1 .. SPDX-License-Identifier: GPL-2.0
5 =======================================================
6 fs-verity: read-only file-based authenticity protection
7 =======================================================
12 fs-verity (``fs/verity/``) is a support layer that filesystems can
13 hook into to support transparent integrity and authenticity protection
14 of read-only files. Currently, it is supported by the ext4 and f2fs
15 filesystems. Like fscrypt, not too much filesystem-specific code is
16 needed to support fs-verity.
18 fs-verity is similar to `dm-verity
19 <https://www.kernel.org/doc/Documentation/device-mapper/verity.txt>`_
20 but works on files rather than block devices. On regular files on
21 filesystems supporting fs-verity, userspace can execute an ioctl that
22 causes the filesystem to build a Merkle tree for the file and persist
23 it to a filesystem-specific location associated with the file.
25 After this, the file is made readonly, and all reads from the file are
26 automatically verified against the file's Merkle tree. Reads of any
27 corrupted data, including mmap reads, will fail.
29 Userspace can use another ioctl to retrieve the root hash (actually
30 the "fs-verity file digest", which is a hash that includes the Merkle
31 tree root hash) that fs-verity is enforcing for the file. This ioctl
32 executes in constant time, regardless of the file size.
34 fs-verity is essentially a way to hash a file in constant time,
35 subject to the caveat that reads which would violate the hash will
41 By itself, the base fs-verity feature only provides integrity
42 protection, i.e. detection of accidental (non-malicious) corruption.
44 However, because fs-verity makes retrieving the file hash extremely
45 efficient, it's primarily meant to be used as a tool to support
46 authentication (detection of malicious modifications) or auditing
47 (logging file hashes before use).
49 Trusted userspace code (e.g. operating system code running on a
50 read-only partition that is itself authenticated by dm-verity) can
51 authenticate the contents of an fs-verity file by using the
52 `FS_IOC_MEASURE_VERITY`_ ioctl to retrieve its hash, then verifying a
53 digital signature of it.
55 A standard file hash could be used instead of fs-verity. However,
56 this is inefficient if the file is large and only a small portion may
57 be accessed. This is often the case for Android application package
58 (APK) files, for example. These typically contain many translations,
59 classes, and other resources that are infrequently or even never
60 accessed on a particular device. It would be slow and wasteful to
61 read and hash the entire file before starting the application.
63 Unlike an ahead-of-time hash, fs-verity also re-verifies data each
64 time it's paged in. This ensures that malicious disk firmware can't
65 undetectably change the contents of the file at runtime.
67 fs-verity does not replace or obsolete dm-verity. dm-verity should
68 still be used on read-only filesystems. fs-verity is for files that
69 must live on a read-write filesystem because they are independently
70 updated and potentially user-installed, so dm-verity cannot be used.
72 The base fs-verity feature is a hashing mechanism only; actually
73 authenticating the files is up to userspace. However, to meet some
74 users' needs, fs-verity optionally supports a simple signature
75 verification mechanism where users can configure the kernel to require
76 that all fs-verity files be signed by a key loaded into a keyring; see
77 `Built-in signature verification`_. Support for fs-verity file hashes
78 in IMA (Integrity Measurement Architecture) policies is also planned.
86 The FS_IOC_ENABLE_VERITY ioctl enables fs-verity on a file. It takes
87 in a pointer to a struct fsverity_enable_arg, defined as
90 struct fsverity_enable_arg {
99 __u64 __reserved2[11];
102 This structure contains the parameters of the Merkle tree to build for
103 the file, and optionally contains a signature. It must be initialized
106 - ``version`` must be 1.
107 - ``hash_algorithm`` must be the identifier for the hash algorithm to
108 use for the Merkle tree, such as FS_VERITY_HASH_ALG_SHA256. See
109 ``include/uapi/linux/fsverity.h`` for the list of possible values.
110 - ``block_size`` must be the Merkle tree block size. Currently, this
111 must be equal to the system page size, which is usually 4096 bytes.
112 Other sizes may be supported in the future. This value is not
113 necessarily the same as the filesystem block size.
114 - ``salt_size`` is the size of the salt in bytes, or 0 if no salt is
115 provided. The salt is a value that is prepended to every hashed
116 block; it can be used to personalize the hashing for a particular
117 file or device. Currently the maximum salt size is 32 bytes.
