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 237f4f864c1SEric BiggersPer-file keys 238f4f864c1SEric 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, 271ba13f2c8SEric Biggersper-file keys are not used. Instead, whenever any data (contents or 272ba13f2c8SEric Biggersfilenames) is encrypted, the file's 16-byte nonce is included in the 273ba13f2c8SEric BiggersIV. 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 305f4f864c1SEric BiggersEncryption modes and usage 306f4f864c1SEric Biggers========================== 307f4f864c1SEric Biggers 308f4f864c1SEric Biggersfscrypt allows one encryption mode to be specified for file contents 309f4f864c1SEric Biggersand one encryption mode to be specified for filenames. Different 310f4f864c1SEric Biggersdirectory trees are permitted to use different encryption modes. 311f4f864c1SEric BiggersCurrently, the following pairs of encryption modes are supported: 312f4f864c1SEric Biggers 313f4f864c1SEric Biggers- AES-256-XTS for contents and AES-256-CTS-CBC for filenames 314f4f864c1SEric Biggers- AES-128-CBC for contents and AES-128-CTS-CBC for filenames 3158094c3ceSEric Biggers- Adiantum for both contents and filenames 316f4f864c1SEric Biggers 3178094c3ceSEric BiggersIf unsure, you should use the (AES-256-XTS, AES-256-CTS-CBC) pair. 3188094c3ceSEric Biggers 319f4f864c1SEric BiggersAES-128-CBC was added only for low-powered embedded devices with 320adbd9b4dSEric Biggerscrypto accelerators such as CAAM or CESA that do not support XTS. To 3214006d799SEric Biggersuse AES-128-CBC, CONFIG_CRYPTO_ESSIV and CONFIG_CRYPTO_SHA256 (or 3224006d799SEric Biggersanother SHA-256 implementation) must be enabled so that ESSIV can be 3234006d799SEric Biggersused. 324f4f864c1SEric Biggers 3258094c3ceSEric BiggersAdiantum is a (primarily) stream cipher-based mode that is fast even 3268094c3ceSEric Biggerson CPUs without dedicated crypto instructions. It's also a true 3278094c3ceSEric Biggerswide-block mode, unlike XTS. It can also eliminate the need to derive 3288094c3ceSEric Biggersper-file keys. However, it depends on the security of two primitives, 3298094c3ceSEric BiggersXChaCha12 and AES-256, rather than just one. See the paper 3308094c3ceSEric Biggers"Adiantum: length-preserving encryption for entry-level processors" 3318094c3ceSEric Biggers(https://eprint.iacr.org/2018/720.pdf) for more details. To use 3328094c3ceSEric BiggersAdiantum, CONFIG_CRYPTO_ADIANTUM must be enabled. Also, fast 3338094c3ceSEric Biggersimplementations of ChaCha and NHPoly1305 should be enabled, e.g. 3348094c3ceSEric BiggersCONFIG_CRYPTO_CHACHA20_NEON and CONFIG_CRYPTO_NHPOLY1305_NEON for ARM. 3358094c3ceSEric Biggers 336f4f864c1SEric BiggersNew encryption modes can be added relatively easily, without changes 337f4f864c1SEric Biggersto individual filesystems. However, authenticated encryption (AE) 338f4f864c1SEric Biggersmodes are not currently supported because of the difficulty of dealing 339f4f864c1SEric Biggerswith ciphertext expansion. 340f4f864c1SEric Biggers 3418094c3ceSEric BiggersContents encryption 3428094c3ceSEric Biggers------------------- 3438094c3ceSEric Biggers 344f4f864c1SEric BiggersFor file contents, each filesystem block is encrypted independently. 345196624e1SChandan RajendraStarting from Linux kernel 5.5, encryption of filesystems with block 346196624e1SChandan Rajendrasize less than system's page size is supported. 347f4f864c1SEric Biggers 3488094c3ceSEric BiggersEach block's IV is set to the logical block number within the file as 3498094c3ceSEric Biggersa little endian number, except that: 350f4f864c1SEric Biggers 3518094c3ceSEric Biggers- With CBC mode encryption, ESSIV is also used. Specifically, each IV 3528094c3ceSEric Biggers is encrypted with AES-256 where the AES-256 key is the SHA-256 hash 3538094c3ceSEric Biggers of the file's data encryption key. 3548094c3ceSEric Biggers 355b103fb76SEric Biggers- With `DIRECT_KEY policies`_, the file's nonce is appended to the IV. 356b103fb76SEric Biggers Currently this is only allowed with the Adiantum encryption mode. 357b103fb76SEric Biggers 358b103fb76SEric Biggers- With `IV_INO_LBLK_64 policies`_, the logical block number is limited 359b103fb76SEric Biggers to 32 bits and is placed in bits 0-31 of the IV. The inode number 360b103fb76SEric Biggers (which is also limited to 32 bits) is placed in bits 32-63. 361b103fb76SEric Biggers 362b103fb76SEric BiggersNote that because file logical block numbers are included in the IVs, 363b103fb76SEric Biggersfilesystems must enforce that blocks are never shifted around within 364b103fb76SEric Biggersencrypted files, e.g. via "collapse range" or "insert range". 3658094c3ceSEric Biggers 3668094c3ceSEric BiggersFilenames encryption 3678094c3ceSEric Biggers-------------------- 3688094c3ceSEric Biggers 3698094c3ceSEric BiggersFor filenames, each full filename is encrypted at once. Because of 3708094c3ceSEric Biggersthe requirements to retain support for efficient directory lookups and 3718094c3ceSEric Biggersfilenames of up to 255 bytes, the same IV is used for every filename 3728094c3ceSEric Biggersin a directory. 3738094c3ceSEric Biggers 374b103fb76SEric BiggersHowever, each encrypted directory still uses a unique key, or 375b103fb76SEric Biggersalternatively has the file's nonce (for `DIRECT_KEY policies`_) or 376b103fb76SEric Biggersinode number (for `IV_INO_LBLK_64 policies`_) included in the IVs. 377b103fb76SEric BiggersThus, IV reuse is limited to within a single directory. 3788094c3ceSEric Biggers 3798094c3ceSEric BiggersWith CTS-CBC, the IV reuse means that when the plaintext filenames 3808094c3ceSEric Biggersshare a common prefix at least as long as the cipher block size (16 3818094c3ceSEric Biggersbytes for AES), the corresponding encrypted filenames will also share 3828094c3ceSEric Biggersa common prefix. This is undesirable. Adiantum does not have this 3838094c3ceSEric Biggersweakness, as it is a wide-block encryption mode. 3848094c3ceSEric Biggers 3858094c3ceSEric BiggersAll supported filenames encryption modes accept any plaintext length 3868094c3ceSEric Biggers>= 16 bytes; cipher block alignment is not required. However, 3878094c3ceSEric Biggersfilenames shorter than 16 bytes are NUL-padded to 16 bytes before 3888094c3ceSEric Biggersbeing encrypted. In addition, to reduce leakage of filename lengths 3898094c3ceSEric Biggersvia their ciphertexts, all filenames are NUL-padded to the next 4, 8, 3908094c3ceSEric Biggers16, or 32-byte boundary (configurable). 32 is recommended since this 3918094c3ceSEric Biggersprovides the best confidentiality, at the cost of making directory 3928094c3ceSEric Biggersentries consume slightly more space. Note that since NUL (``\0``) is 3938094c3ceSEric Biggersnot otherwise a valid character in filenames, the padding will never 3948094c3ceSEric Biggersproduce duplicate plaintexts. 395f4f864c1SEric Biggers 396f4f864c1SEric BiggersSymbolic link targets are considered a type of filename and are 3978094c3ceSEric Biggersencrypted in the same way as filenames in directory entries, except 3988094c3ceSEric Biggersthat IV reuse is not a problem as each symlink has its own inode. 