1 =====================================
2 Filesystem-level encryption (fscrypt)
3 =====================================
8 fscrypt is a library which filesystems can hook into to support
9 transparent encryption of files and directories.
11 Note: "fscrypt" in this document refers to the kernel-level portion,
12 implemented in ``fs/crypto/``, as opposed to the userspace tool
13 `fscrypt <https://github.com/google/fscrypt>`_. This document only
14 covers the kernel-level portion. For command-line examples of how to
15 use encryption, see the documentation for the userspace tool `fscrypt
16 <https://github.com/google/fscrypt>`_. Also, it is recommended to use
17 the fscrypt userspace tool, or other existing userspace tools such as
18 `fscryptctl <https://github.com/google/fscryptctl>`_ or `Android's key
20 <https://source.android.com/security/encryption/file-based>`_, over
21 using the kernel's API directly. Using existing tools reduces the
22 chance of introducing your own security bugs. (Nevertheless, for
23 completeness this documentation covers the kernel's API anyway.)
25 Unlike dm-crypt, fscrypt operates at the filesystem level rather than
26 at the block device level. This allows it to encrypt different files
27 with different keys and to have unencrypted files on the same
28 filesystem. This is useful for multi-user systems where each user's
29 data-at-rest needs to be cryptographically isolated from the others.
30 However, except for filenames, fscrypt does not encrypt filesystem
33 Unlike eCryptfs, which is a stacked filesystem, fscrypt is integrated
34 directly into supported filesystems --- currently ext4, F2FS, and
35 UBIFS. This allows encrypted files to be read and written without
36 caching both the decrypted and encrypted pages in the pagecache,
37 thereby nearly halving the memory used and bringing it in line with
38 unencrypted files. Similarly, half as many dentries and inodes are
39 needed. eCryptfs also limits encrypted filenames to 143 bytes,
40 causing application compatibility issues; fscrypt allows the full 255
41 bytes (NAME_MAX). Finally, unlike eCryptfs, the fscrypt API can be
42 used by unprivileged users, with no need to mount anything.
44 fscrypt does not support encrypting files in-place. Instead, it
45 supports marking an empty directory as encrypted. Then, after
46 userspace provides the key, all regular files, directories, and
47 symbolic links created in that directory tree are transparently
56 Provided that userspace chooses a strong encryption key, fscrypt
57 protects the confidentiality of file contents and filenames in the
58 event of a single point-in-time permanent offline compromise of the
59 block device content. fscrypt does not protect the confidentiality of
60 non-filename metadata, e.g. file sizes, file permissions, file
61 timestamps, and extended attributes. Also, the existence and location
62 of holes (unallocated blocks which logically contain all zeroes) in
63 files is not protected.
65 fscrypt is not guaranteed to protect confidentiality or authenticity
66 if an attacker is able to manipulate the filesystem offline prior to
67 an authorized user later accessing the filesystem.
72 fscrypt (and storage encryption in general) can only provide limited
73 protection, if any at all, against online attacks. In detail:
78 fscrypt is only resistant to side-channel attacks, such as timing or
79 electromagnetic attacks, to the extent that the underlying Linux
80 Cryptographic API algorithms are. If a vulnerable algorithm is used,
81 such as a table-based implementation of AES, it may be possible for an
82 attacker to mount a side channel attack against the online system.
83 Side channel attacks may also be mounted against applications
84 consuming decrypted data.
86 Unauthorized file access
87 ~~~~~~~~~~~~~~~~~~~~~~~~
89 After an encryption key has been added, fscrypt does not hide the
90 plaintext file contents or filenames from other users on the same
91 system. Instead, existing access control mechanisms such as file mode
92 bits, POSIX ACLs, LSMs, or namespaces should be used for this purpose.
94 (For the reasoning behind this, understand that while the key is
95 added, the confidentiality of the data, from the perspective of the
96 system itself, is *not* protected by the mathematical properties of
97 encryption but rather only by the correctness of the kernel.
98 Therefore, any encryption-specific access control checks would merely
99 be enforced by kernel *code* and therefore would be largely redundant
100 with the wide variety of access control mechanisms already available.)
102 Kernel memory compromise
103 ~~~~~~~~~~~~~~~~~~~~~~~~
105 An attacker who compromises the system enough to read from arbitrary
106 memory, e.g. by mounting a physical attack or by exploiting a kernel
107 security vulnerability, can compromise all encryption keys that are
110 However, fscrypt allows encryption keys to be removed from the kernel,
111 which may protect them from later compromise.
113 In more detail, the FS_IOC_REMOVE_ENCRYPTION_KEY ioctl (or the
114 FS_IOC_REMOVE_ENCRYPTION_KEY_ALL_USERS ioctl) can wipe a master
115 encryption key from kernel memory. If it does so, it will also try to
116 evict all cached inodes which had been "unlocked" using the key,
117 thereby wiping their per-file keys and making them once again appear
118 "locked", i.e. in ciphertext or encrypted form.
120 However, these ioctls have some limitations:
122 - Per-file keys for in-use files will *not* be removed or wiped.
123 Therefore, for maximum effect, userspace should close the relevant
124 encrypted files and directories before removing a master key, as
125 well as kill any processes whose working directory is in an affected
128 - The kernel cannot magically wipe copies of the master key(s) that
129 userspace might have as well. Therefore, userspace must wipe all
130 copies of the master key(s) it makes as well; normally this should
131 be done immediately after FS_IOC_ADD_ENCRYPTION_KEY, without waiting
132 for FS_IOC_REMOVE_ENCRYPTION_KEY. Naturally, the same also applies
133 to all higher levels in the key hierarchy. Userspace should also
134 follow other security precautions such as mlock()ing memory
135 containing keys to prevent it from being swapped out.
137 - In general, decrypted contents and filenames in the kernel VFS
138 caches are freed but not wiped. Therefore, portions thereof may be
139 recoverable from freed memory, even after the corresponding key(s)
140 were wiped. To partially solve this, you can set
141 CONFIG_PAGE_POISONING=y in your kernel config and add page_poison=1
142 to your kernel command line. However, this has a performance cost.
144 - Secret keys might still exist in CPU registers, in crypto
145 accelerator hardware (if used by the crypto API to implement any of
146 the algorithms), or in other places not explicitly considered here.
148 Limitations of v1 policies
149 ~~~~~~~~~~~~~~~~~~~~~~~~~~
151 v1 encryption policies have some weaknesses with respect to online
154 - There is no verification that the provided master key is correct.
155 Therefore, a malicious user can temporarily associate the wrong key
156 with another user's encrypted files to which they have read-only
157 access. Because of filesystem caching, the wrong key will then be
158 used by the other user's accesses to those files, even if the other
159 user has the correct key in their own keyring. This violates the
160 meaning of "read-only access".
