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 "file measurement", which is a hash that includes the root hash)
31 that fs-verity is enforcing for the file. This ioctl executes in
32 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 :c:type:`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 measurement of a verity
181 file. The file measurement is a digest that cryptographically
182 identifies the file contents that are being enforced on reads.
184 This ioctl takes in a pointer to a variable-length structure::
186 struct fsverity_digest {
187 __u16 digest_algorithm;
188 __u16 digest_size; /* input/output */
192 ``digest_size`` is an input/output field. On input, it must be
193 initialized to the number of bytes allocated for the variable-length
196 On success, 0 is returned and the kernel fills in the structure as
199 - ``digest_algorithm`` will be the hash algorithm used for the file
200 measurement. It will match ``fsverity_enable_arg::hash_algorithm``.
201 - ``digest_size`` will be the size of the digest in bytes, e.g. 32
202 for SHA-256. (This can be redundant with ``digest_algorithm``.)
203 - ``digest`` will be the actual bytes of the digest.
205 FS_IOC_MEASURE_VERITY is guaranteed to execute in constant time,
206 regardless of the size of the file.
208 FS_IOC_MEASURE_VERITY can fail with the following errors:
210 - ``EFAULT``: the caller provided inaccessible memory
211 - ``ENODATA``: the file is not a verity file
212 - ``ENOTTY``: this type of filesystem does not implement fs-verity
213 - ``EOPNOTSUPP``: the kernel was not configured with fs-verity
214 support, or the filesystem superblock has not had the 'verity'
215 feature enabled on it. (See `Filesystem support`_.)
216 - ``EOVERFLOW``: the digest is longer than the specified
217 ``digest_size`` bytes. Try providing a larger buffer.
222 The existing ioctl FS_IOC_GETFLAGS (which isn't specific to fs-verity)
223 can also be used to check whether a file has fs-verity enabled or not.
224 To do so, check for FS_VERITY_FL (0x00100000) in the returned flags.
226 The verity flag is not settable via FS_IOC_SETFLAGS. You must use
227 FS_IOC_ENABLE_VERITY instead, since parameters must be provided.
232 Since Linux v5.5, the statx() system call sets STATX_ATTR_VERITY if
233 the file has fs-verity enabled. This can perform better than
234 FS_IOC_GETFLAGS and FS_IOC_MEASURE_VERITY because it doesn't require
235 opening the file, and opening verity files can be expensive.
237 Accessing verity files
238 ======================
240 Applications can transparently access a verity file just like a
241 non-verity one, with the following exceptions:
243 - Verity files are readonly. They cannot be opened for writing or
244 truncate()d, even if the file mode bits allow it. Attempts to do
245 one of these things will fail with EPERM. However, changes to
246 metadata such as owner, mode, timestamps, and xattrs are still
247 allowed, since these are not measured by fs-verity. Verity files
248 can also still be renamed, deleted, and linked to.
250 - Direct I/O is not supported on verity files. Attempts to use direct
251 I/O on such files will fall back to buffered I/O.
253 - DAX (Direct Access) is not supported on verity files, because this
254 would circumvent the data verification.
256 - Reads of data that doesn't match the verity Merkle tree will fail
257 with EIO (for read()) or SIGBUS (for mmap() reads).
259 - If the sysctl "fs.verity.require_signatures" is set to 1 and the
260 file's verity measurement is not signed by a key in the fs-verity
261 keyring, then opening the file will fail. See `Built-in signature
264 Direct access to the Merkle tree is not supported. Therefore, if a
265 verity file is copied, or is backed up and restored, then it will lose
266 its "verity"-ness. fs-verity is primarily meant for files like
267 executables that are managed by a package manager.
269 File measurement computation
270 ============================
272 This section describes how fs-verity hashes the file contents using a
273 Merkle tree to produce the "file measurement" which cryptographically
274 identifies the file contents. This algorithm is the same for all
275 filesystems that support fs-verity.
277 Userspace only needs to be aware of this algorithm if it needs to
278 compute the file measurement itself, e.g. in order to sign the file.