399f4f864c1SEric Biggers 400f4f864c1SEric BiggersUser API 401f4f864c1SEric Biggers======== 402f4f864c1SEric Biggers 403f4f864c1SEric BiggersSetting an encryption policy 404f4f864c1SEric Biggers---------------------------- 405f4f864c1SEric Biggers 406ba13f2c8SEric BiggersFS_IOC_SET_ENCRYPTION_POLICY 407ba13f2c8SEric Biggers~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 408ba13f2c8SEric Biggers 409f4f864c1SEric BiggersThe FS_IOC_SET_ENCRYPTION_POLICY ioctl sets an encryption policy on an 410f4f864c1SEric Biggersempty directory or verifies that a directory or regular file already 411f4f864c1SEric Biggershas the specified encryption policy. It takes in a pointer to a 412ba13f2c8SEric Biggers:c:type:`struct fscrypt_policy_v1` or a :c:type:`struct 413ba13f2c8SEric Biggersfscrypt_policy_v2`, defined as follows:: 414f4f864c1SEric Biggers 415ba13f2c8SEric Biggers #define FSCRYPT_POLICY_V1 0 4162336d0deSEric Biggers #define FSCRYPT_KEY_DESCRIPTOR_SIZE 8 417ba13f2c8SEric Biggers struct fscrypt_policy_v1 { 418f4f864c1SEric Biggers __u8 version; 419f4f864c1SEric Biggers __u8 contents_encryption_mode; 420f4f864c1SEric Biggers __u8 filenames_encryption_mode; 421f4f864c1SEric Biggers __u8 flags; 4222336d0deSEric Biggers __u8 master_key_descriptor[FSCRYPT_KEY_DESCRIPTOR_SIZE]; 423f4f864c1SEric Biggers }; 424ba13f2c8SEric Biggers #define fscrypt_policy fscrypt_policy_v1 425ba13f2c8SEric Biggers 426ba13f2c8SEric Biggers #define FSCRYPT_POLICY_V2 2 427ba13f2c8SEric Biggers #define FSCRYPT_KEY_IDENTIFIER_SIZE 16 428ba13f2c8SEric Biggers struct fscrypt_policy_v2 { 429ba13f2c8SEric Biggers __u8 version; 430ba13f2c8SEric Biggers __u8 contents_encryption_mode; 431ba13f2c8SEric Biggers __u8 filenames_encryption_mode; 432ba13f2c8SEric Biggers __u8 flags; 433ba13f2c8SEric Biggers __u8 __reserved[4]; 434ba13f2c8SEric Biggers __u8 master_key_identifier[FSCRYPT_KEY_IDENTIFIER_SIZE]; 435ba13f2c8SEric Biggers }; 436f4f864c1SEric Biggers 437f4f864c1SEric BiggersThis structure must be initialized as follows: 438f4f864c1SEric Biggers 439ba13f2c8SEric Biggers- ``version`` must be FSCRYPT_POLICY_V1 (0) if the struct is 440ba13f2c8SEric Biggers :c:type:`fscrypt_policy_v1` or FSCRYPT_POLICY_V2 (2) if the struct 441ba13f2c8SEric Biggers is :c:type:`fscrypt_policy_v2`. (Note: we refer to the original 442ba13f2c8SEric Biggers policy version as "v1", though its version code is really 0.) For 443ba13f2c8SEric Biggers new encrypted directories, use v2 policies. 444f4f864c1SEric Biggers 445f4f864c1SEric Biggers- ``contents_encryption_mode`` and ``filenames_encryption_mode`` must 4462336d0deSEric Biggers be set to constants from ``<linux/fscrypt.h>`` which identify the 4472336d0deSEric Biggers encryption modes to use. If unsure, use FSCRYPT_MODE_AES_256_XTS 4482336d0deSEric Biggers (1) for ``contents_encryption_mode`` and FSCRYPT_MODE_AES_256_CTS 4492336d0deSEric Biggers (4) for ``filenames_encryption_mode``. 450f4f864c1SEric Biggers 451b103fb76SEric Biggers- ``flags`` contains optional flags from ``<linux/fscrypt.h>``: 452b103fb76SEric Biggers 453b103fb76SEric Biggers - FSCRYPT_POLICY_FLAGS_PAD_*: The amount of NUL padding to use when 454b103fb76SEric Biggers encrypting filenames. If unsure, use FSCRYPT_POLICY_FLAGS_PAD_32 455b103fb76SEric Biggers (0x3). 456b103fb76SEric Biggers - FSCRYPT_POLICY_FLAG_DIRECT_KEY: See `DIRECT_KEY policies`_. 457b103fb76SEric Biggers - FSCRYPT_POLICY_FLAG_IV_INO_LBLK_64: See `IV_INO_LBLK_64 458b103fb76SEric Biggers policies`_. This is mutually exclusive with DIRECT_KEY and is not 459b103fb76SEric Biggers supported on v1 policies. 460f4f864c1SEric Biggers 461ba13f2c8SEric Biggers- For v2 encryption policies, ``__reserved`` must be zeroed. 462ba13f2c8SEric Biggers 463ba13f2c8SEric Biggers- For v1 encryption policies, ``master_key_descriptor`` specifies how 464ba13f2c8SEric Biggers to find the master key in a keyring; see `Adding keys`_. It is up 465ba13f2c8SEric Biggers to userspace to choose a unique ``master_key_descriptor`` for each 466ba13f2c8SEric Biggers master key. The e4crypt and fscrypt tools use the first 8 bytes of 467f4f864c1SEric Biggers ``SHA-512(SHA-512(master_key))``, but this particular scheme is not 468f4f864c1SEric Biggers required. Also, the master key need not be in the keyring yet when 469f4f864c1SEric Biggers FS_IOC_SET_ENCRYPTION_POLICY is executed. However, it must be added 470f4f864c1SEric Biggers before any files can be created in the encrypted directory. 471f4f864c1SEric Biggers 472ba13f2c8SEric Biggers For v2 encryption policies, ``master_key_descriptor`` has been 473ba13f2c8SEric Biggers replaced with ``master_key_identifier``, which is longer and cannot 474ba13f2c8SEric Biggers be arbitrarily chosen. Instead, the key must first be added using 475ba13f2c8SEric Biggers `FS_IOC_ADD_ENCRYPTION_KEY`_. Then, the ``key_spec.u.identifier`` 476ba13f2c8SEric Biggers the kernel returned in the :c:type:`struct fscrypt_add_key_arg` must 477ba13f2c8SEric Biggers be used as the ``master_key_identifier`` in the :c:type:`struct 478ba13f2c8SEric Biggers fscrypt_policy_v2`. 479ba13f2c8SEric Biggers 480f4f864c1SEric BiggersIf the file is not yet encrypted, then FS_IOC_SET_ENCRYPTION_POLICY 481f4f864c1SEric Biggersverifies that the file is an empty directory. If so, the specified 482f4f864c1SEric Biggersencryption policy is assigned to the directory, turning it into an 483f4f864c1SEric Biggersencrypted directory. After that, and after providing the 484f4f864c1SEric Biggerscorresponding master key as described in `Adding keys`_, all regular 485f4f864c1SEric Biggersfiles, directories (recursively), and symlinks created in the 486f4f864c1SEric Biggersdirectory will be encrypted, inheriting the same encryption policy. 487f4f864c1SEric BiggersThe filenames in the directory's entries will be encrypted as well. 488f4f864c1SEric Biggers 489f4f864c1SEric BiggersAlternatively, if the file is already encrypted, then 490f4f864c1SEric BiggersFS_IOC_SET_ENCRYPTION_POLICY validates that the specified encryption 491f4f864c1SEric Biggerspolicy exactly matches the actual one. If they match, then the ioctl 492f4f864c1SEric Biggersreturns 0. Otherwise, it fails with EEXIST. This works on both 493f4f864c1SEric Biggersregular files and directories, including nonempty directories. 494f4f864c1SEric Biggers 495ba13f2c8SEric BiggersWhen a v2 encryption policy is assigned to a directory, it is also 496ba13f2c8SEric Biggersrequired that either the specified key has been added by the current 497ba13f2c8SEric Biggersuser or that the caller has CAP_FOWNER in the initial user namespace. 498ba13f2c8SEric Biggers(This is needed to prevent a user from encrypting their data with 499ba13f2c8SEric Biggersanother user's key.) The key must remain added while 500ba13f2c8SEric BiggersFS_IOC_SET_ENCRYPTION_POLICY is executing. However, if the new 501ba13f2c8SEric Biggersencrypted directory does not need to be accessed immediately, then the 502ba13f2c8SEric Biggerskey can be removed right away afterwards. 503ba13f2c8SEric Biggers 504f4f864c1SEric BiggersNote that the ext4 filesystem does not allow the root directory to be 505f4f864c1SEric Biggersencrypted, even if it is empty. Users who want to encrypt an entire 506f4f864c1SEric Biggersfilesystem with one key should consider using dm-crypt instead. 507f4f864c1SEric Biggers 508f4f864c1SEric BiggersFS_IOC_SET_ENCRYPTION_POLICY can fail with the following errors: 509f4f864c1SEric Biggers 510f4f864c1SEric Biggers- ``EACCES``: the file is not owned by the process's uid, nor does the 511f4f864c1SEric Biggers process have the CAP_FOWNER capability in a namespace with the file 512f4f864c1SEric Biggers owner's uid mapped 513f4f864c1SEric Biggers- ``EEXIST``: the file is already encrypted with an encryption policy 514f4f864c1SEric Biggers different from the one specified 515f4f864c1SEric Biggers- ``EINVAL``: an invalid encryption policy was specified (invalid 516*6e1918cfSDaniel Rosenberg version, mode(s), or flags; or reserved bits were set); or a v1 517*6e1918cfSDaniel Rosenberg encryption policy was specified but the directory has the casefold 518*6e1918cfSDaniel Rosenberg flag enabled (casefolding is incompatible with v1 policies). 519ba13f2c8SEric Biggers- ``ENOKEY``: a v2 encryption policy was specified, but the key with 520ba13f2c8SEric Biggers the specified ``master_key_identifier`` has not been added, nor does 521ba13f2c8SEric Biggers the process have the CAP_FOWNER capability in the initial user 522ba13f2c8SEric Biggers namespace 523f4f864c1SEric Biggers- ``ENOTDIR``: the file is unencrypted and is a regular file, not a 524f4f864c1SEric Biggers directory 525f4f864c1SEric Biggers- ``ENOTEMPTY``: the file is unencrypted and is a nonempty directory 526f4f864c1SEric Biggers- ``ENOTTY``: this type of filesystem does not implement encryption 527f4f864c1SEric Biggers- ``EOPNOTSUPP``: the kernel was not configured with encryption 528643fa961SChandan Rajendra support for filesystems, or the filesystem superblock has not 529f4f864c1SEric Biggers had encryption enabled on it. (For example, to use encryption on an 530643fa961SChandan Rajendra ext4 filesystem, CONFIG_FS_ENCRYPTION must be enabled in the 531f4f864c1SEric Biggers kernel config, and the superblock must have had the "encrypt" 532f4f864c1SEric Biggers feature flag enabled using ``tune2fs -O encrypt`` or ``mkfs.ext4 -O 533f4f864c1SEric Biggers encrypt``.) 