1.. SPDX-License-Identifier: GPL-2.0 2 3.. _fsverity: 4 5======================================================= 6fs-verity: read-only file-based authenticity protection 7======================================================= 8 9Introduction 10============ 11 12fs-verity (``fs/verity/``) is a support layer that filesystems can 13hook into to support transparent integrity and authenticity protection 14of read-only files. Currently, it is supported by the ext4 and f2fs 15filesystems. Like fscrypt, not too much filesystem-specific code is 16needed to support fs-verity. 17 18fs-verity is similar to `dm-verity 19<https://www.kernel.org/doc/Documentation/device-mapper/verity.txt>`_ 20but works on files rather than block devices. On regular files on 21filesystems supporting fs-verity, userspace can execute an ioctl that 22causes the filesystem to build a Merkle tree for the file and persist 23it to a filesystem-specific location associated with the file. 24 25After this, the file is made readonly, and all reads from the file are 26automatically verified against the file's Merkle tree. Reads of any 27corrupted data, including mmap reads, will fail. 28 29Userspace can use another ioctl to retrieve the root hash (actually 30the "fs-verity file digest", which is a hash that includes the Merkle 31tree root hash) that fs-verity is enforcing for the file. This ioctl 32executes in constant time, regardless of the file size. 33 34fs-verity is essentially a way to hash a file in constant time, 35subject to the caveat that reads which would violate the hash will 36fail at runtime. 37 38Use cases 39========= 40 41By itself, the base fs-verity feature only provides integrity 42protection, i.e. detection of accidental (non-malicious) corruption. 43 44However, because fs-verity makes retrieving the file hash extremely 45efficient, it's primarily meant to be used as a tool to support 46authentication (detection of malicious modifications) or auditing 47(logging file hashes before use). 48 49Trusted userspace code (e.g. operating system code running on a 50read-only partition that is itself authenticated by dm-verity) can 51authenticate the contents of an fs-verity file by using the 52`FS_IOC_MEASURE_VERITY`_ ioctl to retrieve its hash, then verifying a 53digital signature of it. 54 55A standard file hash could be used instead of fs-verity. However, 56this is inefficient if the file is large and only a small portion may 57be accessed. This is often the case for Android application package 58(APK) files, for example. These typically contain many translations, 59classes, and other resources that are infrequently or even never 60accessed on a particular device. It would be slow and wasteful to 61read and hash the entire file before starting the application. 62 63Unlike an ahead-of-time hash, fs-verity also re-verifies data each 64time it's paged in. This ensures that malicious disk firmware can't 65undetectably change the contents of the file at runtime. 66 67fs-verity does not replace or obsolete dm-verity. dm-verity should 68still be used on read-only filesystems. fs-verity is for files that 69must live on a read-write filesystem because they are independently 70updated and potentially user-installed, so dm-verity cannot be used. 71 72The base fs-verity feature is a hashing mechanism only; actually 73authenticating the files is up to userspace. However, to meet some 74users' needs, fs-verity optionally supports a simple signature 75verification mechanism where users can configure the kernel to require 76that 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 78in IMA (Integrity Measurement Architecture) policies is also planned. 79 80User API 81======== 82 83FS_IOC_ENABLE_VERITY 84-------------------- 85 86The FS_IOC_ENABLE_VERITY ioctl enables fs-verity on a file. It takes 87in a pointer to a struct fsverity_enable_arg, defined as 88follows:: 89 90 struct fsverity_enable_arg { 91 __u32 version; 92 __u32 hash_algorithm; 93 __u32 block_size; 94 __u32 salt_size; 95 __u64 salt_ptr; 96 __u32 sig_size; 97 __u32 __reserved1; 98 __u64 sig_ptr; 99 __u64 __reserved2[11]; 100 }; 101 102This structure contains the parameters of the Merkle tree to build for 103the file, and optionally contains a signature. It must be initialized 104as follows: 105 106- ``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 119 provided. 