biggers encrypted files can be renamed within an encrypted directory, or 1017f4f864c1SEric Biggers into an unencrypted directory. 1018f4f864c1SEric Biggers 1019f5e55e77SEric Biggers Note: "moving" an unencrypted file into an encrypted directory, e.g. 1020f5e55e77SEric Biggers with the `mv` program, is implemented in userspace by a copy 1021f5e55e77SEric Biggers followed by a delete. Be aware that the original unencrypted data 1022f5e55e77SEric Biggers may remain recoverable from free space on the disk; prefer to keep 1023f5e55e77SEric Biggers all files encrypted from the very beginning. The `shred` program 1024f5e55e77SEric Biggers may be used to overwrite the source files but isn't guaranteed to be 1025f5e55e77SEric Biggers effective on all filesystems and storage devices. 1026f5e55e77SEric Biggers 1027f4f864c1SEric Biggers- Direct I/O is not supported on encrypted files. Attempts to use 1028f4f864c1SEric Biggers direct I/O on such files will fall back to buffered I/O. 1029f4f864c1SEric Biggers 1030457b1e35SEric Biggers- The fallocate operations FALLOC_FL_COLLAPSE_RANGE and 1031457b1e35SEric Biggers FALLOC_FL_INSERT_RANGE are not supported on encrypted files and will 1032457b1e35SEric Biggers fail with EOPNOTSUPP. 1033f4f864c1SEric Biggers 1034f4f864c1SEric Biggers- Online defragmentation of encrypted files is not supported. The 1035f4f864c1SEric Biggers EXT4_IOC_MOVE_EXT and F2FS_IOC_MOVE_RANGE ioctls will fail with 1036f4f864c1SEric Biggers EOPNOTSUPP. 1037f4f864c1SEric Biggers 1038f4f864c1SEric Biggers- The ext4 filesystem does not support data journaling with encrypted 1039f4f864c1SEric Biggers regular files. It will fall back to ordered data mode instead. 1040f4f864c1SEric Biggers 1041f4f864c1SEric Biggers- DAX (Direct Access) is not supported on encrypted files. 1042f4f864c1SEric Biggers 1043f4f864c1SEric Biggers- The st_size of an encrypted symlink will not necessarily give the 1044f4f864c1SEric Biggers length of the symlink target as required by POSIX. It will actually 10452f46a2bcSEric Biggers give the length of the ciphertext, which will be slightly longer 10462f46a2bcSEric Biggers than the plaintext due to NUL-padding and an extra 2-byte overhead. 10472f46a2bcSEric Biggers 10482f46a2bcSEric Biggers- The maximum length of an encrypted symlink is 2 bytes shorter than 10492f46a2bcSEric Biggers the maximum length of an unencrypted symlink. For example, on an 10502f46a2bcSEric Biggers EXT4 filesystem with a 4K block size, unencrypted symlinks can be up 10512f46a2bcSEric Biggers to 4095 bytes long, while encrypted symlinks can only be up to 4093 10522f46a2bcSEric Biggers bytes long (both lengths excluding the terminating null). 1053f4f864c1SEric Biggers 1054f4f864c1SEric BiggersNote that mmap *is* supported. This is possible because the pagecache 1055f4f864c1SEric Biggersfor an encrypted file contains the plaintext, not the ciphertext. 1056f4f864c1SEric Biggers 1057f4f864c1SEric BiggersWithout the key 1058f4f864c1SEric Biggers--------------- 1059f4f864c1SEric Biggers 1060f4f864c1SEric BiggersSome filesystem operations may be performed on encrypted regular 1061f4f864c1SEric Biggersfiles, directories, and symlinks even before their encryption key has 1062ba13f2c8SEric Biggersbeen added, or after their encryption key has been removed: 1063f4f864c1SEric Biggers 1064f4f864c1SEric Biggers- File metadata may be read, e.g. using stat(). 