280 .. _fsverity_merkle_tree:
285 The file contents is divided into blocks, where the block size is
286 configurable but is usually 4096 bytes. The end of the last block is
287 zero-padded if needed. Each block is then hashed, producing the first
288 level of hashes. Then, the hashes in this first level are grouped
289 into 'blocksize'-byte blocks (zero-padding the ends as needed) and
290 these blocks are hashed, producing the second level of hashes. This
291 proceeds up the tree until only a single block remains. The hash of
292 this block is the "Merkle tree root hash".
294 If the file fits in one block and is nonempty, then the "Merkle tree
295 root hash" is simply the hash of the single data block. If the file
296 is empty, then the "Merkle tree root hash" is all zeroes.
298 The "blocks" here are not necessarily the same as "filesystem blocks".
300 If a salt was specified, then it's zero-padded to the closest multiple
301 of the input size of the hash algorithm's compression function, e.g.
302 64 bytes for SHA-256 or 128 bytes for SHA-512. The padded salt is
303 prepended to every data or Merkle tree block that is hashed.
305 The purpose of the block padding is to cause every hash to be taken
306 over the same amount of data, which simplifies the implementation and
307 keeps open more possibilities for hardware acceleration. The purpose
308 of the salt padding is to make the salting "free" when the salted hash
309 state is precomputed, then imported for each hash.
311 Example: in the recommended configuration of SHA-256 and 4K blocks,
312 128 hash values fit in each block. Thus, each level of the Merkle
313 tree is approximately 128 times smaller than the previous, and for
314 large files the Merkle tree's size converges to approximately 1/127 of
315 the original file size. However, for small files, the padding is
316 significant, making the space overhead proportionally more.
318 .. _fsverity_descriptor:
323 By itself, the Merkle tree root hash is ambiguous. For example, it
324 can't a distinguish a large file from a small second file whose data
325 is exactly the top-level hash block of the first file. Ambiguities
326 also arise from the convention of padding to the next block boundary.
328 To solve this problem, the verity file measurement is actually
329 computed as a hash of the following structure, which contains the
330 Merkle tree root hash as well as other fields such as the file size::
332 struct fsverity_descriptor {
333 __u8 version; /* must be 1 */
334 __u8 hash_algorithm; /* Merkle tree hash algorithm */
335 __u8 log_blocksize; /* log2 of size of data and tree blocks */
336 __u8 salt_size; /* size of salt in bytes; 0 if none */
337 __le32 sig_size; /* must be 0 */
338 __le64 data_size; /* size of file the Merkle tree is built over */
339 __u8 root_hash[64]; /* Merkle tree root hash */
340 __u8 salt[32]; /* salt prepended to each hashed block */
341 __u8 __reserved[144]; /* must be 0's */
344 Note that the ``sig_size`` field must be set to 0 for the purpose of
345 computing the file measurement, even if a signature was provided (or
346 will be provided) to `FS_IOC_ENABLE_VERITY`_.
348 Built-in signature verification
349 ===============================
351 With CONFIG_FS_VERITY_BUILTIN_SIGNATURES=y, fs-verity supports putting
352 a portion of an authentication policy (see `Use cases`_) in the
353 kernel. Specifically, it adds support for:
355 1. At fs-verity module initialization time, a keyring ".fs-verity" is
356 created. The root user can add trusted X.509 certificates to this
357 keyring using the add_key() system call, then (when done)
358 optionally use keyctl_restrict_keyring() to prevent additional
359 certificates from being added.
361 2. `FS_IOC_ENABLE_VERITY`_ accepts a pointer to a PKCS#7 formatted
362 detached signature in DER format of the file measurement. On
363 success, this signature is persisted alongside the Merkle tree.
364 Then, any time the file is opened, the kernel will verify the
365 file's actual measurement against this signature, using the
366 certificates in the ".fs-verity" keyring.
368 3. A new sysctl "fs.verity.require_signatures" is made available.
369 When set to 1, the kernel requires that all verity files have a
370 correctly signed file measurement as described in (2).