534f4f864c1SEric Biggers- ``EPERM``: this directory may not be encrypted, e.g. because it is 535f4f864c1SEric Biggers the root directory of an ext4 filesystem 536f4f864c1SEric Biggers- ``EROFS``: the filesystem is readonly 537f4f864c1SEric Biggers 538f4f864c1SEric BiggersGetting an encryption policy 539f4f864c1SEric Biggers---------------------------- 540f4f864c1SEric Biggers 541ba13f2c8SEric BiggersTwo ioctls are available to get a file's encryption policy: 542f4f864c1SEric Biggers 543ba13f2c8SEric Biggers- `FS_IOC_GET_ENCRYPTION_POLICY_EX`_ 544ba13f2c8SEric Biggers- `FS_IOC_GET_ENCRYPTION_POLICY`_ 545ba13f2c8SEric Biggers 546ba13f2c8SEric BiggersThe extended (_EX) version of the ioctl is more general and is 547ba13f2c8SEric Biggersrecommended to use when possible. However, on older kernels only the 548ba13f2c8SEric Biggersoriginal ioctl is available. Applications should try the extended 549ba13f2c8SEric Biggersversion, and if it fails with ENOTTY fall back to the original 550ba13f2c8SEric Biggersversion. 551ba13f2c8SEric Biggers 552ba13f2c8SEric BiggersFS_IOC_GET_ENCRYPTION_POLICY_EX 553ba13f2c8SEric Biggers~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 554ba13f2c8SEric Biggers 555ba13f2c8SEric BiggersThe FS_IOC_GET_ENCRYPTION_POLICY_EX ioctl retrieves the encryption 556ba13f2c8SEric Biggerspolicy, if any, for a directory or regular file. No additional 557ba13f2c8SEric Biggerspermissions are required beyond the ability to open the file. It 558ba13f2c8SEric Biggerstakes in a pointer to a :c:type:`struct fscrypt_get_policy_ex_arg`, 559ba13f2c8SEric Biggersdefined as follows:: 560ba13f2c8SEric Biggers 561ba13f2c8SEric Biggers struct fscrypt_get_policy_ex_arg { 562ba13f2c8SEric Biggers __u64 policy_size; /* input/output */ 563ba13f2c8SEric Biggers union { 564ba13f2c8SEric Biggers __u8 version; 565ba13f2c8SEric Biggers struct fscrypt_policy_v1 v1; 566ba13f2c8SEric Biggers struct fscrypt_policy_v2 v2; 567ba13f2c8SEric Biggers } policy; /* output */ 568ba13f2c8SEric Biggers }; 569ba13f2c8SEric Biggers 570ba13f2c8SEric BiggersThe caller must initialize ``policy_size`` to the size available for 571ba13f2c8SEric Biggersthe policy struct, i.e. ``sizeof(arg.policy)``. 572ba13f2c8SEric Biggers 573ba13f2c8SEric BiggersOn success, the policy struct is returned in ``policy``, and its 574ba13f2c8SEric Biggersactual size is returned in ``policy_size``. ``policy.version`` should 575ba13f2c8SEric Biggersbe checked to determine the version of policy returned. Note that the 576ba13f2c8SEric Biggersversion code for the "v1" policy is actually 0 (FSCRYPT_POLICY_V1). 577ba13f2c8SEric Biggers 578ba13f2c8SEric BiggersFS_IOC_GET_ENCRYPTION_POLICY_EX can fail with the following errors: 579f4f864c1SEric Biggers 580f4f864c1SEric Biggers- ``EINVAL``: the file is encrypted, but it uses an unrecognized 581ba13f2c8SEric Biggers encryption policy version 582f4f864c1SEric Biggers- ``ENODATA``: the file is not encrypted 583ba13f2c8SEric Biggers- ``ENOTTY``: this type of filesystem does not implement encryption, 584ba13f2c8SEric Biggers or this kernel is too old to support FS_IOC_GET_ENCRYPTION_POLICY_EX 585ba13f2c8SEric Biggers (try FS_IOC_GET_ENCRYPTION_POLICY instead) 586f4f864c1SEric Biggers- ``EOPNOTSUPP``: the kernel was not configured with encryption 5870642ea24SChao Yu support for this filesystem, or the filesystem superblock has not 5880642ea24SChao Yu had encryption enabled on it 589ba13f2c8SEric Biggers- ``EOVERFLOW``: the file is encrypted and uses a recognized 590ba13f2c8SEric Biggers encryption policy version, but the policy struct does not fit into 591ba13f2c8SEric Biggers the provided buffer 592f4f864c1SEric Biggers 593f4f864c1SEric BiggersNote: if you only need to know whether a file is encrypted or not, on 594f4f864c1SEric Biggersmost filesystems it is also possible to use the FS_IOC_GETFLAGS ioctl 595f4f864c1SEric Biggersand check for FS_ENCRYPT_FL, or to use the statx() system call and 596f4f864c1SEric Biggerscheck for STATX_ATTR_ENCRYPTED in stx_attributes. 597f4f864c1SEric Biggers 598ba13f2c8SEric BiggersFS_IOC_GET_ENCRYPTION_POLICY 599ba13f2c8SEric Biggers~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 600ba13f2c8SEric Biggers 601ba13f2c8SEric BiggersThe FS_IOC_GET_ENCRYPTION_POLICY ioctl can also retrieve the 602ba13f2c8SEric Biggersencryption policy, if any, for a directory or regular file. However, 603ba13f2c8SEric Biggersunlike `FS_IOC_GET_ENCRYPTION_POLICY_EX`_, 604ba13f2c8SEric BiggersFS_IOC_GET_ENCRYPTION_POLICY only supports the original policy 605ba13f2c8SEric Biggersversion. It takes in a pointer directly to a :c:type:`struct 606ba13f2c8SEric Biggersfscrypt_policy_v1` rather than a :c:type:`struct 607ba13f2c8SEric Biggersfscrypt_get_policy_ex_arg`. 608ba13f2c8SEric Biggers 609ba13f2c8SEric BiggersThe error codes for FS_IOC_GET_ENCRYPTION_POLICY are the same as those 610ba13f2c8SEric Biggersfor FS_IOC_GET_ENCRYPTION_POLICY_EX, except that 611ba13f2c8SEric BiggersFS_IOC_GET_ENCRYPTION_POLICY also returns ``EINVAL`` if the file is 612ba13f2c8SEric Biggersencrypted using a newer encryption policy version. 613ba13f2c8SEric Biggers 614f4f864c1SEric BiggersGetting the per-filesystem salt 615f4f864c1SEric Biggers------------------------------- 616f4f864c1SEric Biggers 617f4f864c1SEric BiggersSome filesystems, such as ext4 and F2FS, also support the deprecated 618f4f864c1SEric Biggersioctl FS_IOC_GET_ENCRYPTION_PWSALT. This ioctl retrieves a randomly 619f4f864c1SEric Biggersgenerated 16-byte value stored in the filesystem superblock. This 620f4f864c1SEric Biggersvalue is intended to used as a salt when deriving an encryption key 621f4f864c1SEric Biggersfrom a passphrase or other low-entropy user credential. 622f4f864c1SEric Biggers 623f4f864c1SEric BiggersFS_IOC_GET_ENCRYPTION_PWSALT is deprecated. Instead, prefer to 624f4f864c1SEric Biggersgenerate and manage any needed salt(s) in userspace. 625f4f864c1SEric Biggers 626f4f864c1SEric BiggersAdding keys 627f4f864c1SEric Biggers----------- 628f4f864c1SEric Biggers 629ba13f2c8SEric BiggersFS_IOC_ADD_ENCRYPTION_KEY 630ba13f2c8SEric Biggers~~~~~~~~~~~~~~~~~~~~~~~~~ 631ba13f2c8SEric Biggers 632ba13f2c8SEric BiggersThe FS_IOC_ADD_ENCRYPTION_KEY ioctl adds a master encryption key to 633ba13f2c8SEric Biggersthe filesystem, making all files on the filesystem which were 634ba13f2c8SEric Biggersencrypted using that key appear "unlocked", i.e. in plaintext form. 635ba13f2c8SEric BiggersIt can be executed on any file or directory on the target filesystem, 636ba13f2c8SEric Biggersbut using the filesystem's root directory is recommended. It takes in 637ba13f2c8SEric Biggersa pointer to a :c:type:`struct fscrypt_add_key_arg`, defined as 638ba13f2c8SEric Biggersfollows:: 639ba13f2c8SEric Biggers 640ba13f2c8SEric Biggers struct fscrypt_add_key_arg { 641ba13f2c8SEric Biggers struct fscrypt_key_specifier key_spec; 642ba13f2c8SEric Biggers __u32 raw_size; 64393edd392SEric Biggers __u32 key_id; 64493edd392SEric Biggers __u32 __reserved[8]; 645ba13f2c8SEric Biggers __u8 raw[]; 646ba13f2c8SEric Biggers }; 647ba13f2c8SEric Biggers 648ba13f2c8SEric Biggers #define FSCRYPT_KEY_SPEC_TYPE_DESCRIPTOR 1 649ba13f2c8SEric Biggers #define FSCRYPT_KEY_SPEC_TYPE_IDENTIFIER 2 650ba13f2c8SEric Biggers 651ba13f2c8SEric Biggers struct fscrypt_key_specifier { 652ba13f2c8SEric Biggers __u32 type; /* one of FSCRYPT_KEY_SPEC_TYPE_* */ 653ba13f2c8SEric Biggers __u32 __reserved; 654ba13f2c8SEric