118 - ``salt_ptr`` is the pointer to the salt, or NULL if no salt is
120 - ``sig_size`` is the size of the signature in bytes, or 0 if no
121 signature is provided. Currently the signature is (somewhat
122 arbitrarily) limited to 16128 bytes. See `Built-in signature
123 verification`_ for more information.
124 - ``sig_ptr`` is the pointer to the signature, or NULL if no
125 signature is provided.
126 - All reserved fields must be zeroed.
128 FS_IOC_ENABLE_VERITY causes the filesystem to build a Merkle tree for
129 the file and persist it to a filesystem-specific location associated
130 with the file, then mark the file as a verity file. This ioctl may
131 take a long time to execute on large files, and it is interruptible by
134 FS_IOC_ENABLE_VERITY checks for write access to the inode. However,
135 it must be executed on an O_RDONLY file descriptor and no processes
136 can have the file open for writing. Attempts to open the file for
137 writing while this ioctl is executing will fail with ETXTBSY. (This
138 is necessary to guarantee that no writable file descriptors will exist
139 after verity is enabled, and to guarantee that the file's contents are
140 stable while the Merkle tree is being built over it.)
142 On success, FS_IOC_ENABLE_VERITY returns 0, and the file becomes a
143 verity file. On failure (including the case of interruption by a
144 fatal signal), no changes are made to the file.
146 FS_IOC_ENABLE_VERITY can fail with the following errors:
148 - ``EACCES``: the process does not have write access to the file
149 - ``EBADMSG``: the signature is malformed
150 - ``EBUSY``: this ioctl is already running on the file
151 - ``EEXIST``: the file already has verity enabled
152 - ``EFAULT``: the caller provided inaccessible memory
153 - ``EINTR``: the operation was interrupted by a fatal signal
154 - ``EINVAL``: unsupported version, hash algorithm, or block size; or
155 reserved bits are set; or the file descriptor refers to neither a
156 regular file nor a directory.
157 - ``EISDIR``: the file descriptor refers to a directory
158 - ``EKEYREJECTED``: the signature doesn't match the file
159 - ``EMSGSIZE``: the salt or signature is too long
160 - ``ENOKEY``: the fs-verity keyring doesn't contain the certificate
161 needed to verify the signature
162 - ``ENOPKG``: fs-verity recognizes the hash algorithm, but it's not
163 available in the kernel's crypto API as currently configured (e.g.
164 for SHA-512, missing CONFIG_CRYPTO_SHA512).
165 - ``ENOTTY``: this type of filesystem does not implement fs-verity
166 - ``EOPNOTSUPP``: the kernel was not configured with fs-verity
167 support; or the filesystem superblock has not had the 'verity'
168 feature enabled on it; or the filesystem does not support fs-verity
169 on this file. (See `Filesystem support`_.)
170 - ``EPERM``: the file is append-only; or, a signature is required and
171 one was not provided.
172 - ``EROFS``: the filesystem is read-only
173 - ``ETXTBSY``: someone has the file open for writing. This can be the
174 caller's file descriptor, another open file descriptor, or the file
175 reference held by a writable memory map.
177 FS_IOC_MEASURE_VERITY
178 ---------------------
180 The FS_IOC_MEASURE_VERITY ioctl retrieves the digest of a verity file.
181 The fs-verity file digest is a cryptographic digest that identifies
182 the file contents that are being enforced on reads; it is computed via
183 a Merkle tree and is different from a traditional full-file digest.
185 This ioctl takes in a pointer to a variable-length structure::
187 struct fsverity_digest {
188 __u16 digest_algorithm;
189 __u16 digest_size; /* input/output */
193 ``digest_size`` is an input/output field. On input, it must be
194 initialized to the number of bytes allocated for the variable-length
197 On success, 0 is returned and the kernel fills in the structure as
200 - ``digest_algorithm`` will be the hash algorithm used for the file
201 digest. It will match ``fsverity_enable_arg::hash_algorithm``.
202 - ``digest_size`` will be the size of the digest in bytes, e.g. 32
203 for SHA-256. (This can be redundant with ``digest_algorithm``.)
204 - ``digest`` will be the actual bytes of the digest.
206 FS_IOC_MEASURE_VERITY is guaranteed to execute in constant time,
207 regardless of the size of the file.