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. 127 128FS_IOC_ENABLE_VERITY causes the filesystem to build a Merkle tree for 129the file and persist it to a filesystem-specific location associated 130with the file, then mark the file as a verity file. This ioctl may 131take a long time to execute on large files, and it is interruptible by 132fatal signals. 133 134FS_IOC_ENABLE_VERITY checks for write access to the inode. However, 135it must be executed on an O_RDONLY file descriptor and no processes 136can have the file open for writing. Attempts to open the file for 137writing while this ioctl is executing will fail with ETXTBSY. (This 138is necessary to guarantee that no writable file descriptors will exist 139after verity is enabled, and to guarantee that the file's contents are 140stable while the Merkle tree is being built over it.) 141 142On success, FS_IOC_ENABLE_VERITY returns 0, and the file becomes a 143verity file. On failure (including the case of interruption by a 144fatal signal), no changes are made to the file. 145 146FS_IOC_ENABLE_VERITY can fail with the following errors: 147 148- ``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. 176 177FS_IOC_MEASURE_VERITY 178--------------------- 179 180The FS_IOC_MEASURE_VERITY ioctl retrieves the digest of a verity file. 181The fs-verity file digest is a cryptographic digest that identifies 182the file contents that are being enforced on reads; it is computed via 183a Merkle tree and is different from a traditional full-file digest. 184 185This ioctl takes in a pointer to a variable-length structure:: 186 187 struct fsverity_digest { 188 __u16 digest_algorithm; 189 __u16 digest_size; /* input/output */ 190 __u8 digest[]; 191 }; 192 193``digest_size`` is an input/output field. On input, it must be 194initialized to the number of bytes allocated for the variable-length 195``digest`` field. 196 197On success, 0 is returned and the kernel fills in the structure as 198follows: 199 200- ``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. 205 206FS_IOC_MEASURE_VERITY is guaranteed to execute in constant time, 207regardless of the size of the file. 208 209FS_IOC_MEASURE_VERITY can fail with the following errors: 210 211- ``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. 219 220FS_IOC_GETFLAGS 221--------------- 222 223The existing ioctl FS_IOC_GETFLAGS (which isn't specific to fs-verity) 224can also be used to check whether a file has fs-verity enabled or not. 225To do so, check for FS_VERITY_FL (0x00100000) in the returned flags. 226 227The verity flag is not settable via FS_IOC_SETFLAGS. You must use 228FS_IOC_ENABLE_VERITY instead, since parameters must be provided. 229 230statx 231----- 232 233Since Linux v5.5, the statx() system call sets STATX_ATTR_VERITY if 234the file has fs-verity enabled. This can perform better than 235FS_IOC_GETFLAGS and FS_IOC_MEASURE_VERITY because it doesn't require 236opening the file, and opening verity files can be expensive. 237 238Accessing verity files 239====================== 240 241Applications can transparently access a verity file just like a 242non-verity one, with the following exceptions: 243 244- Verity files are readonly. They cannot be opened for writing or 245 truncate()d, even if the file mode bits allow it. Attempts to do 246 one of these things will fail with EPERM. However, changes to 247 metadata such as owner, mode, timestamps, and xattrs are still 248 allowed, since these are not measured by fs-verity. Verity files 249 can also still be renamed, deleted, and linked to. 250 251- Direct I/O is not supported on verity files. Attempts to use direct 252 I/O on such files will fall back to buffered I/O. 253 254- DAX (Direct Access) is not supported on verity files, because this 255 would circumvent the data verification. 256 257- Reads of data that doesn't match the verity Merkle tree will fail 258 with EIO (for read()) or SIGBUS (for mmap() reads). 259 260- If the sysctl "fs.verity.require_signatures" is set to 1 and the 261 file is not signed by a key in the fs-verity keyring, then opening 262 the file will fail. See `Built-in signature verification`_. 