372 File measurements must be signed in the following format, which is
373 similar to the structure used by `FS_IOC_MEASURE_VERITY`_::
375 struct fsverity_signed_digest {
376 char magic[8]; /* must be "FSVerity" */
377 __le16 digest_algorithm;
382 fs-verity's built-in signature verification support is meant as a
383 relatively simple mechanism that can be used to provide some level of
384 authenticity protection for verity files, as an alternative to doing
385 the signature verification in userspace or using IMA-appraisal.
386 However, with this mechanism, userspace programs still need to check
387 that the verity bit is set, and there is no protection against verity
388 files being swapped around.
393 fs-verity is currently supported by the ext4 and f2fs filesystems.
394 The CONFIG_FS_VERITY kconfig option must be enabled to use fs-verity
395 on either filesystem.
397 ``include/linux/fsverity.h`` declares the interface between the
398 ``fs/verity/`` support layer and filesystems. Briefly, filesystems
399 must provide an ``fsverity_operations`` structure that provides
400 methods to read and write the verity metadata to a filesystem-specific
401 location, including the Merkle tree blocks and
402 ``fsverity_descriptor``. Filesystems must also call functions in
403 ``fs/verity/`` at certain times, such as when a file is opened or when
404 pages have been read into the pagecache. (See `Verifying data`_.)
409 ext4 supports fs-verity since Linux v5.4 and e2fsprogs v1.45.2.
411 To create verity files on an ext4 filesystem, the filesystem must have
412 been formatted with ``-O verity`` or had ``tune2fs -O verity`` run on
413 it. "verity" is an RO_COMPAT filesystem feature, so once set, old
414 kernels will only be able to mount the filesystem readonly, and old
415 versions of e2fsck will be unable to check the filesystem. Moreover,
416 currently ext4 only supports mounting a filesystem with the "verity"
417 feature when its block size is equal to PAGE_SIZE (often 4096 bytes).
419 ext4 sets the EXT4_VERITY_FL on-disk inode flag on verity files. It
420 can only be set by `FS_IOC_ENABLE_VERITY`_, and it cannot be cleared.
422 ext4 also supports encryption, which can be used simultaneously with
423 fs-verity. In this case, the plaintext data is verified rather than
424 the ciphertext. This is necessary in order to make the file
425 measurement meaningful, since every file is encrypted differently.
427 ext4 stores the verity metadata (Merkle tree and fsverity_descriptor)
428 past the end of the file, starting at the first 64K boundary beyond
429 i_size. This approach works because (a) verity files are readonly,
430 and (b) pages fully beyond i_size aren't visible to userspace but can
431 be read/written internally by ext4 with only some relatively small
432 changes to ext4. This approach avoids having to depend on the
433 EA_INODE feature and on rearchitecturing ext4's xattr support to
434 support paging multi-gigabyte xattrs into memory, and to support
435 encrypting xattrs. Note that the verity metadata *must* be encrypted
436 when the file is, since it contains hashes of the plaintext data.
438 Currently, ext4 verity only supports the case where the Merkle tree
439 block size, filesystem block size, and page size are all the same. It
440 also only supports extent-based files.
445 f2fs supports fs-verity since Linux v5.4 and f2fs-tools v1.11.0.
447 To create verity files on an f2fs filesystem, the filesystem must have
448 been formatted with ``-O verity``.
450 f2fs sets the FADVISE_VERITY_BIT on-disk inode flag on verity files.
451 It can only be set by `FS_IOC_ENABLE_VERITY`_, and it cannot be
454 Like ext4, f2fs stores the verity metadata (Merkle tree and
455 fsverity_descriptor) past the end of the file, starting at the first
456 64K boundary beyond i_size. See explanation for ext4 above.
457 Moreover, f2fs supports at most 4096 bytes of xattr entries per inode
458 which wouldn't be enough for even a single Merkle tree block.
460 Currently, f2fs verity only supports a Merkle tree block size of 4096.
461 Also, f2fs doesn't support enabling verity on files that currently
462 have atomic or volatile writes pending.