209 FS_IOC_MEASURE_VERITY can fail with the following errors:
211 - ``EFAULT``: the caller provided inaccessible memory
212 - ``ENODATA``: the file is not a verity file
213 - ``ENOTTY``: this type of filesystem does not implement fs-verity
214 - ``EOPNOTSUPP``: the kernel was not configured with fs-verity
215 support, or the filesystem superblock has not had the 'verity'
216 feature enabled on it. (See `Filesystem support`_.)
217 - ``EOVERFLOW``: the digest is longer than the specified
218 ``digest_size`` bytes. Try providing a larger buffer.
220 FS_IOC_READ_VERITY_METADATA
221 ---------------------------
223 The FS_IOC_READ_VERITY_METADATA ioctl reads verity metadata from a
224 verity file. This ioctl is available since Linux v5.12.
226 This ioctl allows writing a server program that takes a verity file
227 and serves it to a client program, such that the client can do its own
228 fs-verity compatible verification of the file. This only makes sense
229 if the client doesn't trust the server and if the server needs to
230 provide the storage for the client.
232 This is a fairly specialized use case, and most fs-verity users won't
235 This ioctl takes in a pointer to the following structure::
237 #define FS_VERITY_METADATA_TYPE_MERKLE_TREE 1
238 #define FS_VERITY_METADATA_TYPE_DESCRIPTOR 2
239 #define FS_VERITY_METADATA_TYPE_SIGNATURE 3
241 struct fsverity_read_metadata_arg {
249 ``metadata_type`` specifies the type of metadata to read:
251 - ``FS_VERITY_METADATA_TYPE_MERKLE_TREE`` reads the blocks of the
252 Merkle tree. The blocks are returned in order from the root level
253 to the leaf level. Within each level, the blocks are returned in
254 the same order that their hashes are themselves hashed.
255 See `Merkle tree`_ for more information.
257 - ``FS_VERITY_METADATA_TYPE_DESCRIPTOR`` reads the fs-verity
258 descriptor. See `fs-verity descriptor`_.
260 - ``FS_VERITY_METADATA_TYPE_SIGNATURE`` reads the signature which was
261 passed to FS_IOC_ENABLE_VERITY, if any. See `Built-in signature
264 The semantics are similar to those of ``pread()``. ``offset``
265 specifies the offset in bytes into the metadata item to read from, and
266 ``length`` specifies the maximum number of bytes to read from the
267 metadata item. ``buf_ptr`` is the pointer to the buffer to read into,
268 cast to a 64-bit integer. ``__reserved`` must be 0. On success, the
269 number of bytes read is returned. 0 is returned at the end of the
270 metadata item. The returned length may be less than ``length``, for
271 example if the ioctl is interrupted.
273 The metadata returned by FS_IOC_READ_VERITY_METADATA isn't guaranteed
274 to be authenticated against the file digest that would be returned by
275 `FS_IOC_MEASURE_VERITY`_, as the metadata is expected to be used to
276 implement fs-verity compatible verification anyway (though absent a
277 malicious disk, the metadata will indeed match). E.g. to implement
278 this ioctl, the filesystem is allowed to just read the Merkle tree
279 blocks from disk without actually verifying the path to the root node.
281 FS_IOC_READ_VERITY_METADATA can fail with the following errors:
283 - ``EFAULT``: the caller provided inaccessible memory
284 - ``EINTR``: the ioctl was interrupted before any data was read
285 - ``EINVAL``: reserved fields were set, or ``offset + length``
287 - ``ENODATA``: the file is not a verity file, or
288 FS_VERITY_METADATA_TYPE_SIGNATURE was requested but the file doesn't
289 have a built-in signature
290 - ``ENOTTY``: this type of filesystem does not implement fs-verity, or
291 this ioctl is not yet implemented on it
292 - ``EOPNOTSUPP``: the kernel was not configured with fs-verity
293 support, or the filesystem superblock has not had the 'verity'
294 feature enabled on it. (See `Filesystem support`_.)
299 The existing ioctl FS_IOC_GETFLAGS (which isn't specific to fs-verity)
300 can also be used to check whether a file has fs-verity enabled or not.
301 To do so, check for FS_VERITY_FL (0x00100000) in the returned flags.
303 The verity flag is not settable via FS_IOC_SETFLAGS. You must use
304 FS_IOC_ENABLE_VERITY instead, since parameters must be provided.
309 Since Linux v5.5, the statx() system call sets STATX_ATTR_VERITY if
310 the file has fs-verity enabled. This can perform better than
311 FS_IOC_GETFLAGS and FS_IOC_MEASURE_VERITY because it doesn't require
312 opening the file, and opening verity files can be expensive.