Biggers union { 655ba13f2c8SEric Biggers __u8 __reserved[32]; /* reserve some extra space */ 656ba13f2c8SEric Biggers __u8 descriptor[FSCRYPT_KEY_DESCRIPTOR_SIZE]; 657ba13f2c8SEric Biggers __u8 identifier[FSCRYPT_KEY_IDENTIFIER_SIZE]; 658ba13f2c8SEric Biggers } u; 659ba13f2c8SEric Biggers }; 660ba13f2c8SEric Biggers 66193edd392SEric Biggers struct fscrypt_provisioning_key_payload { 66293edd392SEric Biggers __u32 type; 66393edd392SEric Biggers __u32 __reserved; 66493edd392SEric Biggers __u8 raw[]; 66593edd392SEric Biggers }; 66693edd392SEric Biggers 667ba13f2c8SEric Biggers:c:type:`struct fscrypt_add_key_arg` must be zeroed, then initialized 668ba13f2c8SEric Biggersas follows: 669ba13f2c8SEric Biggers 670ba13f2c8SEric Biggers- If the key is being added for use by v1 encryption policies, then 671ba13f2c8SEric Biggers ``key_spec.type`` must contain FSCRYPT_KEY_SPEC_TYPE_DESCRIPTOR, and 672ba13f2c8SEric Biggers ``key_spec.u.descriptor`` must contain the descriptor of the key 673ba13f2c8SEric Biggers being added, corresponding to the value in the 674ba13f2c8SEric Biggers ``master_key_descriptor`` field of :c:type:`struct 675ba13f2c8SEric Biggers fscrypt_policy_v1`. To add this type of key, the calling process 676ba13f2c8SEric Biggers must have the CAP_SYS_ADMIN capability in the initial user 677ba13f2c8SEric Biggers namespace. 678ba13f2c8SEric Biggers 679ba13f2c8SEric Biggers Alternatively, if the key is being added for use by v2 encryption 680ba13f2c8SEric Biggers policies, then ``key_spec.type`` must contain 681ba13f2c8SEric Biggers FSCRYPT_KEY_SPEC_TYPE_IDENTIFIER, and ``key_spec.u.identifier`` is 682ba13f2c8SEric Biggers an *output* field which the kernel fills in with a cryptographic 683ba13f2c8SEric Biggers hash of the key. To add this type of key, the calling process does 684ba13f2c8SEric Biggers not need any privileges. However, the number of keys that can be 685ba13f2c8SEric Biggers added is limited by the user's quota for the keyrings service (see 686ba13f2c8SEric Biggers ``Documentation/security/keys/core.rst``). 687ba13f2c8SEric Biggers 688ba13f2c8SEric Biggers- ``raw_size`` must be the size of the ``raw`` key provided, in bytes. 68993edd392SEric Biggers Alternatively, if ``key_id`` is nonzero, this field must be 0, since 69093edd392SEric Biggers in that case the size is implied by the specified Linux keyring key. 69193edd392SEric Biggers 69293edd392SEric Biggers- ``key_id`` is 0 if the raw key is given directly in the ``raw`` 69393edd392SEric Biggers field. Otherwise ``key_id`` is the ID of a Linux keyring key of 69493edd392SEric Biggers type "fscrypt-provisioning" whose payload is a :c:type:`struct 69593edd392SEric Biggers fscrypt_provisioning_key_payload` whose ``raw`` field contains the 69693edd392SEric Biggers raw key and whose ``type`` field matches ``key_spec.type``. Since 69793edd392SEric Biggers ``raw`` is variable-length, the total size of this key's payload 69893edd392SEric Biggers must be ``sizeof(struct fscrypt_provisioning_key_payload)`` plus the 69993edd392SEric Biggers raw key size. The process must have Search permission on this key. 70093edd392SEric Biggers 70193edd392SEric Biggers Most users should leave this 0 and specify the raw key directly. 70293edd392SEric Biggers The support for specifying a Linux keyring key is intended mainly to 70393edd392SEric Biggers allow re-adding keys after a filesystem is unmounted and re-mounted, 70493edd392SEric Biggers without having to store the raw keys in userspace memory. 705ba13f2c8SEric Biggers 706ba13f2c8SEric Biggers- ``raw`` is a variable-length field which must contain the actual 70793edd392SEric Biggers key, ``raw_size`` bytes long. Alternatively, if ``key_id`` is 70893edd392SEric Biggers nonzero, then this field is unused. 709ba13f2c8SEric Biggers 710ba13f2c8SEric BiggersFor v2 policy keys, the kernel keeps track of which user (identified 711ba13f2c8SEric Biggersby effective user ID) added the key, and only allows the key to be 712ba13f2c8SEric Biggersremoved by that user --- or by "root", if they use 713ba13f2c8SEric Biggers`FS_IOC_REMOVE_ENCRYPTION_KEY_ALL_USERS`_. 714ba13f2c8SEric Biggers 715ba13f2c8SEric BiggersHowever, if another user has added the key, it may be desirable to 716ba13f2c8SEric Biggersprevent that other user from unexpectedly removing it. Therefore, 717ba13f2c8SEric BiggersFS_IOC_ADD_ENCRYPTION_KEY may also be used to add a v2 policy key 718ba13f2c8SEric Biggers*again*, even if it's already added by other user(s). In this case, 719ba13f2c8SEric BiggersFS_IOC_ADD_ENCRYPTION_KEY will just install a claim to the key for the 720ba13f2c8SEric Biggerscurrent user, rather than actually add the key again (but the raw key 721ba13f2c8SEric Biggersmust still be provided, as a proof of knowledge). 722ba13f2c8SEric Biggers 723ba13f2c8SEric BiggersFS_IOC_ADD_ENCRYPTION_KEY returns 0 if either the key or a claim to 724ba13f2c8SEric Biggersthe key was either added or already exists. 725ba13f2c8SEric Biggers 726ba13f2c8SEric BiggersFS_IOC_ADD_ENCRYPTION_KEY can fail with the following errors: 727ba13f2c8SEric Biggers 728ba13f2c8SEric Biggers- ``EACCES``: FSCRYPT_KEY_SPEC_TYPE_DESCRIPTOR was specified, but the 729ba13f2c8SEric Biggers caller does not have the CAP_SYS_ADMIN capability in the initial 73093edd392SEric Biggers user namespace; or the raw key was specified by Linux key ID but the 73193edd392SEric Biggers process lacks Search permission on the key. 732ba13f2c8SEric Biggers- ``EDQUOT``: the key quota for this user would be exceeded by adding 733ba13f2c8SEric Biggers the key 734ba13f2c8SEric Biggers- ``EINVAL``: invalid key size or key specifier type, or reserved bits 735ba13f2c8SEric Biggers were set 73693edd392SEric Biggers- ``EKEYREJECTED``: the raw key was specified by Linux key ID, but the 73793edd392SEric Biggers key has the wrong type 73893edd392SEric Biggers- ``ENOKEY``: the raw key was specified by Linux key ID, but no key 73993edd392SEric Biggers exists with that ID 740ba13f2c8SEric Biggers- ``ENOTTY``: this type of filesystem does not implement encryption 741ba13f2c8SEric Biggers- ``EOPNOTSUPP``: the kernel was not configured with encryption 742ba13f2c8SEric Biggers support for this filesystem, or the filesystem superblock has not 743ba13f2c8SEric Biggers had encryption enabled on it 744ba13f2c8SEric Biggers 745ba13f2c8SEric BiggersLegacy method 746ba13f2c8SEric Biggers~~~~~~~~~~~~~ 747ba13f2c8SEric Biggers 748ba13f2c8SEric BiggersFor v1 encryption policies, a master encryption key can also be 749ba13f2c8SEric Biggersprovided by adding it to a process-subscribed keyring, e.g. to a 750ba13f2c8SEric Biggerssession keyring, or to a user keyring if the user keyring is linked 751ba13f2c8SEric Biggersinto the session keyring. 752ba13f2c8SEric Biggers 753ba13f2c8SEric BiggersThis method is deprecated (and not supported for v2 encryption 754ba13f2c8SEric Biggerspolicies) for several reasons. First, it cannot be used in 755ba13f2c8SEric Biggerscombination with FS_IOC_REMOVE_ENCRYPTION_KEY (see `Removing keys`_), 756ba13f2c8SEric Biggersso for removing a key a workaround such as keyctl_unlink() in 757ba13f2c8SEric Biggerscombination with ``sync; echo 2 > /proc/sys/vm/drop_caches`` would 758ba13f2c8SEric Biggershave to be used. Second, it doesn't match the fact that the 759ba13f2c8SEric Biggerslocked/unlocked status of encrypted files (i.e. whether they appear to 760ba13f2c8SEric Biggersbe in plaintext form or in ciphertext form) is global. This mismatch 761ba13f2c8SEric Biggershas caused much confusion as well as real problems when processes 762ba13f2c8SEric Biggersrunning under different UIDs, such as a ``sudo`` command, need to 763ba13f2c8SEric Biggersaccess encrypted files. 