464 Implementation details
465 ======================
470 fs-verity ensures that all reads of a verity file's data are verified,
471 regardless of which syscall is used to do the read (e.g. mmap(),
472 read(), pread()) and regardless of whether it's the first read or a
473 later read (unless the later read can return cached data that was
474 already verified). Below, we describe how filesystems implement this.
479 For filesystems using Linux's pagecache, the ``->readpage()`` and
480 ``->readpages()`` methods must be modified to verify pages before they
481 are marked Uptodate. Merely hooking ``->read_iter()`` would be
482 insufficient, since ``->read_iter()`` is not used for memory maps.
484 Therefore, fs/verity/ provides a function fsverity_verify_page() which
485 verifies a page that has been read into the pagecache of a verity
486 inode, but is still locked and not Uptodate, so it's not yet readable
487 by userspace. As needed to do the verification,
488 fsverity_verify_page() will call back into the filesystem to read
489 Merkle tree pages via fsverity_operations::read_merkle_tree_page().
491 fsverity_verify_page() returns false if verification failed; in this
492 case, the filesystem must not set the page Uptodate. Following this,
493 as per the usual Linux pagecache behavior, attempts by userspace to
494 read() from the part of the file containing the page will fail with
495 EIO, and accesses to the page within a memory map will raise SIGBUS.
497 fsverity_verify_page() currently only supports the case where the
498 Merkle tree block size is equal to PAGE_SIZE (often 4096 bytes).
500 In principle, fsverity_verify_page() verifies the entire path in the
501 Merkle tree from the data page to the root hash. However, for
502 efficiency the filesystem may cache the hash pages. Therefore,
503 fsverity_verify_page() only ascends the tree reading hash pages until
504 an already-verified hash page is seen, as indicated by the PageChecked
505 bit being set. It then verifies the path to that page.
507 This optimization, which is also used by dm-verity, results in
508 excellent sequential read performance. This is because usually (e.g.
509 127 in 128 times for 4K blocks and SHA-256) the hash page from the
510 bottom level of the tree will already be cached and checked from
511 reading a previous data page. However, random reads perform worse.
513 Block device based filesystems
514 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
516 Block device based filesystems (e.g. ext4 and f2fs) in Linux also use
517 the pagecache, so the above subsection applies too. However, they
518 also usually read many pages from a file at once, grouped into a
519 structure called a "bio". To make it easier for these types of
520 filesystems to support fs-verity, fs/verity/ also provides a function
521 fsverity_verify_bio() which verifies all pages in a bio.
523 ext4 and f2fs also support encryption. If a verity file is also
524 encrypted, the pages must be decrypted before being verified. To
525 support this, these filesystems allocate a "post-read context" for
526 each bio and store it in ``->bi_private``::
528 struct bio_post_read_ctx {
530 struct work_struct work;
531 unsigned int cur_step;
532 unsigned int enabled_steps;
535 ``enabled_steps`` is a bitmask that specifies whether decryption,
536 verity, or both is enabled. After the bio completes, for each needed
537 postprocessing step the filesystem enqueues the bio_post_read_ctx on a
538 workqueue, and then the workqueue work does the decryption or
539 verification. Finally, pages where no decryption or verity error
540 occurred are marked Uptodate, and the pages are unlocked.
542 Files on ext4 and f2fs may contain holes. Normally, ``->readpages()``
543 simply zeroes holes and sets the corresponding pages Uptodate; no bios
544 are issued. To prevent this case from bypassing fs-verity, these
545 filesystems use fsverity_verify_page() to verify hole pages.
547 ext4 and f2fs disable direct I/O on verity files, since otherwise
548 direct I/O would bypass fs-verity. (They also do the same for
554 This document focuses on the kernel, but a userspace utility for
555 fs-verity can be found at:
557 https://git.kernel.org/pub/scm/linux/kernel/git/ebiggers/fsverity-utils.git
559 See the README.md file in the fsverity-utils source tree for details,
560 including examples of setting up fs-verity protected files.