1065f4f864c1SEric Biggers 1066f4f864c1SEric Biggers- Directories may be listed, in which case the filenames will be 1067f4f864c1SEric Biggers listed in an encoded form derived from their ciphertext. The 1068f4f864c1SEric Biggers current encoding algorithm is described in `Filename hashing and 1069f4f864c1SEric Biggers encoding`_. The algorithm is subject to change, but it is 1070f4f864c1SEric Biggers guaranteed that the presented filenames will be no longer than 1071f4f864c1SEric Biggers NAME_MAX bytes, will not contain the ``/`` or ``\0`` characters, and 1072f4f864c1SEric Biggers will uniquely identify directory entries. 1073f4f864c1SEric Biggers 1074f4f864c1SEric Biggers The ``.`` and ``..`` directory entries are special. They are always 1075f4f864c1SEric Biggers present and are not encrypted or encoded. 1076f4f864c1SEric Biggers 1077f4f864c1SEric Biggers- Files may be deleted. That is, nondirectory files may be deleted 1078f4f864c1SEric Biggers with unlink() as usual, and empty directories may be deleted with 1079f4f864c1SEric Biggers rmdir() as usual. Therefore, ``rm`` and ``rm -r`` will work as 1080f4f864c1SEric Biggers expected. 1081f4f864c1SEric Biggers 1082f4f864c1SEric Biggers- Symlink targets may be read and followed, but they will be presented 1083f4f864c1SEric Biggers in encrypted form, similar to filenames in directories. Hence, they 1084f4f864c1SEric Biggers are unlikely to point to anywhere useful. 1085f4f864c1SEric Biggers 1086f4f864c1SEric BiggersWithout the key, regular files cannot be opened or truncated. 1087f4f864c1SEric BiggersAttempts to do so will fail with ENOKEY. This implies that any 1088f4f864c1SEric Biggersregular file operations that require a file descriptor, such as 1089f4f864c1SEric Biggersread(), write(), mmap(), fallocate(), and ioctl(), are also forbidden. 1090f4f864c1SEric Biggers 1091f4f864c1SEric BiggersAlso without the key, files of any type (including directories) cannot 1092f4f864c1SEric Biggersbe created or linked into an encrypted directory, nor can a name in an 1093f4f864c1SEric Biggersencrypted directory be the source or target of a rename, nor can an 1094f4f864c1SEric BiggersO_TMPFILE temporary file be created in an encrypted directory. All 1095f4f864c1SEric Biggerssuch operations will fail with ENOKEY. 1096f4f864c1SEric Biggers 1097f4f864c1SEric BiggersIt is not currently possible to backup and restore encrypted files 1098f4f864c1SEric Biggerswithout the encryption key. This would require special APIs which 1099f4f864c1SEric Biggershave not yet been implemented. 1100f4f864c1SEric Biggers 1101f4f864c1SEric BiggersEncryption policy enforcement 1102f4f864c1SEric Biggers============================= 1103f4f864c1SEric Biggers 1104f4f864c1SEric BiggersAfter an encryption policy has been set on a directory, all regular 1105f4f864c1SEric Biggersfiles, directories, and symbolic links created in that directory 1106f4f864c1SEric Biggers(recursively) will inherit that encryption policy. Special files --- 1107f4f864c1SEric Biggersthat is, named pipes, device nodes, and UNIX domain sockets --- will 1108f4f864c1SEric Biggersnot be encrypted. 