764ba13f2c8SEric Biggers 765ba13f2c8SEric BiggersNevertheless, to add a key to one of the process-subscribed keyrings, 766ba13f2c8SEric Biggersthe add_key() system call can be used (see: 767f4f864c1SEric Biggers``Documentation/security/keys/core.rst``). The key type must be 768f4f864c1SEric Biggers"logon"; keys of this type are kept in kernel memory and cannot be 769f4f864c1SEric Biggersread back by userspace. The key description must be "fscrypt:" 770f4f864c1SEric Biggersfollowed by the 16-character lower case hex representation of the 771f4f864c1SEric Biggers``master_key_descriptor`` that was set in the encryption policy. The 772f4f864c1SEric Biggerskey payload must conform to the following structure:: 773f4f864c1SEric Biggers 7742336d0deSEric Biggers #define FSCRYPT_MAX_KEY_SIZE 64 775f4f864c1SEric Biggers 776f4f864c1SEric Biggers struct fscrypt_key { 777ba13f2c8SEric Biggers __u32 mode; 778ba13f2c8SEric Biggers __u8 raw[FSCRYPT_MAX_KEY_SIZE]; 779ba13f2c8SEric Biggers __u32 size; 780f4f864c1SEric Biggers }; 781f4f864c1SEric Biggers 782f4f864c1SEric Biggers``mode`` is ignored; just set it to 0. The actual key is provided in 783f4f864c1SEric Biggers``raw`` with ``size`` indicating its size in bytes. That is, the 784f4f864c1SEric Biggersbytes ``raw[0..size-1]`` (inclusive) are the actual key. 785f4f864c1SEric Biggers 786f4f864c1SEric BiggersThe key description prefix "fscrypt:" may alternatively be replaced 787f4f864c1SEric Biggerswith a filesystem-specific prefix such as "ext4:". However, the 788f4f864c1SEric Biggersfilesystem-specific prefixes are deprecated and should not be used in 789f4f864c1SEric Biggersnew programs. 790f4f864c1SEric Biggers 791ba13f2c8SEric BiggersRemoving keys 792ba13f2c8SEric Biggers------------- 793f4f864c1SEric Biggers 794ba13f2c8SEric BiggersTwo ioctls are available for removing a key that was added by 795ba13f2c8SEric Biggers`FS_IOC_ADD_ENCRYPTION_KEY`_: 796ba13f2c8SEric Biggers 797ba13f2c8SEric Biggers- `FS_IOC_REMOVE_ENCRYPTION_KEY`_ 798ba13f2c8SEric Biggers- `FS_IOC_REMOVE_ENCRYPTION_KEY_ALL_USERS`_ 799ba13f2c8SEric Biggers 800ba13f2c8SEric BiggersThese two ioctls differ only in cases where v2 policy keys are added 801ba13f2c8SEric Biggersor removed by non-root users. 802ba13f2c8SEric Biggers 803ba13f2c8SEric BiggersThese ioctls don't work on keys that were added via the legacy 804ba13f2c8SEric Biggersprocess-subscribed keyrings mechanism. 805ba13f2c8SEric Biggers 806ba13f2c8SEric BiggersBefore using these ioctls, read the `Kernel memory compromise`_ 807ba13f2c8SEric Biggerssection for a discussion of the security goals and limitations of 808ba13f2c8SEric Biggersthese ioctls. 809ba13f2c8SEric Biggers 810ba13f2c8SEric BiggersFS_IOC_REMOVE_ENCRYPTION_KEY 811ba13f2c8SEric Biggers~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 812ba13f2c8SEric Biggers 813ba13f2c8SEric BiggersThe FS_IOC_REMOVE_ENCRYPTION_KEY ioctl removes a claim to a master 814ba13f2c8SEric Biggersencryption key from the filesystem, and possibly removes the key 815ba13f2c8SEric Biggersitself. It can be executed on any file or directory on the target 816ba13f2c8SEric Biggersfilesystem, but using the filesystem's root directory is recommended. 817ba13f2c8SEric BiggersIt takes in a pointer to a :c:type:`struct fscrypt_remove_key_arg`, 818ba13f2c8SEric Biggersdefined as follows:: 819ba13f2c8SEric Biggers 820ba13f2c8SEric Biggers struct fscrypt_remove_key_arg { 821ba13f2c8SEric Biggers struct fscrypt_key_specifier key_spec; 822ba13f2c8SEric Biggers #define FSCRYPT_KEY_REMOVAL_STATUS_FLAG_FILES_BUSY 0x00000001 823ba13f2c8SEric Biggers #define FSCRYPT_KEY_REMOVAL_STATUS_FLAG_OTHER_USERS 0x00000002 824ba13f2c8SEric Biggers __u32 removal_status_flags; /* output */ 825ba13f2c8SEric Biggers __u32 __reserved[5]; 826ba13f2c8SEric Biggers }; 827ba13f2c8SEric Biggers 828ba13f2c8SEric BiggersThis structure must be zeroed, then initialized as follows: 829ba13f2c8SEric Biggers 830ba13f2c8SEric Biggers- The key to remove is specified by ``key_spec``: 831ba13f2c8SEric Biggers 832ba13f2c8SEric Biggers - To remove a key used by v1 encryption policies, set 833ba13f2c8SEric Biggers ``key_spec.type`` to FSCRYPT_KEY_SPEC_TYPE_DESCRIPTOR and fill 834ba13f2c8SEric Biggers in ``key_spec.u.descriptor``. To remove this type of key, the 835ba13f2c8SEric Biggers calling process must have the CAP_SYS_ADMIN capability in the 836ba13f2c8SEric Biggers initial user namespace. 837ba13f2c8SEric Biggers 838ba13f2c8SEric Biggers - To remove a key used by v2 encryption policies, set 839ba13f2c8SEric Biggers ``key_spec.type`` to FSCRYPT_KEY_SPEC_TYPE_IDENTIFIER and fill 840ba13f2c8SEric Biggers in ``key_spec.u.identifier``. 841ba13f2c8SEric Biggers 842ba13f2c8SEric BiggersFor v2 policy keys, this ioctl is usable by non-root users. However, 843ba13f2c8SEric Biggersto make this possible, it actually just removes the current user's 844ba13f2c8SEric Biggersclaim to the key, undoing a single call to FS_IOC_ADD_ENCRYPTION_KEY. 845ba13f2c8SEric BiggersOnly after all claims are removed is the key really removed. 846ba13f2c8SEric Biggers 847ba13f2c8SEric BiggersFor example, if FS_IOC_ADD_ENCRYPTION_KEY was called with uid 1000, 848ba13f2c8SEric Biggersthen the key will be "claimed" by uid 1000, and 849ba13f2c8SEric BiggersFS_IOC_REMOVE_ENCRYPTION_KEY will only succeed as uid 1000. Or, if 850ba13f2c8SEric Biggersboth uids 1000 and 2000 added the key, then for each uid 851ba13f2c8SEric BiggersFS_IOC_REMOVE_ENCRYPTION_KEY will only remove their own claim. Only 852ba13f2c8SEric Biggersonce *both* are removed is the key really removed. (Think of it like 853ba13f2c8SEric Biggersunlinking a file that may have hard links.) 854ba13f2c8SEric Biggers 855ba13f2c8SEric BiggersIf FS_IOC_REMOVE_ENCRYPTION_KEY really removes the key, it will also 856ba13f2c8SEric Biggerstry to "lock" all files that had been unlocked with the key. It won't 857ba13f2c8SEric Biggerslock files that are still in-use, so this ioctl is expected to be used 858ba13f2c8SEric Biggersin cooperation with userspace ensuring that none of the files are 859ba13f2c8SEric Biggersstill open. However, if necessary, this ioctl can be executed again 860ba13f2c8SEric Biggerslater to retry locking any remaining files. 861ba13f2c8SEric Biggers 862ba13f2c8SEric BiggersFS_IOC_REMOVE_ENCRYPTION_KEY returns 0 if either the key was removed 863ba13f2c8SEric Biggers(but may still have files remaining to be locked), the user's claim to 864ba13f2c8SEric Biggersthe key was removed, or the key was already removed but had files 865ba13f2c8SEric Biggersremaining to be the locked so the ioctl retried locking them. In any 866ba13f2c8SEric Biggersof these cases, ``removal_status_flags`` is filled in with the 867ba13f2c8SEric Biggersfollowing informational status flags: 868ba13f2c8SEric Biggers 869ba13f2c8SEric Biggers- ``FSCRYPT_KEY_REMOVAL_STATUS_FLAG_FILES_BUSY``: set if some file(s) 870ba13f2c8SEric Biggers are still in-use. Not guaranteed to be set in the case where only 871ba13f2c8SEric Biggers the user's claim to the key was removed. 872ba13f2c8SEric Biggers- ``FSCRYPT_KEY_REMOVAL_STATUS_FLAG_OTHER_USERS``: set if only the 873ba13f2c8SEric Biggers user's claim to the key was removed, not the key itself 874ba13f2c8SEric Biggers 875ba13f2c8SEric BiggersFS_IOC_REMOVE_ENCRYPTION_KEY can fail with the following errors: 876ba13f2c8SEric Biggers 877ba13f2c8SEric Biggers- ``EACCES``: The FSCRYPT_KEY_SPEC_TYPE_DESCRIPTOR key specifier type 878ba13f2c8SEric Biggers was specified, but the caller does not have the CAP_SYS_ADMIN 879ba13f2c8SEric Biggers capability in the initial user namespace 880ba13f2c8SEric Biggers- ``EINVAL``: invalid key specifier type, or reserved bits were set 881ba13f2c8SEric Biggers- ``ENOKEY``: the key object was not found at all, i.e. it was never 882ba13f2c8SEric Biggers added in the first place or was already fully removed including all 883ba13f2c8SEric Biggers files locked; or, the user does not have a claim to the key (but 884ba13f2c8SEric Biggers someone else does). 