314 Accessing verity files
315 ======================
317 Applications can transparently access a verity file just like a
318 non-verity one, with the following exceptions:
320 - Verity files are readonly. They cannot be opened for writing or
321 truncate()d, even if the file mode bits allow it. Attempts to do
322 one of these things will fail with EPERM. However, changes to
323 metadata such as owner, mode, timestamps, and xattrs are still
324 allowed, since these are not measured by fs-verity. Verity files
325 can also still be renamed, deleted, and linked to.
327 - Direct I/O is not supported on verity files. Attempts to use direct
328 I/O on such files will fall back to buffered I/O.
330 - DAX (Direct Access) is not supported on verity files, because this
331 would circumvent the data verification.
333 - Reads of data that doesn't match the verity Merkle tree will fail
334 with EIO (for read()) or SIGBUS (for mmap() reads).
336 - If the sysctl "fs.verity.require_signatures" is set to 1 and the
337 file is not signed by a key in the fs-verity keyring, then opening
338 the file will fail. See `Built-in signature verification`_.
340 Direct access to the Merkle tree is not supported. Therefore, if a
341 verity file is copied, or is backed up and restored, then it will lose
342 its "verity"-ness. fs-verity is primarily meant for files like
343 executables that are managed by a package manager.
345 File digest computation
346 =======================
348 This section describes how fs-verity hashes the file contents using a
349 Merkle tree to produce the digest which cryptographically identifies
350 the file contents. This algorithm is the same for all filesystems
351 that support fs-verity.
353 Userspace only needs to be aware of this algorithm if it needs to
354 compute fs-verity file digests itself, e.g. in order to sign files.
356 .. _fsverity_merkle_tree:
361 The file contents is divided into blocks, where the block size is
362 configurable but is usually 4096 bytes. The end of the last block is
363 zero-padded if needed. Each block is then hashed, producing the first
364 level of hashes. Then, the hashes in this first level are grouped
365 into 'blocksize'-byte blocks (zero-padding the ends as needed) and
366 these blocks are hashed, producing the second level of hashes. This
367 proceeds up the tree until only a single block remains. The hash of
368 this block is the "Merkle tree root hash".
370 If the file fits in one block and is nonempty, then the "Merkle tree
371 root hash" is simply the hash of the single data block. If the file
372 is empty, then the "Merkle tree root hash" is all zeroes.
374 The "blocks" here are not necessarily the same as "filesystem blocks".
376 If a salt was specified, then it's zero-padded to the closest multiple
377 of the input size of the hash algorithm's compression function, e.g.
378 64 bytes for SHA-256 or 128 bytes for SHA-512. The padded salt is
379 prepended to every data or Merkle tree block that is hashed.
381 The purpose of the block padding is to cause every hash to be taken
382 over the same amount of data, which simplifies the implementation and
383 keeps open more possibilities for hardware acceleration. The purpose
384 of the salt padding is to make the salting "free" when the salted hash
385 state is precomputed, then imported for each hash.
387 Example: in the recommended configuration of SHA-256 and 4K blocks,
388 128 hash values fit in each block. Thus, each level of the Merkle
389 tree is approximately 128 times smaller than the previous, and for
390 large files the Merkle tree's size converges to approximately 1/127 of
391 the original file size. However, for small files, the padding is
392 significant, making the space overhead proportionally more.
394 .. _fsverity_descriptor:
399 By itself, the Merkle tree root hash is ambiguous. For example, it
400 can't a distinguish a large file from a small second file whose data
401 is exactly the top-level hash block of the first file. Ambiguities
402 also arise from the convention of padding to the next block boundary.