263 264Direct access to the Merkle tree is not supported. Therefore, if a 265verity file is copied, or is backed up and restored, then it will lose 266its "verity"-ness. fs-verity is primarily meant for files like 267executables that are managed by a package manager. 268 269File digest computation 270======================= 271 272This section describes how fs-verity hashes the file contents using a 273Merkle tree to produce the digest which cryptographically identifies 274the file contents. This algorithm is the same for all filesystems 275that support fs-verity. 276 277Userspace only needs to be aware of this algorithm if it needs to 278compute fs-verity file digests itself, e.g. in order to sign files. 279 280.. _fsverity_merkle_tree: 281 282Merkle tree 283----------- 284 285The file contents is divided into blocks, where the block size is 286configurable but is usually 4096 bytes. The end of the last block is 287zero-padded if needed. Each block is then hashed, producing the first 288level of hashes. Then, the hashes in this first level are grouped 289into 'blocksize'-byte blocks (zero-padding the ends as needed) and 290these blocks are hashed, producing the second level of hashes. This 291proceeds up the tree until only a single block remains. The hash of 292this block is the "Merkle tree root hash". 293 294If the file fits in one block and is nonempty, then the "Merkle tree 295root hash" is simply the hash of the single data block. If the file 296is empty, then the "Merkle tree root hash" is all zeroes. 297 298The "blocks" here are not necessarily the same as "filesystem blocks". 299 300If a salt was specified, then it's zero-padded to the closest multiple 301of the input size of the hash algorithm's compression function, e.g. 30264 bytes for SHA-256 or 128 bytes for SHA-512. The padded salt is 303prepended to every data or Merkle tree block that is hashed. 304 305The purpose of the block padding is to cause every hash to be taken 306over the same amount of data, which simplifies the implementation and 307keeps open more possibilities for hardware acceleration. The purpose 308of the salt padding is to make the salting "free" when the salted hash 309state is precomputed, then imported for each hash. 310 311Example: in the recommended configuration of SHA-256 and 4K blocks, 312128 hash values fit in each block. Thus, each level of the Merkle 313tree is approximately 128 times smaller than the previous, and for 314large files the Merkle tree's size converges to approximately 1/127 of 315the original file size. However, for small files, the padding is 316significant, making the space overhead proportionally more. 317 318.. _fsverity_descriptor: 319 320fs-verity descriptor 321-------------------- 322 323By itself, the Merkle tree root hash is ambiguous. For example, it 324can't a distinguish a large file from a small second file whose data 325is exactly the top-level hash block of the first file. Ambiguities 326also arise from the convention of padding to the next block boundary. 327 328To solve this problem, the fs-verity file digest is actually computed 329as a hash of the following structure, which contains the Merkle tree 330root hash as well as other fields such as the file size:: 331 332 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 __reserved_0x04; /* 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 */ 342 }; 343 344Built-in signature verification 345=============================== 346 347With CONFIG_FS_VERITY_BUILTIN_SIGNATURES=y, fs-verity supports putting 348a portion of an authentication policy (see `Use cases`_) in the 349kernel. Specifically, it adds support for: 350 3511. At fs-verity module initialization time, a keyring ".fs-verity" is 352 created. The root user can add trusted X.509 certificates to this 353 keyring using the add_key() system call, then (when done) 354 optionally use keyctl_restrict_keyring() to prevent additional 355 certificates from being added. 356 3572. `FS_IOC_ENABLE_VERITY`_ accepts a pointer to a PKCS#7 formatted 358 detached signature in DER format of the file's fs-verity digest. 359 On success, this signature is persisted alongside the Merkle tree. 360 Then, any time the file is opened, the kernel will verify the 361 file's actual digest against this signature, using the certificates 362 in the ".fs-verity" keyring. 