1109f4f864c1SEric Biggers 1110f4f864c1SEric BiggersExcept for those special files, it is forbidden to have unencrypted 1111f4f864c1SEric Biggersfiles, or files encrypted with a different encryption policy, in an 1112f4f864c1SEric Biggersencrypted directory tree. Attempts to link or rename such a file into 1113f5e55e77SEric Biggersan encrypted directory will fail with EXDEV. This is also enforced 1114f4f864c1SEric Biggersduring ->lookup() to provide limited protection against offline 1115f4f864c1SEric Biggersattacks that try to disable or downgrade encryption in known locations 1116f4f864c1SEric Biggerswhere applications may later write sensitive data. It is recommended 1117f4f864c1SEric Biggersthat systems implementing a form of "verified boot" take advantage of 1118f4f864c1SEric Biggersthis by validating all top-level encryption policies prior to access. 1119f4f864c1SEric Biggers 1120f4f864c1SEric BiggersImplementation details 1121f4f864c1SEric Biggers====================== 1122f4f864c1SEric Biggers 1123f4f864c1SEric BiggersEncryption context 1124f4f864c1SEric Biggers------------------ 1125f4f864c1SEric Biggers 1126f4f864c1SEric BiggersAn encryption policy is represented on-disk by a :c:type:`struct 1127ba13f2c8SEric Biggersfscrypt_context_v1` or a :c:type:`struct fscrypt_context_v2`. It is 1128ba13f2c8SEric Biggersup to individual filesystems to decide where to store it, but normally 1129ba13f2c8SEric Biggersit would be stored in a hidden extended attribute. It should *not* be 1130ba13f2c8SEric Biggersexposed by the xattr-related system calls such as getxattr() and 1131ba13f2c8SEric Biggerssetxattr() because of the special semantics of the encryption xattr. 1132ba13f2c8SEric Biggers(In particular, there would be much confusion if an encryption policy 1133ba13f2c8SEric Biggerswere to be added to or removed from anything other than an empty 1134ba13f2c8SEric Biggersdirectory.) These structs are defined as follows:: 1135f4f864c1SEric Biggers 1136f4f864c1SEric Biggers #define FS_KEY_DERIVATION_NONCE_SIZE 16 1137f4f864c1SEric Biggers 1138ba13f2c8SEric Biggers #define FSCRYPT_KEY_DESCRIPTOR_SIZE 8 1139ba13f2c8SEric Biggers struct fscrypt_context_v1 { 1140ba13f2c8SEric Biggers u8 version; 1141f4f864c1SEric Biggers u8 contents_encryption_mode; 1142f4f864c1SEric Biggers u8 filenames_encryption_mode; 1143f4f864c1SEric Biggers u8 flags; 11442336d0deSEric Biggers u8 master_key_descriptor[FSCRYPT_KEY_DESCRIPTOR_SIZE]; 1145f4f864c1SEric Biggers u8 nonce[FS_KEY_DERIVATION_NONCE_SIZE]; 1146f4f864c1SEric Biggers }; 1147f4f864c1SEric Biggers 1148ba13f2c8SEric Biggers #define FSCRYPT_KEY_IDENTIFIER_SIZE 16 1149ba13f2c8SEric Biggers struct fscrypt_context_v2 { 1150ba13f2c8SEric Biggers u8 version; 1151ba13f2c8SEric Biggers u8 contents_encryption_mode; 1152ba13f2c8SEric Biggers u8 filenames_encryption_mode; 1153ba13f2c8SEric Biggers u8 flags; 1154ba13f2c8SEric Biggers u8 __reserved[4]; 1155ba13f2c8SEric Biggers u8 master_key_identifier[FSCRYPT_KEY_IDENTIFIER_SIZE]; 1156ba13f2c8SEric Biggers u8 nonce[FS_KEY_DERIVATION_NONCE_SIZE]; 1157ba13f2c8SEric Biggers }; 1158ba13f2c8SEric Biggers 1159ba13f2c8SEric BiggersThe context structs contain the same information as the corresponding 1160ba13f2c8SEric Biggerspolicy structs (see `Setting an encryption policy`_), except that the 1161ba13f2c8SEric Biggerscontext structs also contain a nonce. The nonce is randomly generated 1162ba13f2c8SEric Biggersby the kernel and is used as KDF input or as a tweak to cause 1163f592efe7SEric Biggersdifferent files to be encrypted differently; see `Per-file encryption 1164f592efe7SEric Biggerskeys`_ and `DIRECT_KEY policies`_. 