162 - A compromise of a per-file key also compromises the master key from
163 which it was derived.
165 - Non-root users cannot securely remove encryption keys.
167 All the above problems are fixed with v2 encryption policies. For
168 this reason among others, it is recommended to use v2 encryption
169 policies on all new encrypted directories.
177 Each encrypted directory tree is protected by a *master key*. Master
178 keys can be up to 64 bytes long, and must be at least as long as the
179 greater of the key length needed by the contents and filenames
180 encryption modes being used. For example, if AES-256-XTS is used for
181 contents encryption, the master key must be 64 bytes (512 bits). Note
182 that the XTS mode is defined to require a key twice as long as that
183 required by the underlying block cipher.
185 To "unlock" an encrypted directory tree, userspace must provide the
186 appropriate master key. There can be any number of master keys, each
187 of which protects any number of directory trees on any number of
190 Master keys must be real cryptographic keys, i.e. indistinguishable
191 from random bytestrings of the same length. This implies that users
192 **must not** directly use a password as a master key, zero-pad a
193 shorter key, or repeat a shorter key. Security cannot be guaranteed
194 if userspace makes any such error, as the cryptographic proofs and
195 analysis would no longer apply.
197 Instead, users should generate master keys either using a
198 cryptographically secure random number generator, or by using a KDF
199 (Key Derivation Function). The kernel does not do any key stretching;
200 therefore, if userspace derives the key from a low-entropy secret such
201 as a passphrase, it is critical that a KDF designed for this purpose
202 be used, such as scrypt, PBKDF2, or Argon2.
204 Key derivation function
205 -----------------------
207 With one exception, fscrypt never uses the master key(s) for
208 encryption directly. Instead, they are only used as input to a KDF
209 (Key Derivation Function) to derive the actual keys.
211 The KDF used for a particular master key differs depending on whether
212 the key is used for v1 encryption policies or for v2 encryption
213 policies. Users **must not** use the same key for both v1 and v2
214 encryption policies. (No real-world attack is currently known on this
215 specific case of key reuse, but its security cannot be guaranteed
216 since the cryptographic proofs and analysis would no longer apply.)
218 For v1 encryption policies, the KDF only supports deriving per-file
219 encryption keys. It works by encrypting the master key with
220 AES-128-ECB, using the file's 16-byte nonce as the AES key. The
221 resulting ciphertext is used as the derived key. If the ciphertext is
222 longer than needed, then it is truncated to the needed length.
224 For v2 encryption policies, the KDF is HKDF-SHA512. The master key is
225 passed as the "input keying material", no salt is used, and a distinct
226 "application-specific information string" is used for each distinct
227 key to be derived. For example, when a per-file encryption key is
228 derived, the application-specific information string is the file's
229 nonce prefixed with "fscrypt\\0" and a context byte. Different
230 context bytes are used for other types of derived keys.
232 HKDF-SHA512 is preferred to the original AES-128-ECB based KDF because
233 HKDF is more flexible, is nonreversible, and evenly distributes
234 entropy from the master key. HKDF is also standardized and widely
235 used by other software, whereas the AES-128-ECB based KDF is ad-hoc.
240 Since each master key can protect many files, it is necessary to
241 "tweak" the encryption of each file so that the same plaintext in two
242 files doesn't map to the same ciphertext, or vice versa. In most
243 cases, fscrypt does this by deriving per-file keys. When a new
244 encrypted inode (regular file, directory, or symlink) is created,
245 fscrypt randomly generates a 16-byte nonce and stores it in the
246 inode's encryption xattr. Then, it uses a KDF (as described in `Key
247 derivation function`_) to derive the file's key from the master key
250 Key derivation was chosen over key wrapping because wrapped keys would
251 require larger xattrs which would be less likely to fit in-line in the
252 filesystem's inode table, and there didn't appear to be any
253 significant advantages to key wrapping. In particular, currently
254 there is no requirement to support unlocking a file with multiple
255 alternative master keys or to support rotating master keys. Instead,
256 the master keys may be wrapped in userspace, e.g. as is done by the
257 `fscrypt <https://github.com/google/fscrypt>`_ tool.
259 Including the inode number in the IVs was considered. However, it was
260 rejected as it would have prevented ext4 filesystems from being
261 resized, and by itself still wouldn't have been sufficient to prevent
262 the same key from being directly reused for both XTS and CTS-CBC.
264 DIRECT_KEY and per-mode keys
265 ----------------------------
267 The Adiantum encryption mode (see `Encryption modes and usage`_) is
268 suitable for both contents and filenames encryption, and it accepts
269 long IVs --- long enough to hold both an 8-byte logical block number
270 and a 16-byte per-file nonce. Also, the overhead of each Adiantum key
271 is greater than that of an AES-256-XTS key.
273 Therefore, to improve performance and save memory, for Adiantum a
274 "direct key" configuration is supported. When the user has enabled
275 this by setting FSCRYPT_POLICY_FLAG_DIRECT_KEY in the fscrypt policy,
276 per-file keys are not used. Instead, whenever any data (contents or
277 filenames) is encrypted, the file's 16-byte nonce is included in the
280 - For v1 encryption policies, the encryption is done directly with the
281 master key. Because of this, users **must not** use the same master
282 key for any other purpose, even for other v1 policies.
284 - For v2 encryption policies, the encryption is done with a per-mode
285 key derived using the KDF. Users may use the same master key for
286 other v2 encryption policies.
291 For master keys used for v2 encryption policies, a unique 16-byte "key
292 identifier" is also derived using the KDF. This value is stored in
293 the clear, since it is needed to reliably identify the key itself.
295 Encryption modes and usage
296 ==========================
298 fscrypt allows one encryption mode to be specified for file contents
299 and one encryption mode to be specified for filenames. Different
300 directory trees are permitted to use different encryption modes.
301 Currently, the following pairs of encryption modes are supported:
303 - AES-256-XTS for contents and AES-256-CTS-CBC for filenames
304 - AES-128-CBC for contents and AES-128-CTS-CBC for filenames
305 - Adiantum for both contents and filenames
307 If unsure, you should use the (AES-256-XTS, AES-256-CTS-CBC) pair.
309 AES-128-CBC was added only for low-powered embedded devices with
310 crypto accelerators such as CAAM or CESA that do not support XTS. To
311 use AES-128-CBC, CONFIG_CRYPTO_SHA256 (or another SHA-256
312 implementation) must be enabled so that ESSIV can be used.