363 3643. A new sysctl "fs.verity.require_signatures" is made available. 365 When set to 1, the kernel requires that all verity files have a 366 correctly signed digest as described in (2). 367 368fs-verity file digests must be signed in the following format, which 369is similar to the structure used by `FS_IOC_MEASURE_VERITY`_:: 370 371 struct fsverity_formatted_digest { 372 char magic[8]; /* must be "FSVerity" */ 373 __le16 digest_algorithm; 374 __le16 digest_size; 375 __u8 digest[]; 376 }; 377 378fs-verity's built-in signature verification support is meant as a 379relatively simple mechanism that can be used to provide some level of 380authenticity protection for verity files, as an alternative to doing 381the signature verification in userspace or using IMA-appraisal. 382However, with this mechanism, userspace programs still need to check 383that the verity bit is set, and there is no protection against verity 384files being swapped around. 385 386Filesystem support 387================== 388 389fs-verity is currently supported by the ext4 and f2fs filesystems. 390The CONFIG_FS_VERITY kconfig option must be enabled to use fs-verity 391on either filesystem. 392 393``include/linux/fsverity.h`` declares the interface between the 394``fs/verity/`` support layer and filesystems. Briefly, filesystems 395must provide an ``fsverity_operations`` structure that provides 396methods to read and write the verity metadata to a filesystem-specific 397location, including the Merkle tree blocks and 398``fsverity_descriptor``. Filesystems must also call functions in 399``fs/verity/`` at certain times, such as when a file is opened or when 400pages have been read into the pagecache. (See `Verifying data`_.) 401 402ext4 403---- 404 405ext4 supports fs-verity since Linux v5.4 and e2fsprogs v1.45.2. 406 407To create verity files on an ext4 filesystem, the filesystem must have 408been formatted with ``-O verity`` or had ``tune2fs -O verity`` run on 409it. "verity" is an RO_COMPAT filesystem feature, so once set, old 410kernels will only be able to mount the filesystem readonly, and old 411versions of e2fsck will be unable to check the filesystem. Moreover, 412currently ext4 only supports mounting a filesystem with the "verity" 413feature when its block size is equal to PAGE_SIZE (often 4096 bytes). 414 415ext4 sets the EXT4_VERITY_FL on-disk inode flag on verity files. It 416can only be set by `FS_IOC_ENABLE_VERITY`_, and it cannot be cleared. 417 418ext4 also supports encryption, which can be used simultaneously with 419fs-verity. In this case, the plaintext data is verified rather than 420the ciphertext. This is necessary in order to make the fs-verity file 421digest meaningful, since every file is encrypted differently. 422 423ext4 stores the verity metadata (Merkle tree and fsverity_descriptor) 424past the end of the file, starting at the first 64K boundary beyond 425i_size. This approach works because (a) verity files are readonly, 426and (b) pages fully beyond i_size aren't visible to userspace but can 427be read/written internally by ext4 with only some relatively small 428changes to ext4. This approach avoids having to depend on the 429EA_INODE feature and on rearchitecturing ext4's xattr support to 430support paging multi-gigabyte xattrs into memory, and to support 431encrypting xattrs. Note that the verity metadata *must* be encrypted 432when the file is, since it contains hashes of the plaintext data. 433 434Currently, ext4 verity only supports the case where the Merkle tree 435block size, filesystem block size, and page size are all the same. It 436also only supports extent-based files. 437 438f2fs 439---- 440 441f2fs supports fs-verity since Linux v5.4 and f2fs-tools v1.11.0. 442 443To create verity files on an f2fs filesystem, the filesystem must have 444been formatted with ``-O verity``. 445 446f2fs sets the FADVISE_VERITY_BIT on-disk inode flag on verity files. 447It can only be set by `FS_IOC_ENABLE_VERITY`_, and it cannot be 448cleared. 449 450Like ext4, f2fs stores the verity metadata (Merkle tree and 451fsverity_descriptor) past the end of the file, starting at the first 45264K boundary beyond i_size. See explanation for ext4 above. 