885ba13f2c8SEric Biggers- ``ENOTTY``: this type of filesystem does not implement encryption 886ba13f2c8SEric Biggers- ``EOPNOTSUPP``: the kernel was not configured with encryption 887ba13f2c8SEric Biggers support for this filesystem, or the filesystem superblock has not 888ba13f2c8SEric Biggers had encryption enabled on it 889ba13f2c8SEric Biggers 890ba13f2c8SEric BiggersFS_IOC_REMOVE_ENCRYPTION_KEY_ALL_USERS 891ba13f2c8SEric Biggers~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 892ba13f2c8SEric Biggers 893ba13f2c8SEric BiggersFS_IOC_REMOVE_ENCRYPTION_KEY_ALL_USERS is exactly the same as 894ba13f2c8SEric Biggers`FS_IOC_REMOVE_ENCRYPTION_KEY`_, except that for v2 policy keys, the 895ba13f2c8SEric BiggersALL_USERS version of the ioctl will remove all users' claims to the 896ba13f2c8SEric Biggerskey, not just the current user's. I.e., the key itself will always be 897ba13f2c8SEric Biggersremoved, no matter how many users have added it. This difference is 898ba13f2c8SEric Biggersonly meaningful if non-root users are adding and removing keys. 899ba13f2c8SEric Biggers 900ba13f2c8SEric BiggersBecause of this, FS_IOC_REMOVE_ENCRYPTION_KEY_ALL_USERS also requires 901ba13f2c8SEric Biggers"root", namely the CAP_SYS_ADMIN capability in the initial user 902ba13f2c8SEric Biggersnamespace. Otherwise it will fail with EACCES. 903ba13f2c8SEric Biggers 904ba13f2c8SEric BiggersGetting key status 905ba13f2c8SEric Biggers------------------ 906ba13f2c8SEric Biggers 907ba13f2c8SEric BiggersFS_IOC_GET_ENCRYPTION_KEY_STATUS 908ba13f2c8SEric Biggers~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 909ba13f2c8SEric Biggers 910ba13f2c8SEric BiggersThe FS_IOC_GET_ENCRYPTION_KEY_STATUS ioctl retrieves the status of a 911ba13f2c8SEric Biggersmaster encryption key. It can be executed on any file or directory on 912ba13f2c8SEric Biggersthe target filesystem, but using the filesystem's root directory is 913ba13f2c8SEric Biggersrecommended. It takes in a pointer to a :c:type:`struct 914ba13f2c8SEric Biggersfscrypt_get_key_status_arg`, defined as follows:: 915ba13f2c8SEric Biggers 916ba13f2c8SEric Biggers struct fscrypt_get_key_status_arg { 917ba13f2c8SEric Biggers /* input */ 918ba13f2c8SEric Biggers struct fscrypt_key_specifier key_spec; 919ba13f2c8SEric Biggers __u32 __reserved[6]; 920ba13f2c8SEric Biggers 921ba13f2c8SEric Biggers /* output */ 922ba13f2c8SEric Biggers #define FSCRYPT_KEY_STATUS_ABSENT 1 923ba13f2c8SEric Biggers #define FSCRYPT_KEY_STATUS_PRESENT 2 924ba13f2c8SEric Biggers #define FSCRYPT_KEY_STATUS_INCOMPLETELY_REMOVED 3 925ba13f2c8SEric Biggers __u32 status; 926ba13f2c8SEric Biggers #define FSCRYPT_KEY_STATUS_FLAG_ADDED_BY_SELF 0x00000001 927ba13f2c8SEric Biggers __u32 status_flags; 928ba13f2c8SEric Biggers __u32 user_count; 929ba13f2c8SEric Biggers __u32 __out_reserved[13]; 930ba13f2c8SEric Biggers }; 931ba13f2c8SEric Biggers 932ba13f2c8SEric BiggersThe caller must zero all input fields, then fill in ``key_spec``: 933ba13f2c8SEric Biggers 934ba13f2c8SEric Biggers - To get the status of a key for v1 encryption policies, set 935ba13f2c8SEric Biggers ``key_spec.type`` to FSCRYPT_KEY_SPEC_TYPE_DESCRIPTOR and fill 936ba13f2c8SEric Biggers in ``key_spec.u.descriptor``. 937ba13f2c8SEric Biggers 938ba13f2c8SEric Biggers - To get the status of a key for v2 encryption policies, set 939ba13f2c8SEric Biggers ``key_spec.type`` to FSCRYPT_KEY_SPEC_TYPE_IDENTIFIER and fill 940ba13f2c8SEric Biggers in ``key_spec.u.identifier``. 941ba13f2c8SEric Biggers 942ba13f2c8SEric BiggersOn success, 0 is returned and the kernel fills in the output fields: 943ba13f2c8SEric Biggers 944ba13f2c8SEric Biggers- ``status`` indicates whether the key is absent, present, or 945ba13f2c8SEric Biggers incompletely removed. Incompletely removed means that the master 946ba13f2c8SEric Biggers secret has been removed, but some files are still in use; i.e., 947ba13f2c8SEric Biggers `FS_IOC_REMOVE_ENCRYPTION_KEY`_ returned 0 but set the informational 948ba13f2c8SEric Biggers status flag FSCRYPT_KEY_REMOVAL_STATUS_FLAG_FILES_BUSY. 949ba13f2c8SEric Biggers 950ba13f2c8SEric Biggers- ``status_flags`` can contain the following flags: 951ba13f2c8SEric Biggers 952ba13f2c8SEric Biggers - ``FSCRYPT_KEY_STATUS_FLAG_ADDED_BY_SELF`` indicates that the key 953ba13f2c8SEric Biggers has added by the current user. This is only set for keys 954ba13f2c8SEric Biggers identified by ``identifier`` rather than by ``descriptor``. 955ba13f2c8SEric Biggers 956ba13f2c8SEric Biggers- ``user_count`` specifies the number of users who have added the key. 957ba13f2c8SEric Biggers This is only set for keys identified by ``identifier`` rather than 958ba13f2c8SEric Biggers by ``descriptor``. 959ba13f2c8SEric Biggers 960ba13f2c8SEric BiggersFS_IOC_GET_ENCRYPTION_KEY_STATUS can fail with the following errors: 961ba13f2c8SEric Biggers 962ba13f2c8SEric Biggers- ``EINVAL``: invalid key specifier type, or reserved bits were set 963ba13f2c8SEric Biggers- ``ENOTTY``: this type of filesystem does not implement encryption 964ba13f2c8SEric Biggers- ``EOPNOTSUPP``: the kernel was not configured with encryption 965ba13f2c8SEric Biggers support for this filesystem, or the filesystem superblock has not 966ba13f2c8SEric Biggers had encryption enabled on it 967ba13f2c8SEric Biggers 968ba13f2c8SEric BiggersAmong other use cases, FS_IOC_GET_ENCRYPTION_KEY_STATUS can be useful 969ba13f2c8SEric Biggersfor determining whether the key for a given encrypted directory needs 970ba13f2c8SEric Biggersto be added before prompting the user for the passphrase needed to 971ba13f2c8SEric Biggersderive the key. 972ba13f2c8SEric Biggers 973ba13f2c8SEric BiggersFS_IOC_GET_ENCRYPTION_KEY_STATUS can only get the status of keys in 974ba13f2c8SEric Biggersthe filesystem-level keyring, i.e. the keyring managed by 975ba13f2c8SEric Biggers`FS_IOC_ADD_ENCRYPTION_KEY`_ and `FS_IOC_REMOVE_ENCRYPTION_KEY`_. It 976ba13f2c8SEric Biggerscannot get the status of a key that has only been added for use by v1 977ba13f2c8SEric Biggersencryption policies using the legacy mechanism involving 978ba13f2c8SEric Biggersprocess-subscribed keyrings. 979f4f864c1SEric Biggers 980f4f864c1SEric BiggersAccess semantics 981f4f864c1SEric Biggers================ 982f4f864c1SEric Biggers 983f4f864c1SEric BiggersWith the key 984f4f864c1SEric Biggers------------ 985f4f864c1SEric Biggers 986f4f864c1SEric BiggersWith the encryption key, encrypted regular files, directories, and 987f4f864c1SEric Biggerssymlinks behave very similarly to their unencrypted counterparts --- 988f4f864c1SEric Biggersafter all, the encryption is intended to be transparent. However, 989f4f864c1SEric Biggersastute users may notice some differences in behavior: 990f4f864c1SEric Biggers 991f4f864c1SEric Biggers- Unencrypted files, or files encrypted with a different encryption 992f4f864c1SEric Biggers policy (i.e. different key, modes, or flags), cannot be renamed or 993f4f864c1SEric Biggers linked into an encrypted directory; see `Encryption policy 994f5e55e77SEric Biggers enforcement`_. Attempts to do so will fail with EXDEV. However, 995f4f864c1SEric Biggers encrypted files can be renamed within an encrypted directory, or 996f4f864c1SEric Biggers into an unencrypted directory. 997f4f864c1SEric Biggers 998f5e55e77SEric Biggers Note: "moving" an unencrypted file into an encrypted directory, e.g. 999f5e55e77SEric Biggers with the `mv` program, is implemented in userspace by a copy 1000f5e55e77SEric Biggers followed by a delete. Be aware that the original unencrypted data 1001f5e55e77SEric Biggers may remain recoverable from free space on the disk; prefer to keep 1002f5e55e77SEric Biggers all files encrypted from the very beginning. The `shred` program 1003f5e55e77SEric Biggers may be used to overwrite the source files but isn't guaranteed to be 1004f5e55e77SEric Biggers effective on all filesystems and storage devices. 1005f5e55e77SEric Biggers 1006f4f864c1SEric Biggers- Direct I/O is not supported on encrypted files. Attempts to use 1007f4f864c1SEric Biggers direct I/O on such files will fall back to buffered I/O. 1008f4f864c1SEric Biggers 1009f4f864c1SEric Biggers- The fallocate operations FALLOC_FL_COLLAPSE_RANGE, 1010f4f864c1SEric Biggers FALLOC_FL_INSERT_RANGE, and FALLOC_FL_ZERO_RANGE are not supported 1011f4f864c1SEric Biggers on encrypted files and will fail with EOPNOTSUPP. 