565 To test fs-verity, use xfstests. For example, using `kvm-xfstests
566 <https://github.com/tytso/xfstests-bld/blob/master/Documentation/kvm-quickstart.md>`_::
568 kvm-xfstests -c ext4,f2fs -g verity
573 This section answers frequently asked questions about fs-verity that
574 weren't already directly answered in other parts of this document.
576 :Q: Why isn't fs-verity part of IMA?
577 :A: fs-verity and IMA (Integrity Measurement Architecture) have
578 different focuses. fs-verity is a filesystem-level mechanism for
579 hashing individual files using a Merkle tree. In contrast, IMA
580 specifies a system-wide policy that specifies which files are
581 hashed and what to do with those hashes, such as log them,
582 authenticate them, or add them to a measurement list.
584 IMA is planned to support the fs-verity hashing mechanism as an
585 alternative to doing full file hashes, for people who want the
586 performance and security benefits of the Merkle tree based hash.
587 But it doesn't make sense to force all uses of fs-verity to be
588 through IMA. As a standalone filesystem feature, fs-verity
589 already meets many users' needs, and it's testable like other
590 filesystem features e.g. with xfstests.
592 :Q: Isn't fs-verity useless because the attacker can just modify the
593 hashes in the Merkle tree, which is stored on-disk?
594 :A: To verify the authenticity of an fs-verity file you must verify
595 the authenticity of the "file measurement", which is basically the
596 root hash of the Merkle tree. See `Use cases`_.
598 :Q: Isn't fs-verity useless because the attacker can just replace a
599 verity file with a non-verity one?
600 :A: See `Use cases`_. In the initial use case, it's really trusted
601 userspace code that authenticates the files; fs-verity is just a
602 tool to do this job efficiently and securely. The trusted
603 userspace code will consider non-verity files to be inauthentic.
605 :Q: Why does the Merkle tree need to be stored on-disk? Couldn't you
606 store just the root hash?
607 :A: If the Merkle tree wasn't stored on-disk, then you'd have to
608 compute the entire tree when the file is first accessed, even if
609 just one byte is being read. This is a fundamental consequence of
610 how Merkle tree hashing works. To verify a leaf node, you need to
611 verify the whole path to the root hash, including the root node
612 (the thing which the root hash is a hash of). But if the root
613 node isn't stored on-disk, you have to compute it by hashing its
614 children, and so on until you've actually hashed the entire file.
616 That defeats most of the point of doing a Merkle tree-based hash,
617 since if you have to hash the whole file ahead of time anyway,
618 then you could simply do sha256(file) instead. That would be much
619 simpler, and a bit faster too.
621 It's true that an in-memory Merkle tree could still provide the
622 advantage of verification on every read rather than just on the
623 first read. However, it would be inefficient because every time a
624 hash page gets evicted (you can't pin the entire Merkle tree into
625 memory, since it may be very large), in order to restore it you
626 again need to hash everything below it in the tree. This again
627 defeats most of the point of doing a Merkle tree-based hash, since
628 a single block read could trigger re-hashing gigabytes of data.
630 :Q: But couldn't you store just the leaf nodes and compute the rest?
631 :A: See previous answer; this really just moves up one level, since
632 one could alternatively interpret the data blocks as being the
633 leaf nodes of the Merkle tree. It's true that the tree can be
634 computed much faster if the leaf level is stored rather than just
635 the data, but that's only because each level is less than 1% the
636 size of the level below (assuming the recommended settings of
637 SHA-256 and 4K blocks). For the exact same reason, by storing
638 "just the leaf nodes" you'd already be storing over 99% of the
639 tree, so you might as well simply store the whole tree.
641 :Q: Can the Merkle tree be built ahead of time, e.g. distributed as
642 part of a package that is installed to many computers?
643 :A: This isn't currently supported. It was part of the original
644 design, but was removed to simplify the kernel UAPI and because it
645 wasn't a critical use case. Files are usually installed once and
646 used many times, and cryptographic hashing is somewhat fast on
647 most modern processors.
649 :Q: Why doesn't fs-verity support writes?