404 To solve this problem, the fs-verity file digest is actually computed
405 as a hash of the following structure, which contains the Merkle tree
406 root hash as well as other fields such as the file size::
408 struct fsverity_descriptor {
409 __u8 version; /* must be 1 */
410 __u8 hash_algorithm; /* Merkle tree hash algorithm */
411 __u8 log_blocksize; /* log2 of size of data and tree blocks */
412 __u8 salt_size; /* size of salt in bytes; 0 if none */
413 __le32 __reserved_0x04; /* must be 0 */
414 __le64 data_size; /* size of file the Merkle tree is built over */
415 __u8 root_hash[64]; /* Merkle tree root hash */
416 __u8 salt[32]; /* salt prepended to each hashed block */
417 __u8 __reserved[144]; /* must be 0's */
420 Built-in signature verification
421 ===============================
423 With CONFIG_FS_VERITY_BUILTIN_SIGNATURES=y, fs-verity supports putting
424 a portion of an authentication policy (see `Use cases`_) in the
425 kernel. Specifically, it adds support for:
427 1. At fs-verity module initialization time, a keyring ".fs-verity" is
428 created. The root user can add trusted X.509 certificates to this
429 keyring using the add_key() system call, then (when done)
430 optionally use keyctl_restrict_keyring() to prevent additional
431 certificates from being added.
433 2. `FS_IOC_ENABLE_VERITY`_ accepts a pointer to a PKCS#7 formatted
434 detached signature in DER format of the file's fs-verity digest.
435 On success, this signature is persisted alongside the Merkle tree.
436 Then, any time the file is opened, the kernel will verify the
437 file's actual digest against this signature, using the certificates
438 in the ".fs-verity" keyring.
440 3. A new sysctl "fs.verity.require_signatures" is made available.
441 When set to 1, the kernel requires that all verity files have a
442 correctly signed digest as described in (2).
444 fs-verity file digests must be signed in the following format, which
445 is similar to the structure used by `FS_IOC_MEASURE_VERITY`_::
447 struct fsverity_formatted_digest {
448 char magic[8]; /* must be "FSVerity" */
449 __le16 digest_algorithm;
454 fs-verity's built-in signature verification support is meant as a
455 relatively simple mechanism that can be used to provide some level of
456 authenticity protection for verity files, as an alternative to doing
457 the signature verification in userspace or using IMA-appraisal.
458 However, with this mechanism, userspace programs still need to check
459 that the verity bit is set, and there is no protection against verity
460 files being swapped around.
465 fs-verity is currently supported by the ext4 and f2fs filesystems.
466 The CONFIG_FS_VERITY kconfig option must be enabled to use fs-verity
467 on either filesystem.
469 ``include/linux/fsverity.h`` declares the interface between the
470 ``fs/verity/`` support layer and filesystems. Briefly, filesystems
471 must provide an ``fsverity_operations`` structure that provides
472 methods to read and write the verity metadata to a filesystem-specific
473 location, including the Merkle tree blocks and
474 ``fsverity_descriptor``. Filesystems must also call functions in
475 ``fs/verity/`` at certain times, such as when a file is opened or when
476 pages have been read into the pagecache. (See `Verifying data`_.)
481 ext4 supports fs-verity since Linux v5.4 and e2fsprogs v1.45.2.
483 To create verity files on an ext4 filesystem, the filesystem must have
484 been formatted with ``-O verity`` or had ``tune2fs -O verity`` run on
485 it. "verity" is an RO_COMPAT filesystem feature, so once set, old
486 kernels will only be able to mount the filesystem readonly, and old
487 versions of e2fsck will be unable to check the filesystem. Moreover,
488 currently ext4 only supports mounting a filesystem with the "verity"
489 feature when its block size is equal to PAGE_SIZE (often 4096 bytes).
491 ext4 sets the EXT4_VERITY_FL on-disk inode flag on verity files. It
492 can only be set by `FS_IOC_ENABLE_VERITY`_, and it cannot be cleared.
494 ext4 also supports encryption, which can be used simultaneously with
495 fs-verity. In this case, the plaintext data is verified rather than
496 the ciphertext. This is necessary in order to make the fs-verity file
497 digest meaningful, since every file is encrypted differently.
499 ext4 stores the verity metadata (Merkle tree and fsverity_descriptor)
500 past the end of the file, starting at the first 64K boundary beyond
501 i_size. This approach works because (a) verity files are readonly,
502 and (b) pages fully beyond i_size aren't visible to userspace but can
503 be read/written internally by ext4 with only some relatively small
504 changes to ext4. This approach avoids having to depend on the
505 EA_INODE feature and on rearchitecturing ext4's xattr support to
506 support paging multi-gigabyte xattrs into memory, and to support
507 encrypting xattrs. Note that the verity metadata *must* be encrypted
508 when the file is, since it contains hashes of the plaintext data.
510 Currently, ext4 verity only supports the case where the Merkle tree
511 block size, filesystem block size, and page size are all the same. It
512 also only supports extent-based files.