314 Adiantum is a (primarily) stream cipher-based mode that is fast even
315 on CPUs without dedicated crypto instructions. It's also a true
316 wide-block mode, unlike XTS. It can also eliminate the need to derive
317 per-file keys. However, it depends on the security of two primitives,
318 XChaCha12 and AES-256, rather than just one. See the paper
319 "Adiantum: length-preserving encryption for entry-level processors"
320 (https://eprint.iacr.org/2018/720.pdf) for more details. To use
321 Adiantum, CONFIG_CRYPTO_ADIANTUM must be enabled. Also, fast
322 implementations of ChaCha and NHPoly1305 should be enabled, e.g.
323 CONFIG_CRYPTO_CHACHA20_NEON and CONFIG_CRYPTO_NHPOLY1305_NEON for ARM.
325 New encryption modes can be added relatively easily, without changes
326 to individual filesystems. However, authenticated encryption (AE)
327 modes are not currently supported because of the difficulty of dealing
328 with ciphertext expansion.
333 For file contents, each filesystem block is encrypted independently.
334 Currently, only the case where the filesystem block size is equal to
335 the system's page size (usually 4096 bytes) is supported.
337 Each block's IV is set to the logical block number within the file as
338 a little endian number, except that:
340 - With CBC mode encryption, ESSIV is also used. Specifically, each IV
341 is encrypted with AES-256 where the AES-256 key is the SHA-256 hash
342 of the file's data encryption key.
344 - In the "direct key" configuration (FSCRYPT_POLICY_FLAG_DIRECT_KEY
345 set in the fscrypt_policy), the file's nonce is also appended to the
346 IV. Currently this is only allowed with the Adiantum encryption
352 For filenames, each full filename is encrypted at once. Because of
353 the requirements to retain support for efficient directory lookups and
354 filenames of up to 255 bytes, the same IV is used for every filename
357 However, each encrypted directory still uses a unique key; or
358 alternatively (for the "direct key" configuration) has the file's
359 nonce included in the IVs. Thus, IV reuse is limited to within a
362 With CTS-CBC, the IV reuse means that when the plaintext filenames
363 share a common prefix at least as long as the cipher block size (16
364 bytes for AES), the corresponding encrypted filenames will also share
365 a common prefix. This is undesirable. Adiantum does not have this
366 weakness, as it is a wide-block encryption mode.
368 All supported filenames encryption modes accept any plaintext length
369 >= 16 bytes; cipher block alignment is not required. However,
370 filenames shorter than 16 bytes are NUL-padded to 16 bytes before
371 being encrypted. In addition, to reduce leakage of filename lengths
372 via their ciphertexts, all filenames are NUL-padded to the next 4, 8,
373 16, or 32-byte boundary (configurable). 32 is recommended since this
374 provides the best confidentiality, at the cost of making directory
375 entries consume slightly more space. Note that since NUL (``\0``) is
376 not otherwise a valid character in filenames, the padding will never
377 produce duplicate plaintexts.
379 Symbolic link targets are considered a type of filename and are
380 encrypted in the same way as filenames in directory entries, except
381 that IV reuse is not a problem as each symlink has its own inode.
386 Setting an encryption policy
387 ----------------------------
389 FS_IOC_SET_ENCRYPTION_POLICY
390 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
392 The FS_IOC_SET_ENCRYPTION_POLICY ioctl sets an encryption policy on an
393 empty directory or verifies that a directory or regular file already
394 has the specified encryption policy. It takes in a pointer to a
395 :c:type:`struct fscrypt_policy_v1` or a :c:type:`struct
396 fscrypt_policy_v2`, defined as follows::
398 #define FSCRYPT_POLICY_V1 0
399 #define FSCRYPT_KEY_DESCRIPTOR_SIZE 8
400 struct fscrypt_policy_v1 {
402 __u8 contents_encryption_mode;
403 __u8 filenames_encryption_mode;
405 __u8 master_key_descriptor[FSCRYPT_KEY_DESCRIPTOR_SIZE];
407 #define fscrypt_policy fscrypt_policy_v1
409 #define FSCRYPT_POLICY_V2 2
410 #define FSCRYPT_KEY_IDENTIFIER_SIZE 16
411 struct fscrypt_policy_v2 {
413 __u8 contents_encryption_mode;
414 __u8 filenames_encryption_mode;
417 __u8 master_key_identifier[FSCRYPT_KEY_IDENTIFIER_SIZE];
420 This structure must be initialized as follows:
422 - ``version`` must be FSCRYPT_POLICY_V1 (0) if the struct is
423 :c:type:`fscrypt_policy_v1` or FSCRYPT_POLICY_V2 (2) if the struct
424 is :c:type:`fscrypt_policy_v2`. (Note: we refer to the original
425 policy version as "v1", though its version code is really 0.) For
426 new encrypted directories, use v2 policies.
428 - ``contents_encryption_mode`` and ``filenames_encryption_mode`` must
429 be set to constants from ``<linux/fscrypt.h>`` which identify the
430 encryption modes to use. If unsure, use FSCRYPT_MODE_AES_256_XTS
431 (1) for ``contents_encryption_mode`` and FSCRYPT_MODE_AES_256_CTS
432 (4) for ``filenames_encryption_mode``.
434 - ``flags`` must contain a value from ``<linux/fscrypt.h>`` which
435 identifies the amount of NUL-padding to use when encrypting
436 filenames. If unsure, use FSCRYPT_POLICY_FLAGS_PAD_32 (0x3).
437 Additionally, if the encryption modes are both
438 FSCRYPT_MODE_ADIANTUM, this can contain
439 FSCRYPT_POLICY_FLAG_DIRECT_KEY; see `DIRECT_KEY and per-mode keys`_.
441 - For v2 encryption policies, ``__reserved`` must be zeroed.
443 - For v1 encryption policies, ``master_key_descriptor`` specifies how
444 to find the master key in a keyring; see `Adding keys`_. It is up
445 to userspace to choose a unique ``master_key_descriptor`` for each
446 master key. The e4crypt and fscrypt tools use the first 8 bytes of
447 ``SHA-512(SHA-512(master_key))``, but this particular scheme is not
448 required. Also, the master key need not be in the keyring yet when
449 FS_IOC_SET_ENCRYPTION_POLICY is executed. However, it must be added
450 before any files can be created in the encrypted directory.