650 :A: Write support would be very difficult and would require a
651 completely different design, so it's well outside the scope of
652 fs-verity. Write support would require:
654 - A way to maintain consistency between the data and hashes,
655 including all levels of hashes, since corruption after a crash
656 (especially of potentially the entire file!) is unacceptable.
657 The main options for solving this are data journalling,
658 copy-on-write, and log-structured volume. But it's very hard to
659 retrofit existing filesystems with new consistency mechanisms.
660 Data journalling is available on ext4, but is very slow.
662 - Rebuilding the the Merkle tree after every write, which would be
663 extremely inefficient. Alternatively, a different authenticated
664 dictionary structure such as an "authenticated skiplist" could
665 be used. However, this would be far more complex.
667 Compare it to dm-verity vs. dm-integrity. dm-verity is very
668 simple: the kernel just verifies read-only data against a
669 read-only Merkle tree. In contrast, dm-integrity supports writes
670 but is slow, is much more complex, and doesn't actually support
671 full-device authentication since it authenticates each sector
672 independently, i.e. there is no "root hash". It doesn't really
673 make sense for the same device-mapper target to support these two
674 very different cases; the same applies to fs-verity.
676 :Q: Since verity files are immutable, why isn't the immutable bit set?
677 :A: The existing "immutable" bit (FS_IMMUTABLE_FL) already has a
678 specific set of semantics which not only make the file contents
679 read-only, but also prevent the file from being deleted, renamed,
680 linked to, or having its owner or mode changed. These extra
681 properties are unwanted for fs-verity, so reusing the immutable
682 bit isn't appropriate.
684 :Q: Why does the API use ioctls instead of setxattr() and getxattr()?
685 :A: Abusing the xattr interface for basically arbitrary syscalls is
686 heavily frowned upon by most of the Linux filesystem developers.
687 An xattr should really just be an xattr on-disk, not an API to
688 e.g. magically trigger construction of a Merkle tree.
690 :Q: Does fs-verity support remote filesystems?
691 :A: Only ext4 and f2fs support is implemented currently, but in
692 principle any filesystem that can store per-file verity metadata
693 can support fs-verity, regardless of whether it's local or remote.
694 Some filesystems may have fewer options of where to store the
695 verity metadata; one possibility is to store it past the end of
696 the file and "hide" it from userspace by manipulating i_size. The
697 data verification functions provided by ``fs/verity/`` also assume
698 that the filesystem uses the Linux pagecache, but both local and
699 remote filesystems normally do so.
701 :Q: Why is anything filesystem-specific at all? Shouldn't fs-verity
702 be implemented entirely at the VFS level?
703 :A: There are many reasons why this is not possible or would be very
704 difficult, including the following:
706 - To prevent bypassing verification, pages must not be marked
707 Uptodate until they've been verified. Currently, each
708 filesystem is responsible for marking pages Uptodate via
709 ``->readpages()``. Therefore, currently it's not possible for
710 the VFS to do the verification on its own. Changing this would
711 require significant changes to the VFS and all filesystems.
713 - It would require defining a filesystem-independent way to store
714 the verity metadata. Extended attributes don't work for this
715 because (a) the Merkle tree may be gigabytes, but many
716 filesystems assume that all xattrs fit into a single 4K
717 filesystem block, and (b) ext4 and f2fs encryption doesn't
718 encrypt xattrs, yet the Merkle tree *must* be encrypted when the
719 file contents are, because it stores hashes of the plaintext
722 So the verity metadata would have to be stored in an actual
723 file. Using a separate file would be very ugly, since the
724 metadata is fundamentally part of the file to be protected, and
725 it could cause problems where users could delete the real file
726 but not the metadata file or vice versa. On the other hand,
727 having it be in the same file would break applications unless
728 filesystems' notion of i_size were divorced from the VFS's,
729 which would be complex and require changes to all filesystems.
731 - It's desirable that FS_IOC_ENABLE_VERITY uses the filesystem's
732 transaction mechanism so that either the file ends up with
733 verity enabled, or no changes were made. Allowing intermediate
734 states to occur after a crash may cause problems.
projects / mirror_ubuntu-hirsute-kernel.git / blob