517 f2fs supports fs-verity since Linux v5.4 and f2fs-tools v1.11.0.
519 To create verity files on an f2fs filesystem, the filesystem must have
520 been formatted with ``-O verity``.
522 f2fs sets the FADVISE_VERITY_BIT on-disk inode flag on verity files.
523 It can only be set by `FS_IOC_ENABLE_VERITY`_, and it cannot be
526 Like ext4, f2fs stores the verity metadata (Merkle tree and
527 fsverity_descriptor) past the end of the file, starting at the first
528 64K boundary beyond i_size. See explanation for ext4 above.
529 Moreover, f2fs supports at most 4096 bytes of xattr entries per inode
530 which wouldn't be enough for even a single Merkle tree block.
532 Currently, f2fs verity only supports a Merkle tree block size of 4096.
533 Also, f2fs doesn't support enabling verity on files that currently
534 have atomic or volatile writes pending.
536 Implementation details
537 ======================
542 fs-verity ensures that all reads of a verity file's data are verified,
543 regardless of which syscall is used to do the read (e.g. mmap(),
544 read(), pread()) and regardless of whether it's the first read or a
545 later read (unless the later read can return cached data that was
546 already verified). Below, we describe how filesystems implement this.
551 For filesystems using Linux's pagecache, the ``->readpage()`` and
552 ``->readpages()`` methods must be modified to verify pages before they
553 are marked Uptodate. Merely hooking ``->read_iter()`` would be
554 insufficient, since ``->read_iter()`` is not used for memory maps.
556 Therefore, fs/verity/ provides a function fsverity_verify_page() which
557 verifies a page that has been read into the pagecache of a verity
558 inode, but is still locked and not Uptodate, so it's not yet readable
559 by userspace. As needed to do the verification,
560 fsverity_verify_page() will call back into the filesystem to read
561 Merkle tree pages via fsverity_operations::read_merkle_tree_page().
563 fsverity_verify_page() returns false if verification failed; in this
564 case, the filesystem must not set the page Uptodate. Following this,
565 as per the usual Linux pagecache behavior, attempts by userspace to
566 read() from the part of the file containing the page will fail with
567 EIO, and accesses to the page within a memory map will raise SIGBUS.
569 fsverity_verify_page() currently only supports the case where the
570 Merkle tree block size is equal to PAGE_SIZE (often 4096 bytes).
572 In principle, fsverity_verify_page() verifies the entire path in the
573 Merkle tree from the data page to the root hash. However, for
574 efficiency the filesystem may cache the hash pages. Therefore,
575 fsverity_verify_page() only ascends the tree reading hash pages until
576 an already-verified hash page is seen, as indicated by the PageChecked
577 bit being set. It then verifies the path to that page.
579 This optimization, which is also used by dm-verity, results in
580 excellent sequential read performance. This is because usually (e.g.
581 127 in 128 times for 4K blocks and SHA-256) the hash page from the
582 bottom level of the tree will already be cached and checked from
583 reading a previous data page. However, random reads perform worse.
585 Block device based filesystems
586 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
588 Block device based filesystems (e.g. ext4 and f2fs) in Linux also use
589 the pagecache, so the above subsection applies too. However, they
590 also usually read many pages from a file at once, grouped into a
591 structure called a "bio". To make it easier for these types of
592 filesystems to support fs-verity, fs/verity/ also provides a function
593 fsverity_verify_bio() which verifies all pages in a bio.
595 ext4 and f2fs also support encryption. If a verity file is also
596 encrypted, the pages must be decrypted before being verified. To
597 support this, these filesystems allocate a "post-read context" for
598 each bio and store it in ``->bi_private``::
600 struct bio_post_read_ctx {
602 struct work_struct work;
603 unsigned int cur_step;
604 unsigned int enabled_steps;
607 ``enabled_steps`` is a bitmask that specifies whether decryption,
608 verity, or both is enabled. After the bio completes, for each needed
609 postprocessing step the filesystem enqueues the bio_post_read_ctx on a
610 workqueue, and then the workqueue work does the decryption or
611 verification. Finally, pages where no decryption or verity error
612 occurred are marked Uptodate, and the pages are unlocked.
614 Files on ext4 and f2fs may contain holes. Normally, ``->readpages()``
615 simply zeroes holes and sets the corresponding pages Uptodate; no bios
616 are issued. To prevent this case from bypassing fs-verity, these
617 filesystems use fsverity_verify_page() to verify hole pages.