452 For v2 encryption policies, ``master_key_descriptor`` has been
453 replaced with ``master_key_identifier``, which is longer and cannot
454 be arbitrarily chosen. Instead, the key must first be added using
455 `FS_IOC_ADD_ENCRYPTION_KEY`_. Then, the ``key_spec.u.identifier``
456 the kernel returned in the :c:type:`struct fscrypt_add_key_arg` must
457 be used as the ``master_key_identifier`` in the :c:type:`struct
460 If the file is not yet encrypted, then FS_IOC_SET_ENCRYPTION_POLICY
461 verifies that the file is an empty directory. If so, the specified
462 encryption policy is assigned to the directory, turning it into an
463 encrypted directory. After that, and after providing the
464 corresponding master key as described in `Adding keys`_, all regular
465 files, directories (recursively), and symlinks created in the
466 directory will be encrypted, inheriting the same encryption policy.
467 The filenames in the directory's entries will be encrypted as well.
469 Alternatively, if the file is already encrypted, then
470 FS_IOC_SET_ENCRYPTION_POLICY validates that the specified encryption
471 policy exactly matches the actual one. If they match, then the ioctl
472 returns 0. Otherwise, it fails with EEXIST. This works on both
473 regular files and directories, including nonempty directories.
475 When a v2 encryption policy is assigned to a directory, it is also
476 required that either the specified key has been added by the current
477 user or that the caller has CAP_FOWNER in the initial user namespace.
478 (This is needed to prevent a user from encrypting their data with
479 another user's key.) The key must remain added while
480 FS_IOC_SET_ENCRYPTION_POLICY is executing. However, if the new
481 encrypted directory does not need to be accessed immediately, then the
482 key can be removed right away afterwards.
484 Note that the ext4 filesystem does not allow the root directory to be
485 encrypted, even if it is empty. Users who want to encrypt an entire
486 filesystem with one key should consider using dm-crypt instead.
488 FS_IOC_SET_ENCRYPTION_POLICY can fail with the following errors:
490 - ``EACCES``: the file is not owned by the process's uid, nor does the
491 process have the CAP_FOWNER capability in a namespace with the file
493 - ``EEXIST``: the file is already encrypted with an encryption policy
494 different from the one specified
495 - ``EINVAL``: an invalid encryption policy was specified (invalid
496 version, mode(s), or flags; or reserved bits were set)
497 - ``ENOKEY``: a v2 encryption policy was specified, but the key with
498 the specified ``master_key_identifier`` has not been added, nor does
499 the process have the CAP_FOWNER capability in the initial user
501 - ``ENOTDIR``: the file is unencrypted and is a regular file, not a
503 - ``ENOTEMPTY``: the file is unencrypted and is a nonempty directory
504 - ``ENOTTY``: this type of filesystem does not implement encryption
505 - ``EOPNOTSUPP``: the kernel was not configured with encryption
506 support for filesystems, or the filesystem superblock has not
507 had encryption enabled on it. (For example, to use encryption on an
508 ext4 filesystem, CONFIG_FS_ENCRYPTION must be enabled in the
509 kernel config, and the superblock must have had the "encrypt"
510 feature flag enabled using ``tune2fs -O encrypt`` or ``mkfs.ext4 -O
512 - ``EPERM``: this directory may not be encrypted, e.g. because it is
513 the root directory of an ext4 filesystem
514 - ``EROFS``: the filesystem is readonly
516 Getting an encryption policy
517 ----------------------------
519 Two ioctls are available to get a file's encryption policy:
521 - `FS_IOC_GET_ENCRYPTION_POLICY_EX`_
522 - `FS_IOC_GET_ENCRYPTION_POLICY`_
524 The extended (_EX) version of the ioctl is more general and is
525 recommended to use when possible. However, on older kernels only the
526 original ioctl is available. Applications should try the extended
527 version, and if it fails with ENOTTY fall back to the original
530 FS_IOC_GET_ENCRYPTION_POLICY_EX
531 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
533 The FS_IOC_GET_ENCRYPTION_POLICY_EX ioctl retrieves the encryption
534 policy, if any, for a directory or regular file. No additional
535 permissions are required beyond the ability to open the file. It
536 takes in a pointer to a :c:type:`struct fscrypt_get_policy_ex_arg`,
539 struct fscrypt_get_policy_ex_arg {
540 __u64 policy_size; /* input/output */
543 struct fscrypt_policy_v1 v1;
544 struct fscrypt_policy_v2 v2;
545 } policy; /* output */
548 The caller must initialize ``policy_size`` to the size available for
549 the policy struct, i.e. ``sizeof(arg.policy)``.
551 On success, the policy struct is returned in ``policy``, and its
552 actual size is returned in ``policy_size``. ``policy.version`` should
553 be checked to determine the version of policy returned. Note that the
554 version code for the "v1" policy is actually 0 (FSCRYPT_POLICY_V1).
556 FS_IOC_GET_ENCRYPTION_POLICY_EX can fail with the following errors:
558 - ``EINVAL``: the file is encrypted, but it uses an unrecognized
559 encryption policy version
560 - ``ENODATA``: the file is not encrypted
561 - ``ENOTTY``: this type of filesystem does not implement encryption,
562 or this kernel is too old to support FS_IOC_GET_ENCRYPTION_POLICY_EX
563 (try FS_IOC_GET_ENCRYPTION_POLICY instead)
564 - ``EOPNOTSUPP``: the kernel was not configured with encryption
565 support for this filesystem, or the filesystem superblock has not
566 had encryption enabled on it
567 - ``EOVERFLOW``: the file is encrypted and uses a recognized
568 encryption policy version, but the policy struct does not fit into
571 Note: if you only need to know whether a file is encrypted or not, on
572 most filesystems it is also possible to use the FS_IOC_GETFLAGS ioctl
573 and check for FS_ENCRYPT_FL, or to use the statx() system call and
574 check for STATX_ATTR_ENCRYPTED in stx_attributes.
576 FS_IOC_GET_ENCRYPTION_POLICY
577 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
579 The FS_IOC_GET_ENCRYPTION_POLICY ioctl can also retrieve the
580 encryption policy, if any, for a directory or regular file. However,
581 unlike `FS_IOC_GET_ENCRYPTION_POLICY_EX`_,
582 FS_IOC_GET_ENCRYPTION_POLICY only supports the original policy
583 version. It takes in a pointer directly to a :c:type:`struct
584 fscrypt_policy_v1` rather than a :c:type:`struct
585 fscrypt_get_policy_ex_arg`.
587 The error codes for FS_IOC_GET_ENCRYPTION_POLICY are the same as those
588 for FS_IOC_GET_ENCRYPTION_POLICY_EX, except that
589 FS_IOC_GET_ENCRYPTION_POLICY also returns ``EINVAL`` if the file is
590 encrypted using a newer encryption policy version.
592 Getting the per-filesystem salt
593 -------------------------------
595 Some filesystems, such as ext4 and F2FS, also support the deprecated
596 ioctl FS_IOC_GET_ENCRYPTION_PWSALT. This ioctl retrieves a randomly
597 generated 16-byte value stored in the filesystem superblock. This
598 value is intended to used as a salt when deriving an encryption key
599 from a passphrase or other low-entropy user credential.