1012f4f864c1SEric Biggers 1013f4f864c1SEric Biggers- Online defragmentation of encrypted files is not supported. The 1014f4f864c1SEric Biggers EXT4_IOC_MOVE_EXT and F2FS_IOC_MOVE_RANGE ioctls will fail with 1015f4f864c1SEric Biggers EOPNOTSUPP. 1016f4f864c1SEric Biggers 1017f4f864c1SEric Biggers- The ext4 filesystem does not support data journaling with encrypted 1018f4f864c1SEric Biggers regular files. It will fall back to ordered data mode instead. 1019f4f864c1SEric Biggers 1020f4f864c1SEric Biggers- DAX (Direct Access) is not supported on encrypted files. 1021f4f864c1SEric Biggers 1022f4f864c1SEric Biggers- The st_size of an encrypted symlink will not necessarily give the 1023f4f864c1SEric Biggers length of the symlink target as required by POSIX. It will actually 10242f46a2bcSEric Biggers give the length of the ciphertext, which will be slightly longer 10252f46a2bcSEric Biggers than the plaintext due to NUL-padding and an extra 2-byte overhead. 10262f46a2bcSEric Biggers 10272f46a2bcSEric Biggers- The maximum length of an encrypted symlink is 2 bytes shorter than 10282f46a2bcSEric Biggers the maximum length of an unencrypted symlink. For example, on an 10292f46a2bcSEric Biggers EXT4 filesystem with a 4K block size, unencrypted symlinks can be up 10302f46a2bcSEric Biggers to 4095 bytes long, while encrypted symlinks can only be up to 4093 10312f46a2bcSEric Biggers bytes long (both lengths excluding the terminating null). 1032f4f864c1SEric Biggers 1033f4f864c1SEric BiggersNote that mmap *is* supported. This is possible because the pagecache 1034f4f864c1SEric Biggersfor an encrypted file contains the plaintext, not the ciphertext. 1035f4f864c1SEric Biggers 1036f4f864c1SEric BiggersWithout the key 1037f4f864c1SEric Biggers--------------- 1038f4f864c1SEric Biggers 1039f4f864c1SEric BiggersSome filesystem operations may be performed on encrypted regular 1040f4f864c1SEric Biggersfiles, directories, and symlinks even before their encryption key has 1041ba13f2c8SEric Biggersbeen added, or after their encryption key has been removed: 1042f4f864c1SEric Biggers 1043f4f864c1SEric Biggers- File metadata may be read, e.g. using stat(). 1044f4f864c1SEric Biggers 1045f4f864c1SEric Biggers- Directories may be listed, in which case the filenames will be 1046f4f864c1SEric Biggers listed in an encoded form derived from their ciphertext. The 1047f4f864c1SEric Biggers current encoding algorithm is described in `Filename hashing and 1048f4f864c1SEric Biggers encoding`_. The algorithm is subject to change, but it is 1049f4f864c1SEric Biggers guaranteed that the presented filenames will be no longer than 1050f4f864c1SEric Biggers NAME_MAX bytes, will not contain the ``/`` or ``\0`` characters, and 1051f4f864c1SEric Biggers will uniquely identify directory entries. 1052f4f864c1SEric Biggers 1053f4f864c1SEric Biggers The ``.`` and ``..`` directory entries are special. They are always 1054f4f864c1SEric Biggers present and are not encrypted or encoded. 1055f4f864c1SEric Biggers 1056f4f864c1SEric Biggers- Files may be deleted. That is, nondirectory files may be deleted 1057f4f864c1SEric Biggers with unlink() as usual, and empty directories may be deleted with 1058f4f864c1SEric Biggers rmdir() as usual. Therefore, ``rm`` and ``rm -r`` will work as 1059f4f864c1SEric Biggers expected. 1060f4f864c1SEric Biggers 1061f4f864c1SEric Biggers- Symlink targets may be read and followed, but they will be presented 1062f4f864c1SEric Biggers in encrypted form, similar to filenames in directories. Hence, they 1063f4f864c1SEric Biggers are unlikely to point to anywhere useful. 1064f4f864c1SEric Biggers 1065f4f864c1SEric BiggersWithout the key, regular files cannot be opened or truncated. 1066f4f864c1SEric BiggersAttempts to do so will fail with ENOKEY. This implies that any 1067f4f864c1SEric Biggersregular file operations that require a file descriptor, such as 1068f4f864c1SEric Biggersread(), write(), mmap(), fallocate(), and ioctl(), are also forbidden. 1069f4f864c1SEric Biggers 1070f4f864c1SEric BiggersAlso without the key, files of any type (including directories) cannot 1071f4f864c1SEric Biggersbe created or linked into an encrypted directory, nor can a name in an 1072f4f864c1SEric Biggersencrypted directory be the source or target of a rename, nor can an 1073f4f864c1SEric BiggersO_TMPFILE temporary file be created in an encrypted directory. All 1074f4f864c1SEric Biggerssuch operations will fail with ENOKEY. 1075f4f864c1SEric Biggers 1076f4f864c1SEric BiggersIt is not currently possible to backup and restore encrypted files 1077f4f864c1SEric Biggerswithout the encryption key. This would require special APIs which 1078f4f864c1SEric Biggershave not yet been implemented. 1079f4f864c1SEric Biggers 1080f4f864c1SEric BiggersEncryption policy enforcement 1081f4f864c1SEric Biggers============================= 1082f4f864c1SEric Biggers 1083f4f864c1SEric BiggersAfter an encryption policy has been set on a directory, all regular 1084f4f864c1SEric Biggersfiles, directories, and symbolic links created in that directory 1085f4f864c1SEric Biggers(recursively) will inherit that encryption policy. Special files --- 1086f4f864c1SEric Biggersthat is, named pipes, device nodes, and UNIX domain sockets --- will 1087f4f864c1SEric Biggersnot be encrypted. 1088f4f864c1SEric Biggers 1089f4f864c1SEric BiggersExcept for those special files, it is forbidden to have unencrypted 1090f4f864c1SEric Biggersfiles, or files encrypted with a different encryption policy, in an 1091f4f864c1SEric Biggersencrypted directory tree. Attempts to link or rename such a file into 1092f5e55e77SEric Biggersan encrypted directory will fail with EXDEV. This is also enforced 1093f4f864c1SEric Biggersduring ->lookup() to provide limited protection against offline 1094f4f864c1SEric Biggersattacks that try to disable or downgrade encryption in known locations 1095f4f864c1SEric Biggerswhere applications may later write sensitive data. It is recommended 1096f4f864c1SEric Biggersthat systems implementing a form of "verified boot" take advantage of 1097f4f864c1SEric Biggersthis by validating all top-level encryption policies prior to access. 1098f4f864c1SEric Biggers 1099f4f864c1SEric BiggersImplementation details 1100f4f864c1SEric Biggers====================== 1101f4f864c1SEric Biggers 1102f4f864c1SEric BiggersEncryption context 1103f4f864c1SEric Biggers------------------ 1104f4f864c1SEric Biggers 1105f4f864c1SEric BiggersAn encryption policy is represented on-disk by a :c:type:`struct 1106ba13f2c8SEric Biggersfscrypt_context_v1` or a :c:type:`struct fscrypt_context_v2`. It is 1107ba13f2c8SEric Biggersup to individual filesystems to decide where to store it, but normally 1108ba13f2c8SEric Biggersit would be stored in a hidden extended attribute. It should *not* be 1109ba13f2c8SEric Biggersexposed by the xattr-related system calls such as getxattr() and 1110ba13f2c8SEric Biggerssetxattr() because of the special semantics of the encryption xattr. 