1165f4f864c1SEric Biggers 1166f4f864c1SEric BiggersData path changes 1167f4f864c1SEric Biggers----------------- 1168f4f864c1SEric Biggers 1169f4f864c1SEric BiggersFor the read path (->readpage()) of regular files, filesystems can 1170f4f864c1SEric Biggersread the ciphertext into the page cache and decrypt it in-place. The 1171f4f864c1SEric Biggerspage lock must be held until decryption has finished, to prevent the 1172f4f864c1SEric Biggerspage from becoming visible to userspace prematurely. 1173f4f864c1SEric Biggers 1174f4f864c1SEric BiggersFor the write path (->writepage()) of regular files, filesystems 1175f4f864c1SEric Biggerscannot encrypt data in-place in the page cache, since the cached 1176f4f864c1SEric Biggersplaintext must be preserved. Instead, filesystems must encrypt into a 1177f4f864c1SEric Biggerstemporary buffer or "bounce page", then write out the temporary 1178f4f864c1SEric Biggersbuffer. Some filesystems, such as UBIFS, already use temporary 1179f4f864c1SEric Biggersbuffers regardless of encryption. Other filesystems, such as ext4 and 1180f4f864c1SEric BiggersF2FS, have to allocate bounce pages specially for encryption. 1181f4f864c1SEric Biggers 1182f4f864c1SEric BiggersFilename hashing and encoding 1183f4f864c1SEric Biggers----------------------------- 1184f4f864c1SEric Biggers 1185f4f864c1SEric BiggersModern filesystems accelerate directory lookups by using indexed 1186f4f864c1SEric Biggersdirectories. An indexed directory is organized as a tree keyed by 1187f4f864c1SEric Biggersfilename hashes. When a ->lookup() is requested, the filesystem 1188f4f864c1SEric Biggersnormally hashes the filename being looked up so that it can quickly 1189f4f864c1SEric Biggersfind the corresponding directory entry, if any. 1190f4f864c1SEric Biggers 1191f4f864c1SEric BiggersWith encryption, lookups must be supported and efficient both with and 1192f4f864c1SEric Biggerswithout the encryption key. Clearly, it would not work to hash the 1193f4f864c1SEric Biggersplaintext filenames, since the plaintext filenames are unavailable 1194f4f864c1SEric Biggerswithout the key. (Hashing the plaintext filenames would also make it 1195f4f864c1SEric Biggersimpossible for the filesystem's fsck tool to optimize encrypted 1196f4f864c1SEric Biggersdirectories.) Instead, filesystems hash the ciphertext filenames, 1197f4f864c1SEric Biggersi.e. the bytes actually stored on-disk in the directory entries. When 1198f4f864c1SEric Biggersasked to do a ->lookup() with the key, the filesystem just encrypts 1199f4f864c1SEric Biggersthe user-supplied name to get the ciphertext. 1200f4f864c1SEric Biggers 1201f4f864c1SEric BiggersLookups without the key are more complicated. The raw ciphertext may 1202f4f864c1SEric Biggerscontain the ``\0`` and ``/`` characters, which are illegal in 1203f4f864c1SEric Biggersfilenames. Therefore, readdir() must base64-encode the ciphertext for 1204f4f864c1SEric Biggerspresentation. For most filenames, this works fine; on ->lookup(), the 1205f4f864c1SEric Biggersfilesystem just base64-decodes the user-supplied name to get back to 1206f4f864c1SEric Biggersthe raw ciphertext. 