601 FS_IOC_GET_ENCRYPTION_PWSALT is deprecated. Instead, prefer to
602 generate and manage any needed salt(s) in userspace.
607 FS_IOC_ADD_ENCRYPTION_KEY
608 ~~~~~~~~~~~~~~~~~~~~~~~~~
610 The FS_IOC_ADD_ENCRYPTION_KEY ioctl adds a master encryption key to
611 the filesystem, making all files on the filesystem which were
612 encrypted using that key appear "unlocked", i.e. in plaintext form.
613 It can be executed on any file or directory on the target filesystem,
614 but using the filesystem's root directory is recommended. It takes in
615 a pointer to a :c:type:`struct fscrypt_add_key_arg`, defined as
618 struct fscrypt_add_key_arg {
619 struct fscrypt_key_specifier key_spec;
625 #define FSCRYPT_KEY_SPEC_TYPE_DESCRIPTOR 1
626 #define FSCRYPT_KEY_SPEC_TYPE_IDENTIFIER 2
628 struct fscrypt_key_specifier {
629 __u32 type; /* one of FSCRYPT_KEY_SPEC_TYPE_* */
632 __u8 __reserved[32]; /* reserve some extra space */
633 __u8 descriptor[FSCRYPT_KEY_DESCRIPTOR_SIZE];
634 __u8 identifier[FSCRYPT_KEY_IDENTIFIER_SIZE];
638 :c:type:`struct fscrypt_add_key_arg` must be zeroed, then initialized
641 - If the key is being added for use by v1 encryption policies, then
642 ``key_spec.type`` must contain FSCRYPT_KEY_SPEC_TYPE_DESCRIPTOR, and
643 ``key_spec.u.descriptor`` must contain the descriptor of the key
644 being added, corresponding to the value in the
645 ``master_key_descriptor`` field of :c:type:`struct
646 fscrypt_policy_v1`. To add this type of key, the calling process
647 must have the CAP_SYS_ADMIN capability in the initial user
650 Alternatively, if the key is being added for use by v2 encryption
651 policies, then ``key_spec.type`` must contain
652 FSCRYPT_KEY_SPEC_TYPE_IDENTIFIER, and ``key_spec.u.identifier`` is
653 an *output* field which the kernel fills in with a cryptographic
654 hash of the key. To add this type of key, the calling process does
655 not need any privileges. However, the number of keys that can be
656 added is limited by the user's quota for the keyrings service (see
657 ``Documentation/security/keys/core.rst``).
659 - ``raw_size`` must be the size of the ``raw`` key provided, in bytes.
661 - ``raw`` is a variable-length field which must contain the actual
662 key, ``raw_size`` bytes long.
664 For v2 policy keys, the kernel keeps track of which user (identified
665 by effective user ID) added the key, and only allows the key to be
666 removed by that user --- or by "root", if they use
667 `FS_IOC_REMOVE_ENCRYPTION_KEY_ALL_USERS`_.
669 However, if another user has added the key, it may be desirable to
670 prevent that other user from unexpectedly removing it. Therefore,
671 FS_IOC_ADD_ENCRYPTION_KEY may also be used to add a v2 policy key
672 *again*, even if it's already added by other user(s). In this case,
673 FS_IOC_ADD_ENCRYPTION_KEY will just install a claim to the key for the
674 current user, rather than actually add the key again (but the raw key
675 must still be provided, as a proof of knowledge).
677 FS_IOC_ADD_ENCRYPTION_KEY returns 0 if either the key or a claim to
678 the key was either added or already exists.
680 FS_IOC_ADD_ENCRYPTION_KEY can fail with the following errors:
682 - ``EACCES``: FSCRYPT_KEY_SPEC_TYPE_DESCRIPTOR was specified, but the
683 caller does not have the CAP_SYS_ADMIN capability in the initial
685 - ``EDQUOT``: the key quota for this user would be exceeded by adding
687 - ``EINVAL``: invalid key size or key specifier type, or reserved bits
689 - ``ENOTTY``: this type of filesystem does not implement encryption
690 - ``EOPNOTSUPP``: the kernel was not configured with encryption
691 support for this filesystem, or the filesystem superblock has not
692 had encryption enabled on it
697 For v1 encryption policies, a master encryption key can also be
698 provided by adding it to a process-subscribed keyring, e.g. to a
699 session keyring, or to a user keyring if the user keyring is linked
700 into the session keyring.
702 This method is deprecated (and not supported for v2 encryption
703 policies) for several reasons. First, it cannot be used in
704 combination with FS_IOC_REMOVE_ENCRYPTION_KEY (see `Removing keys`_),
705 so for removing a key a workaround such as keyctl_unlink() in
706 combination with ``sync; echo 2 > /proc/sys/vm/drop_caches`` would
707 have to be used. Second, it doesn't match the fact that the
708 locked/unlocked status of encrypted files (i.e. whether they appear to
709 be in plaintext form or in ciphertext form) is global. This mismatch
710 has caused much confusion as well as real problems when processes
711 running under different UIDs, such as a ``sudo`` command, need to
712 access encrypted files.
714 Nevertheless, to add a key to one of the process-subscribed keyrings,
715 the add_key() system call can be used (see:
716 ``Documentation/security/keys/core.rst``). The key type must be
717 "logon"; keys of this type are kept in kernel memory and cannot be
718 read back by userspace. The key description must be "fscrypt:"
719 followed by the 16-character lower case hex representation of the
720 ``master_key_descriptor`` that was set in the encryption policy. The
721 key payload must conform to the following structure::
723 #define FSCRYPT_MAX_KEY_SIZE 64
727 __u8 raw[FSCRYPT_MAX_KEY_SIZE];
731 ``mode`` is ignored; just set it to 0. The actual key is provided in
732 ``raw`` with ``size`` indicating its size in bytes. That is, the
733 bytes ``raw[0..size-1]`` (inclusive) are the actual key.
735 The key description prefix "fscrypt:" may alternatively be replaced
736 with a filesystem-specific prefix such as "ext4:". However, the
737 filesystem-specific prefixes are deprecated and should not be used in
743 Two ioctls are available for removing a key that was added by
744 `FS_IOC_ADD_ENCRYPTION_KEY`_:
746 - `FS_IOC_REMOVE_ENCRYPTION_KEY`_
747 - `FS_IOC_REMOVE_ENCRYPTION_KEY_ALL_USERS`_
749 These two ioctls differ only in cases where v2 policy keys are added
750 or removed by non-root users.