619 ext4 and f2fs disable direct I/O on verity files, since otherwise
620 direct I/O would bypass fs-verity. (They also do the same for
626 This document focuses on the kernel, but a userspace utility for
627 fs-verity can be found at:
629 https://git.kernel.org/pub/scm/linux/kernel/git/ebiggers/fsverity-utils.git
631 See the README.md file in the fsverity-utils source tree for details,
632 including examples of setting up fs-verity protected files.
637 To test fs-verity, use xfstests. For example, using `kvm-xfstests
638 <https://github.com/tytso/xfstests-bld/blob/master/Documentation/kvm-quickstart.md>`_::
640 kvm-xfstests -c ext4,f2fs -g verity
645 This section answers frequently asked questions about fs-verity that
646 weren't already directly answered in other parts of this document.
648 :Q: Why isn't fs-verity part of IMA?
649 :A: fs-verity and IMA (Integrity Measurement Architecture) have
650 different focuses. fs-verity is a filesystem-level mechanism for
651 hashing individual files using a Merkle tree. In contrast, IMA
652 specifies a system-wide policy that specifies which files are
653 hashed and what to do with those hashes, such as log them,
654 authenticate them, or add them to a measurement list.
656 IMA is planned to support the fs-verity hashing mechanism as an
657 alternative to doing full file hashes, for people who want the
658 performance and security benefits of the Merkle tree based hash.
659 But it doesn't make sense to force all uses of fs-verity to be
660 through IMA. As a standalone filesystem feature, fs-verity
661 already meets many users' needs, and it's testable like other
662 filesystem features e.g. with xfstests.
664 :Q: Isn't fs-verity useless because the attacker can just modify the
665 hashes in the Merkle tree, which is stored on-disk?
666 :A: To verify the authenticity of an fs-verity file you must verify
667 the authenticity of the "fs-verity file digest", which
668 incorporates the root hash of the Merkle tree. See `Use cases`_.
670 :Q: Isn't fs-verity useless because the attacker can just replace a
671 verity file with a non-verity one?
672 :A: See `Use cases`_. In the initial use case, it's really trusted
673 userspace code that authenticates the files; fs-verity is just a
674 tool to do this job efficiently and securely. The trusted
675 userspace code will consider non-verity files to be inauthentic.
677 :Q: Why does the Merkle tree need to be stored on-disk? Couldn't you
678 store just the root hash?
679 :A: If the Merkle tree wasn't stored on-disk, then you'd have to
680 compute the entire tree when the file is first accessed, even if
681 just one byte is being read. This is a fundamental consequence of
682 how Merkle tree hashing works. To verify a leaf node, you need to
683 verify the whole path to the root hash, including the root node
684 (the thing which the root hash is a hash of). But if the root
685 node isn't stored on-disk, you have to compute it by hashing its
686 children, and so on until you've actually hashed the entire file.
688 That defeats most of the point of doing a Merkle tree-based hash,
689 since if you have to hash the whole file ahead of time anyway,
690 then you could simply do sha256(file) instead. That would be much
691 simpler, and a bit faster too.
693 It's true that an in-memory Merkle tree could still provide the
694 advantage of verification on every read rather than just on the
695 first read. However, it would be inefficient because every time a
696 hash page gets evicted (you can't pin the entire Merkle tree into
697 memory, since it may be very large), in order to restore it you
698 again need to hash everything below it in the tree. This again
699 defeats most of the point of doing a Merkle tree-based hash, since
700 a single block read could trigger re-hashing gigabytes of data.
702 :Q: But couldn't you store just the leaf nodes and compute the rest?
703 :A: See previous answer; this really just moves up one level, since
704 one could alternatively interpret the data blocks as being the
705 leaf nodes of the Merkle tree. It's true that the tree can be
706 computed much faster if the leaf level is stored rather than just
707 the data, but that's only because each level is less than 1% the
708 size of the level below (assuming the recommended settings of
709 SHA-256 and 4K blocks). For the exact same reason, by storing
710 "just the leaf nodes" you'd already be storing over 99% of the
711 tree, so you might as well simply store the whole tree.
713 :Q: Can the Merkle tree be built ahead of time, e.g. distributed as
714 part of a package that is installed to many computers?