1207f4f864c1SEric Biggers 1208f4f864c1SEric BiggersHowever, for very long filenames, base64 encoding would cause the 1209f4f864c1SEric Biggersfilename length to exceed NAME_MAX. To prevent this, readdir() 1210f4f864c1SEric Biggersactually presents long filenames in an abbreviated form which encodes 1211f4f864c1SEric Biggersa strong "hash" of the ciphertext filename, along with the optional 1212f4f864c1SEric Biggersfilesystem-specific hash(es) needed for directory lookups. This 1213f4f864c1SEric Biggersallows the filesystem to still, with a high degree of confidence, map 1214f4f864c1SEric Biggersthe filename given in ->lookup() back to a particular directory entry 1215f4f864c1SEric Biggersthat was previously listed by readdir(). See :c:type:`struct 1216edc440e3SDaniel Rosenbergfscrypt_nokey_name` in the source for more details. 1217f4f864c1SEric Biggers 1218f4f864c1SEric BiggersNote that the precise way that filenames are presented to userspace 1219f4f864c1SEric Biggerswithout the key is subject to change in the future. It is only meant 1220f4f864c1SEric Biggersas a way to temporarily present valid filenames so that commands like 1221f4f864c1SEric Biggers``rm -r`` work as expected on encrypted directories. 122205643363SEric Biggers 122305643363SEric BiggersTests 122405643363SEric Biggers===== 122505643363SEric Biggers 122605643363SEric BiggersTo test fscrypt, use xfstests, which is Linux's de facto standard 122705643363SEric Biggersfilesystem test suite. First, run all the tests in the "encrypt" 122805643363SEric Biggersgroup on the relevant filesystem(s). For example, to test ext4 and 122905643363SEric Biggersf2fs encryption using `kvm-xfstests 123005643363SEric Biggers<https://github.com/tytso/xfstests-bld/blob/master/Documentation/kvm-quickstart.md>`_:: 123105643363SEric Biggers 123205643363SEric Biggers kvm-xfstests -c ext4,f2fs -g encrypt 123305643363SEric Biggers 123405643363SEric BiggersUBIFS encryption can also be tested this way, but it should be done in 123505643363SEric Biggersa separate command, and it takes some time for kvm-xfstests to set up 123605643363SEric Biggersemulated UBI volumes:: 123705643363SEric Biggers 123805643363SEric Biggers kvm-xfstests -c ubifs -g encrypt 123905643363SEric Biggers 124005643363SEric BiggersNo tests should fail. However, tests that use non-default encryption 124105643363SEric Biggersmodes (e.g. generic/549 and generic/550) will be skipped if the needed 124205643363SEric Biggersalgorithms were not built into the kernel's crypto API. Also, tests 124305643363SEric Biggersthat access the raw block device (e.g. generic/399, generic/548, 124405643363SEric Biggersgeneric/549, generic/550) will be skipped on UBIFS. 124505643363SEric Biggers 124605643363SEric BiggersBesides running the "encrypt" group tests, for ext4 and f2fs it's also 124705643363SEric Biggerspossible to run most xfstests with the "test_dummy_encryption" mount 124805643363SEric Biggersoption. This option causes all new files to be automatically 124905643363SEric Biggersencrypted with a dummy key, without having to make any API calls. 125005643363SEric BiggersThis tests the encrypted I/O paths more thoroughly. To do this with 125105643363SEric Biggerskvm-xfstests, use the "encrypt" filesystem configuration:: 125205643363SEric Biggers 125305643363SEric Biggers kvm-xfstests -c ext4/encrypt,f2fs/encrypt -g auto 125405643363SEric Biggers 125505643363SEric BiggersBecause this runs many more tests than "-g encrypt" does, it takes 125605643363SEric Biggersmuch longer to run; so also consider using `gce-xfstests 125705643363SEric Biggers<https://github.com/tytso/xfstests-bld/blob/master/Documentation/gce-xfstests.md>`_ 125805643363SEric Biggersinstead of kvm-xfstests:: 125905643363SEric Biggers 126005643363SEric Biggers gce-xfstests -c ext4/encrypt,f2fs/encrypt -g auto 1261