752 These ioctls don't work on keys that were added via the legacy
753 process-subscribed keyrings mechanism.
755 Before using these ioctls, read the `Kernel memory compromise`_
756 section for a discussion of the security goals and limitations of
759 FS_IOC_REMOVE_ENCRYPTION_KEY
760 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
762 The FS_IOC_REMOVE_ENCRYPTION_KEY ioctl removes a claim to a master
763 encryption key from the filesystem, and possibly removes the key
764 itself. It can be executed on any file or directory on the target
765 filesystem, but using the filesystem's root directory is recommended.
766 It takes in a pointer to a :c:type:`struct fscrypt_remove_key_arg`,
769 struct fscrypt_remove_key_arg {
770 struct fscrypt_key_specifier key_spec;
771 #define FSCRYPT_KEY_REMOVAL_STATUS_FLAG_FILES_BUSY 0x00000001
772 #define FSCRYPT_KEY_REMOVAL_STATUS_FLAG_OTHER_USERS 0x00000002
773 __u32 removal_status_flags; /* output */
777 This structure must be zeroed, then initialized as follows:
779 - The key to remove is specified by ``key_spec``:
781 - To remove a key used by v1 encryption policies, set
782 ``key_spec.type`` to FSCRYPT_KEY_SPEC_TYPE_DESCRIPTOR and fill
783 in ``key_spec.u.descriptor``. To remove this type of key, the
784 calling process must have the CAP_SYS_ADMIN capability in the
785 initial user namespace.
787 - To remove a key used by v2 encryption policies, set
788 ``key_spec.type`` to FSCRYPT_KEY_SPEC_TYPE_IDENTIFIER and fill
789 in ``key_spec.u.identifier``.
791 For v2 policy keys, this ioctl is usable by non-root users. However,
792 to make this possible, it actually just removes the current user's
793 claim to the key, undoing a single call to FS_IOC_ADD_ENCRYPTION_KEY.
794 Only after all claims are removed is the key really removed.
796 For example, if FS_IOC_ADD_ENCRYPTION_KEY was called with uid 1000,
797 then the key will be "claimed" by uid 1000, and
798 FS_IOC_REMOVE_ENCRYPTION_KEY will only succeed as uid 1000. Or, if
799 both uids 1000 and 2000 added the key, then for each uid
800 FS_IOC_REMOVE_ENCRYPTION_KEY will only remove their own claim. Only
801 once *both* are removed is the key really removed. (Think of it like
802 unlinking a file that may have hard links.)
804 If FS_IOC_REMOVE_ENCRYPTION_KEY really removes the key, it will also
805 try to "lock" all files that had been unlocked with the key. It won't
806 lock files that are still in-use, so this ioctl is expected to be used
807 in cooperation with userspace ensuring that none of the files are
808 still open. However, if necessary, this ioctl can be executed again
809 later to retry locking any remaining files.
811 FS_IOC_REMOVE_ENCRYPTION_KEY returns 0 if either the key was removed
812 (but may still have files remaining to be locked), the user's claim to
813 the key was removed, or the key was already removed but had files
814 remaining to be the locked so the ioctl retried locking them. In any
815 of these cases, ``removal_status_flags`` is filled in with the
816 following informational status flags:
818 - ``FSCRYPT_KEY_REMOVAL_STATUS_FLAG_FILES_BUSY``: set if some file(s)
819 are still in-use. Not guaranteed to be set in the case where only
820 the user's claim to the key was removed.
821 - ``FSCRYPT_KEY_REMOVAL_STATUS_FLAG_OTHER_USERS``: set if only the
822 user's claim to the key was removed, not the key itself
824 FS_IOC_REMOVE_ENCRYPTION_KEY can fail with the following errors:
826 - ``EACCES``: The FSCRYPT_KEY_SPEC_TYPE_DESCRIPTOR key specifier type
827 was specified, but the caller does not have the CAP_SYS_ADMIN
828 capability in the initial user namespace
829 - ``EINVAL``: invalid key specifier type, or reserved bits were set
830 - ``ENOKEY``: the key object was not found at all, i.e. it was never
831 added in the first place or was already fully removed including all
832 files locked; or, the user does not have a claim to the key (but
834 - ``ENOTTY``: this type of filesystem does not implement encryption
835 - ``EOPNOTSUPP``: the kernel was not configured with encryption
836 support for this filesystem, or the filesystem superblock has not
837 had encryption enabled on it
839 FS_IOC_REMOVE_ENCRYPTION_KEY_ALL_USERS
840 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
842 FS_IOC_REMOVE_ENCRYPTION_KEY_ALL_USERS is exactly the same as
843 `FS_IOC_REMOVE_ENCRYPTION_KEY`_, except that for v2 policy keys, the
844 ALL_USERS version of the ioctl will remove all users' claims to the
845 key, not just the current user's. I.e., the key itself will always be
846 removed, no matter how many users have added it. This difference is
847 only meaningful if non-root users are adding and removing keys.
849 Because of this, FS_IOC_REMOVE_ENCRYPTION_KEY_ALL_USERS also requires
850 "root", namely the CAP_SYS_ADMIN capability in the initial user
851 namespace. Otherwise it will fail with EACCES.
856 FS_IOC_GET_ENCRYPTION_KEY_STATUS
857 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
859 The FS_IOC_GET_ENCRYPTION_KEY_STATUS ioctl retrieves the status of a
860 master encryption key. It can be executed on any file or directory on
861 the target filesystem, but using the filesystem's root directory is
862 recommended. It takes in a pointer to a :c:type:`struct
863 fscrypt_get_key_status_arg`, defined as follows::
865 struct fscrypt_get_key_status_arg {
867 struct fscrypt_key_specifier key_spec;
871 #define FSCRYPT_KEY_STATUS_ABSENT 1
872 #define FSCRYPT_KEY_STATUS_PRESENT 2
873 #define FSCRYPT_KEY_STATUS_INCOMPLETELY_REMOVED 3
875 #define FSCRYPT_KEY_STATUS_FLAG_ADDED_BY_SELF 0x00000001
878 __u32 __out_reserved[13];
881 The caller must zero all input fields, then fill in ``key_spec``:
883 - To get the status of a key for v1 encryption policies, set
884 ``key_spec.type`` to FSCRYPT_KEY_SPEC_TYPE_DESCRIPTOR and fill
885 in ``key_spec.u.descriptor``.
887 - To get the status of a key for v2 encryption policies, set
888 ``key_spec.type`` to FSCRYPT_KEY_SPEC_TYPE_IDENTIFIER and fill
889 in ``key_spec.u.identifier``.