715 :A: This isn't currently supported. It was part of the original
716 design, but was removed to simplify the kernel UAPI and because it
717 wasn't a critical use case. Files are usually installed once and
718 used many times, and cryptographic hashing is somewhat fast on
719 most modern processors.
721 :Q: Why doesn't fs-verity support writes?
722 :A: Write support would be very difficult and would require a
723 completely different design, so it's well outside the scope of
724 fs-verity. Write support would require:
726 - A way to maintain consistency between the data and hashes,
727 including all levels of hashes, since corruption after a crash
728 (especially of potentially the entire file!) is unacceptable.
729 The main options for solving this are data journalling,
730 copy-on-write, and log-structured volume. But it's very hard to
731 retrofit existing filesystems with new consistency mechanisms.
732 Data journalling is available on ext4, but is very slow.
734 - Rebuilding the Merkle tree after every write, which would be
735 extremely inefficient. Alternatively, a different authenticated
736 dictionary structure such as an "authenticated skiplist" could
737 be used. However, this would be far more complex.
739 Compare it to dm-verity vs. dm-integrity. dm-verity is very
740 simple: the kernel just verifies read-only data against a
741 read-only Merkle tree. In contrast, dm-integrity supports writes
742 but is slow, is much more complex, and doesn't actually support
743 full-device authentication since it authenticates each sector
744 independently, i.e. there is no "root hash". It doesn't really
745 make sense for the same device-mapper target to support these two
746 very different cases; the same applies to fs-verity.
748 :Q: Since verity files are immutable, why isn't the immutable bit set?
749 :A: The existing "immutable" bit (FS_IMMUTABLE_FL) already has a
750 specific set of semantics which not only make the file contents
751 read-only, but also prevent the file from being deleted, renamed,
752 linked to, or having its owner or mode changed. These extra
753 properties are unwanted for fs-verity, so reusing the immutable
754 bit isn't appropriate.
756 :Q: Why does the API use ioctls instead of setxattr() and getxattr()?
757 :A: Abusing the xattr interface for basically arbitrary syscalls is
758 heavily frowned upon by most of the Linux filesystem developers.
759 An xattr should really just be an xattr on-disk, not an API to
760 e.g. magically trigger construction of a Merkle tree.
762 :Q: Does fs-verity support remote filesystems?
763 :A: Only ext4 and f2fs support is implemented currently, but in
764 principle any filesystem that can store per-file verity metadata
765 can support fs-verity, regardless of whether it's local or remote.
766 Some filesystems may have fewer options of where to store the
767 verity metadata; one possibility is to store it past the end of
768 the file and "hide" it from userspace by manipulating i_size. The
769 data verification functions provided by ``fs/verity/`` also assume
770 that the filesystem uses the Linux pagecache, but both local and
771 remote filesystems normally do so.
773 :Q: Why is anything filesystem-specific at all? Shouldn't fs-verity
774 be implemented entirely at the VFS level?
775 :A: There are many reasons why this is not possible or would be very
776 difficult, including the following:
778 - To prevent bypassing verification, pages must not be marked
779 Uptodate until they've been verified. Currently, each
780 filesystem is responsible for marking pages Uptodate via
781 ``->readpages()``. Therefore, currently it's not possible for
782 the VFS to do the verification on its own. Changing this would
783 require significant changes to the VFS and all filesystems.
785 - It would require defining a filesystem-independent way to store
786 the verity metadata. Extended attributes don't work for this
787 because (a) the Merkle tree may be gigabytes, but many
788 filesystems assume that all xattrs fit into a single 4K
789 filesystem block, and (b) ext4 and f2fs encryption doesn't
790 encrypt xattrs, yet the Merkle tree *must* be encrypted when the
791 file contents are, because it stores hashes of the plaintext
794 So the verity metadata would have to be stored in an actual
795 file. Using a separate file would be very ugly, since the
796 metadata is fundamentally part of the file to be protected, and
797 it could cause problems where users could delete the real file
798 but not the metadata file or vice versa. On the other hand,
799 having it be in the same file would break applications unless
800 filesystems' notion of i_size were divorced from the VFS's,
801 which would be complex and require changes to all filesystems.
803 - It's desirable that FS_IOC_ENABLE_VERITY uses the filesystem's
804 transaction mechanism so that either the file ends up with
805 verity enabled, or no changes were made. Allowing intermediate
806 states to occur after a crash may cause problems.
projects / linux-2.6-microblaze.git / blob