1111ba13f2c8SEric Biggers(In particular, there would be much confusion if an encryption policy 1112ba13f2c8SEric Biggerswere to be added to or removed from anything other than an empty 1113ba13f2c8SEric Biggersdirectory.) These structs are defined as follows:: 1114f4f864c1SEric Biggers 1115f4f864c1SEric Biggers #define FS_KEY_DERIVATION_NONCE_SIZE 16 1116f4f864c1SEric Biggers 1117ba13f2c8SEric Biggers #define FSCRYPT_KEY_DESCRIPTOR_SIZE 8 1118ba13f2c8SEric Biggers struct fscrypt_context_v1 { 1119ba13f2c8SEric Biggers u8 version; 1120f4f864c1SEric Biggers u8 contents_encryption_mode; 1121f4f864c1SEric Biggers u8 filenames_encryption_mode; 1122f4f864c1SEric Biggers u8 flags; 11232336d0deSEric Biggers u8 master_key_descriptor[FSCRYPT_KEY_DESCRIPTOR_SIZE]; 1124f4f864c1SEric Biggers u8 nonce[FS_KEY_DERIVATION_NONCE_SIZE]; 1125f4f864c1SEric Biggers }; 1126f4f864c1SEric Biggers 1127ba13f2c8SEric Biggers #define FSCRYPT_KEY_IDENTIFIER_SIZE 16 1128ba13f2c8SEric Biggers struct fscrypt_context_v2 { 1129ba13f2c8SEric Biggers u8 version; 1130ba13f2c8SEric Biggers u8 contents_encryption_mode; 1131ba13f2c8SEric Biggers u8 filenames_encryption_mode; 1132ba13f2c8SEric Biggers u8 flags; 1133ba13f2c8SEric Biggers u8 __reserved[4]; 1134ba13f2c8SEric Biggers u8 master_key_identifier[FSCRYPT_KEY_IDENTIFIER_SIZE]; 1135ba13f2c8SEric Biggers u8 nonce[FS_KEY_DERIVATION_NONCE_SIZE]; 1136ba13f2c8SEric Biggers }; 1137ba13f2c8SEric Biggers 1138ba13f2c8SEric BiggersThe context structs contain the same information as the corresponding 1139ba13f2c8SEric Biggerspolicy structs (see `Setting an encryption policy`_), except that the 1140ba13f2c8SEric Biggerscontext structs also contain a nonce. The nonce is randomly generated 1141ba13f2c8SEric Biggersby the kernel and is used as KDF input or as a tweak to cause 1142ba13f2c8SEric Biggersdifferent files to be encrypted differently; see `Per-file keys`_ and 1143b103fb76SEric Biggers`DIRECT_KEY policies`_. 1144f4f864c1SEric Biggers 1145f4f864c1SEric BiggersData path changes 1146f4f864c1SEric Biggers----------------- 1147f4f864c1SEric Biggers 1148f4f864c1SEric BiggersFor the read path (->readpage()) of regular files, filesystems can 1149f4f864c1SEric Biggersread the ciphertext into the page cache and decrypt it in-place. The 1150f4f864c1SEric Biggerspage lock must be held until decryption has finished, to prevent the 1151f4f864c1SEric Biggerspage from becoming visible to userspace prematurely. 1152f4f864c1SEric Biggers 1153f4f864c1SEric BiggersFor the write path (->writepage()) of regular files, filesystems 1154f4f864c1SEric Biggerscannot encrypt data in-place in the page cache, since the cached 1155f4f864c1SEric Biggersplaintext must be preserved. Instead, filesystems must encrypt into a 1156f4f864c1SEric Biggerstemporary buffer or "bounce page", then write out the temporary 1157f4f864c1SEric Biggersbuffer. Some filesystems, such as UBIFS, already use temporary 1158f4f864c1SEric Biggersbuffers regardless of encryption. Other filesystems, such as ext4 and 1159f4f864c1SEric BiggersF2FS, have to allocate bounce pages specially for encryption. 1160f4f864c1SEric Biggers 1161f4f864c1SEric BiggersFilename hashing and encoding 1162f4f864c1SEric Biggers----------------------------- 1163f4f864c1SEric Biggers 1164f4f864c1SEric BiggersModern filesystems accelerate directory lookups by using indexed 1165f4f864c1SEric Biggersdirectories. An indexed directory is organized as a tree keyed by 1166f4f864c1SEric Biggersfilename hashes. When a ->lookup() is requested, the filesystem 1167f4f864c1SEric Biggersnormally hashes the filename being looked up so that it can quickly 1168f4f864c1SEric Biggersfind the corresponding directory entry, if any. 1169f4f864c1SEric Biggers 1170f4f864c1SEric BiggersWith encryption, lookups must be supported and efficient both with and 1171f4f864c1SEric Biggerswithout the encryption key. Clearly, it would not work to hash the 1172f4f864c1SEric Biggersplaintext filenames, since the plaintext filenames are unavailable 1173f4f864c1SEric Biggerswithout the key. (Hashing the plaintext filenames would also make it 1174f4f864c1SEric Biggersimpossible for the filesystem's fsck tool to optimize encrypted 1175f4f864c1SEric Biggersdirectories.) Instead, filesystems hash the ciphertext filenames, 1176f4f864c1SEric Biggersi.e. the bytes actually stored on-disk in the directory entries. When 1177f4f864c1SEric Biggersasked to do a ->lookup() with the key, the filesystem just encrypts 1178f4f864c1SEric Biggersthe user-supplied name to get the ciphertext. 1179f4f864c1SEric Biggers 1180f4f864c1SEric BiggersLookups without the key are more complicated. The raw ciphertext may 1181f4f864c1SEric Biggerscontain the ``\0`` and ``/`` characters, which are illegal in 1182f4f864c1SEric Biggersfilenames. Therefore, readdir() must base64-encode the ciphertext for 1183f4f864c1SEric Biggerspresentation. For most filenames, this works fine; on ->lookup(), the 1184f4f864c1SEric Biggersfilesystem just base64-decodes the user-supplied name to get back to 1185f4f864c1SEric Biggersthe raw ciphertext. 1186f4f864c1SEric Biggers 1187f4f864c1SEric BiggersHowever, for very long filenames, base64 encoding would cause the 1188f4f864c1SEric Biggersfilename length to exceed NAME_MAX. To prevent this, readdir() 1189f4f864c1SEric Biggersactually presents long filenames in an abbreviated form which encodes 1190f4f864c1SEric Biggersa strong "hash" of the ciphertext filename, along with the optional 1191f4f864c1SEric Biggersfilesystem-specific hash(es) needed for directory lookups. This 1192f4f864c1SEric Biggersallows the filesystem to still, with a high degree of confidence, map 1193f4f864c1SEric Biggersthe filename given in ->lookup() back to a particular directory entry 1194f4f864c1SEric Biggersthat was previously listed by readdir(). See :c:type:`struct 1195f4f864c1SEric Biggersfscrypt_digested_name` in the source for more details. 1196f4f864c1SEric Biggers 1197f4f864c1SEric BiggersNote that the precise way that filenames are presented to userspace 1198f4f864c1SEric Biggerswithout the key is subject to change in the future. It is only meant 1199f4f864c1SEric Biggersas a way to temporarily present valid filenames so that commands like 1200f4f864c1SEric Biggers``rm -r`` work as expected on encrypted directories. 120105643363SEric Biggers 120205643363SEric BiggersTests 120305643363SEric Biggers===== 120405643363SEric Biggers 120505643363SEric BiggersTo test fscrypt, use xfstests, which is Linux's de facto standard 120605643363SEric Biggersfilesystem test suite. First, run all the tests in the "encrypt" 120705643363SEric Biggersgroup on the relevant filesystem(s). For example, to test ext4 and 120805643363SEric Biggersf2fs encryption using `kvm-xfstests 120905643363SEric Biggers<https://github.com/tytso/xfstests-bld/blob/master/Documentation/kvm-quickstart.md>`_:: 121005643363SEric Biggers 121105643363SEric Biggers kvm-xfstests -c ext4,f2fs -g encrypt 121205643363SEric Biggers 121305643363SEric BiggersUBIFS encryption can also be tested this way, but it should be done in 121405643363SEric Biggersa separate command, and it takes some time for kvm-xfstests to set up 121505643363SEric Biggersemulated UBI volumes:: 121605643363SEric Biggers 121705643363SEric Biggers kvm-xfstests -c ubifs -g encrypt 121805643363SEric Biggers 121905643363SEric BiggersNo tests should fail. However, tests that use non-default encryption 122005643363SEric Biggersmodes (e.g. generic/549 and generic/550) will be skipped if the needed 122105643363SEric Biggersalgorithms were not built into the kernel's crypto API. Also, tests 122205643363SEric Biggersthat access the raw block device (e.g. generic/399, generic/548, 122305643363SEric Biggersgeneric/549, generic/550) will be skipped on UBIFS. 122405643363SEric Biggers 122505643363SEric BiggersBesides running the "encrypt" group tests, for ext4 and f2fs it's also 122605643363SEric Biggerspossible to run most xfstests with the "test_dummy_encryption" mount 122705643363SEric Biggersoption. This option causes all new files to be automatically 122805643363SEric Biggersencrypted with a dummy key, without having to make any API calls. 122905643363SEric BiggersThis tests the encrypted I/O paths more thoroughly. To do this with 123005643363SEric Biggerskvm-xfstests, use the "encrypt" filesystem configuration:: 123105643363SEric Biggers 123205643363SEric Biggers kvm-xfstests -c ext4/encrypt,f2fs/encrypt -g auto 123305643363SEric Biggers 123405643363SEric BiggersBecause this runs many more tests than "-g encrypt" does, it takes 123505643363SEric Biggersmuch longer to run; so also consider using `gce-xfstests 123605643363SEric Biggers<https://github.com/tytso/xfstests-bld/blob/master/Documentation/gce-xfstests.md>`_ 123705643363SEric Biggersinstead of kvm-xfstests:: 123805643363SEric Biggers 123905643363SEric Biggers gce-xfstests -c ext4/encrypt,f2fs/encrypt -g auto 1240