891 On success, 0 is returned and the kernel fills in the output fields:
893 - ``status`` indicates whether the key is absent, present, or
894 incompletely removed. Incompletely removed means that the master
895 secret has been removed, but some files are still in use; i.e.,
896 `FS_IOC_REMOVE_ENCRYPTION_KEY`_ returned 0 but set the informational
897 status flag FSCRYPT_KEY_REMOVAL_STATUS_FLAG_FILES_BUSY.
899 - ``status_flags`` can contain the following flags:
901 - ``FSCRYPT_KEY_STATUS_FLAG_ADDED_BY_SELF`` indicates that the key
902 has added by the current user. This is only set for keys
903 identified by ``identifier`` rather than by ``descriptor``.
905 - ``user_count`` specifies the number of users who have added the key.
906 This is only set for keys identified by ``identifier`` rather than
909 FS_IOC_GET_ENCRYPTION_KEY_STATUS can fail with the following errors:
911 - ``EINVAL``: invalid key specifier type, or reserved bits were set
912 - ``ENOTTY``: this type of filesystem does not implement encryption
913 - ``EOPNOTSUPP``: the kernel was not configured with encryption
914 support for this filesystem, or the filesystem superblock has not
915 had encryption enabled on it
917 Among other use cases, FS_IOC_GET_ENCRYPTION_KEY_STATUS can be useful
918 for determining whether the key for a given encrypted directory needs
919 to be added before prompting the user for the passphrase needed to
922 FS_IOC_GET_ENCRYPTION_KEY_STATUS can only get the status of keys in
923 the filesystem-level keyring, i.e. the keyring managed by
924 `FS_IOC_ADD_ENCRYPTION_KEY`_ and `FS_IOC_REMOVE_ENCRYPTION_KEY`_. It
925 cannot get the status of a key that has only been added for use by v1
926 encryption policies using the legacy mechanism involving
927 process-subscribed keyrings.
935 With the encryption key, encrypted regular files, directories, and
936 symlinks behave very similarly to their unencrypted counterparts ---
937 after all, the encryption is intended to be transparent. However,
938 astute users may notice some differences in behavior:
940 - Unencrypted files, or files encrypted with a different encryption
941 policy (i.e. different key, modes, or flags), cannot be renamed or
942 linked into an encrypted directory; see `Encryption policy
943 enforcement`_. Attempts to do so will fail with EXDEV. However,
944 encrypted files can be renamed within an encrypted directory, or
945 into an unencrypted directory.
947 Note: "moving" an unencrypted file into an encrypted directory, e.g.
948 with the `mv` program, is implemented in userspace by a copy
949 followed by a delete. Be aware that the original unencrypted data
950 may remain recoverable from free space on the disk; prefer to keep
951 all files encrypted from the very beginning. The `shred` program
952 may be used to overwrite the source files but isn't guaranteed to be
953 effective on all filesystems and storage devices.
955 - Direct I/O is not supported on encrypted files. Attempts to use
956 direct I/O on such files will fall back to buffered I/O.
958 - The fallocate operations FALLOC_FL_COLLAPSE_RANGE,
959 FALLOC_FL_INSERT_RANGE, and FALLOC_FL_ZERO_RANGE are not supported
960 on encrypted files and will fail with EOPNOTSUPP.
962 - Online defragmentation of encrypted files is not supported. The
963 EXT4_IOC_MOVE_EXT and F2FS_IOC_MOVE_RANGE ioctls will fail with
966 - The ext4 filesystem does not support data journaling with encrypted
967 regular files. It will fall back to ordered data mode instead.
969 - DAX (Direct Access) is not supported on encrypted files.
971 - The st_size of an encrypted symlink will not necessarily give the
972 length of the symlink target as required by POSIX. It will actually
973 give the length of the ciphertext, which will be slightly longer
974 than the plaintext due to NUL-padding and an extra 2-byte overhead.
976 - The maximum length of an encrypted symlink is 2 bytes shorter than
977 the maximum length of an unencrypted symlink. For example, on an
978 EXT4 filesystem with a 4K block size, unencrypted symlinks can be up
979 to 4095 bytes long, while encrypted symlinks can only be up to 4093
980 bytes long (both lengths excluding the terminating null).
982 Note that mmap *is* supported. This is possible because the pagecache
983 for an encrypted file contains the plaintext, not the ciphertext.
988 Some filesystem operations may be performed on encrypted regular
989 files, directories, and symlinks even before their encryption key has
990 been added, or after their encryption key has been removed:
992 - File metadata may be read, e.g. using stat().
994 - Directories may be listed, in which case the filenames will be
995 listed in an encoded form derived from their ciphertext. The
996 current encoding algorithm is described in `Filename hashing and
997 encoding`_. The algorithm is subject to change, but it is
998 guaranteed that the presented filenames will be no longer than
999 NAME_MAX bytes, will not contain the ``/`` or ``\0`` characters, and
1000 will uniquely identify directory entries.
1002 The ``.`` and ``..`` directory entries are special. They are always
1003 present and are not encrypted or encoded.
1005 - Files may be deleted. That is, nondirectory files may be deleted
1006 with unlink() as usual, and empty directories may be deleted with
1007 rmdir() as usual. Therefore, ``rm`` and ``rm -r`` will work as
1010 - Symlink targets may be read and followed, but they will be presented
1011 in encrypted form, similar to filenames in directories. Hence, they
1012 are unlikely to point to anywhere useful.
1014 Without the key, regular files cannot be opened or truncated.
1015 Attempts to do so will fail with ENOKEY. This implies that any
1016 regular file operations that require a file descriptor, such as
1017 read(), write(), mmap(), fallocate(), and ioctl(), are also forbidden.
1019 Also without the key, files of any type (including directories) cannot
1020 be created or linked into an encrypted directory, nor can a name in an
1021 encrypted directory be the source or target of a rename, nor can an
1022 O_TMPFILE temporary file be created in an encrypted directory. All
1023 such operations will fail with ENOKEY.
1025 It is not currently possible to backup and restore encrypted files
1026 without the encryption key. This would require special APIs which
1027 have not yet been implemented.
1029 Encryption policy enforcement
1030 =============================
1032 After an encryption policy has been set on a directory, all regular
1033 files, directories, and symbolic links created in that directory
1034 (recursively) will inherit that encryption policy. Special files ---
1035 that is, named pipes, device nodes, and UNIX domain sockets --- will
1038 Except for those special files, it is forbidden to have unencrypted
1039 files, or files encrypted with a different encryption policy, in an
1040 encrypted directory tree. Attempts to link or rename such a file into
1041 an encrypted directory will fail with EXDEV. This is also enforced
1042 during ->lookup() to provide limited protection against offline
1043 attacks that try to disable or downgrade encryption in known locations
1044 where applications may later write sensitive data. It is recommended
1045 that systems implementing a form of "verified boot" take advantage of
1046 this by validating all top-level encryption policies prior to access.
1048 Implementation details
1049 ======================
1054 An encryption policy is represented on-disk by a :c:type:`struct
1055 fscrypt_context_v1` or a :c:type:`struct fscrypt_context_v2`. It is
1056 up to individual filesystems to decide where to store it, but normally
1057 it would be stored in a hidden extended attribute. It should *not* be
1058 exposed by the xattr-related system calls such as getxattr() and
1059 setxattr() because of the special semantics of the encryption xattr.
1060 (In particular, there would be much confusion if an encryption policy
1061 were to be added to or removed from anything other than an empty
1062 directory.) These structs are defined as follows::
1064 #define FS_KEY_DERIVATION_NONCE_SIZE 16
1066 #define FSCRYPT_KEY_DESCRIPTOR_SIZE 8
1067 struct fscrypt_context_v1 {
1069 u8 contents_encryption_mode;
1070 u8 filenames_encryption_mode;
1072 u8 master_key_descriptor[FSCRYPT_KEY_DESCRIPTOR_SIZE];
1073 u8 nonce[FS_KEY_DERIVATION_NONCE_SIZE];
1076 #define FSCRYPT_KEY_IDENTIFIER_SIZE 16
1077 struct fscrypt_context_v2 {
1079 u8 contents_encryption_mode;
1080 u8 filenames_encryption_mode;
1083 u8 master_key_identifier[FSCRYPT_KEY_IDENTIFIER_SIZE];
1084 u8 nonce[FS_KEY_DERIVATION_NONCE_SIZE];
1087 The context structs contain the same information as the corresponding
1088 policy structs (see `Setting an encryption policy`_), except that the
1089 context structs also contain a nonce. The nonce is randomly generated
1090 by the kernel and is used as KDF input or as a tweak to cause
1091 different files to be encrypted differently; see `Per-file keys`_ and
1092 `DIRECT_KEY and per-mode keys`_.
1097 For the read path (->readpage()) of regular files, filesystems can
1098 read the ciphertext into the page cache and decrypt it in-place. The
1099 page lock must be held until decryption has finished, to prevent the
1100 page from becoming visible to userspace prematurely.
1102 For the write path (->writepage()) of regular files, filesystems
1103 cannot encrypt data in-place in the page cache, since the cached
1104 plaintext must be preserved. Instead, filesystems must encrypt into a
1105 temporary buffer or "bounce page", then write out the temporary
1106 buffer. Some filesystems, such as UBIFS, already use temporary
1107 buffers regardless of encryption. Other filesystems, such as ext4 and
1108 F2FS, have to allocate bounce pages specially for encryption.
1110 Filename hashing and encoding
1111 -----------------------------
1113 Modern filesystems accelerate directory lookups by using indexed
1114 directories. An indexed directory is organized as a tree keyed by
1115 filename hashes. When a ->lookup() is requested, the filesystem
1116 normally hashes the filename being looked up so that it can quickly
1117 find the corresponding directory entry, if any.
1119 With encryption, lookups must be supported and efficient both with and
1120 without the encryption key. Clearly, it would not work to hash the
1121 plaintext filenames, since the plaintext filenames are unavailable
1122 without the key. (Hashing the plaintext filenames would also make it
1123 impossible for the filesystem's fsck tool to optimize encrypted
1124 directories.) Instead, filesystems hash the ciphertext filenames,
1125 i.e. the bytes actually stored on-disk in the directory entries. When
1126 asked to do a ->lookup() with the key, the filesystem just encrypts
1127 the user-supplied name to get the ciphertext.
1129 Lookups without the key are more complicated. The raw ciphertext may
1130 contain the ``\0`` and ``/`` characters, which are illegal in
1131 filenames. Therefore, readdir() must base64-encode the ciphertext for
1132 presentation. For most filenames, this works fine; on ->lookup(), the
1133 filesystem just base64-decodes the user-supplied name to get back to
1136 However, for very long filenames, base64 encoding would cause the
1137 filename length to exceed NAME_MAX. To prevent this, readdir()
1138 actually presents long filenames in an abbreviated form which encodes
1139 a strong "hash" of the ciphertext filename, along with the optional
1140 filesystem-specific hash(es) needed for directory lookups. This
1141 allows the filesystem to still, with a high degree of confidence, map
1142 the filename given in ->lookup() back to a particular directory entry
1143 that was previously listed by readdir(). See :c:type:`struct
1144 fscrypt_digested_name` in the source for more details.
1146 Note that the precise way that filenames are presented to userspace
1147 without the key is subject to change in the future. It is only meant
1148 as a way to temporarily present valid filenames so that commands like
1149 ``rm -r`` work as expected on encrypted directories.
1154 To test fscrypt, use xfstests, which is Linux's de facto standard
1155 filesystem test suite. First, run all the tests in the "encrypt"
1156 group on the relevant filesystem(s). For example, to test ext4 and
1157 f2fs encryption using `kvm-xfstests
1158 <https://github.com/tytso/xfstests-bld/blob/master/Documentation/kvm-quickstart.md>`_::
1160 kvm-xfstests -c ext4,f2fs -g encrypt
1162 UBIFS encryption can also be tested this way, but it should be done in
1163 a separate command, and it takes some time for kvm-xfstests to set up
1164 emulated UBI volumes::
1166 kvm-xfstests -c ubifs -g encrypt
1168 No tests should fail. However, tests that use non-default encryption
1169 modes (e.g. generic/549 and generic/550) will be skipped if the needed
1170 algorithms were not built into the kernel's crypto API. Also, tests
1171 that access the raw block device (e.g. generic/399, generic/548,
1172 generic/549, generic/550) will be skipped on UBIFS.
1174 Besides running the "encrypt" group tests, for ext4 and f2fs it's also
1175 possible to run most xfstests with the "test_dummy_encryption" mount
1176 option. This option causes all new files to be automatically
1177 encrypted with a dummy key, without having to make any API calls.
1178 This tests the encrypted I/O paths more thoroughly. To do this with
1179 kvm-xfstests, use the "encrypt" filesystem configuration::
1181 kvm-xfstests -c ext4/encrypt,f2fs/encrypt -g auto
1183 Because this runs many more tests than "-g encrypt" does, it takes
1184 much longer to run; so also consider using `gce-xfstests
1185 <https://github.com/tytso/xfstests-bld/blob/master/Documentation/gce-xfstests.md>`_
1186 instead of kvm-xfstests::
1188 gce-xfstests -c ext4/encrypt,f2fs/encrypt -g auto