453Moreover, f2fs supports at most 4096 bytes of xattr entries per inode 454which wouldn't be enough for even a single Merkle tree block. 455 456Currently, f2fs verity only supports a Merkle tree block size of 4096. 457Also, f2fs doesn't support enabling verity on files that currently 458have atomic or volatile writes pending. 459 460Implementation details 461====================== 462 463Verifying data 464-------------- 465 466fs-verity ensures that all reads of a verity file's data are verified, 467regardless of which syscall is used to do the read (e.g. mmap(), 468read(), pread()) and regardless of whether it's the first read or a 469later read (unless the later read can return cached data that was 470already verified). Below, we describe how filesystems implement this. 471 472Pagecache 473~~~~~~~~~ 474 475For filesystems using Linux's pagecache, the ``->readpage()`` and 476``->readpages()`` methods must be modified to verify pages before they 477are marked Uptodate. Merely hooking ``->read_iter()`` would be 478insufficient, since ``->read_iter()`` is not used for memory maps. 479 480Therefore, fs/verity/ provides a function fsverity_verify_page() which 481verifies a page that has been read into the pagecache of a verity 482inode, but is still locked and not Uptodate, so it's not yet readable 483by userspace. As needed to do the verification, 484fsverity_verify_page() will call back into the filesystem to read 485Merkle tree pages via fsverity_operations::read_merkle_tree_page(). 486 487fsverity_verify_page() returns false if verification failed; in this 488case, the filesystem must not set the page Uptodate. Following this, 489as per the usual Linux pagecache behavior, attempts by userspace to 490read() from the part of the file containing the page will fail with 491EIO, and accesses to the page within a memory map will raise SIGBUS. 492 493fsverity_verify_page() currently only supports the case where the 494Merkle tree block size is equal to PAGE_SIZE (often 4096 bytes). 495 496In principle, fsverity_verify_page() verifies the entire path in the 497Merkle tree from the data page to the root hash. However, for 498efficiency the filesystem may cache the hash pages. Therefore, 499fsverity_verify_page() only ascends the tree reading hash pages until 500an already-verified hash page is seen, as indicated by the PageChecked 501bit being set. It then verifies the path to that page. 502 503This optimization, which is also used by dm-verity, results in 504excellent sequential read performance. This is because usually (e.g. 505127 in 128 times for 4K blocks and SHA-256) the hash page from the 506bottom level of the tree will already be cached and checked from 507reading a previous data page. However, random reads perform worse. 508 509Block device based filesystems 510~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 511 512Block device based filesystems (e.g. ext4 and f2fs) in Linux also use 513the pagecache, so the above subsection applies too. However, they 514also usually read many pages from a file at once, grouped into a 515structure called a "bio". To make it easier for these types of 516filesystems to support fs-verity, fs/verity/ also provides a function 517fsverity_verify_bio() which verifies all pages in a bio. 518 519ext4 and f2fs also support encryption. If a verity file is also 520encrypted, the pages must be decrypted before being verified. To 521support this, these filesystems allocate a "post-read context" for 522each bio and store it in ``->bi_private``:: 523 524 struct bio_post_read_ctx { 525 struct bio *bio; 526 struct work_struct work; 527 unsigned int cur_step; 528 unsigned int enabled_steps; 529 }; 530 531``enabled_steps`` is a bitmask that specifies whether decryption, 532verity, or both is enabled. After the bio completes, for each needed 533postprocessing step the filesystem enqueues the bio_post_read_ctx on a 534workqueue, and then the workqueue work does the decryption or 535verification. Finally, pages where no decryption or verity error 536occurred are marked Uptodate, and the pages are unlocked. 537 538Files on ext4 and f2fs may contain holes. Normally, ``->readpages()`` 539simply zeroes holes and sets the corresponding pages Uptodate; no bios 540are issued. To prevent this case from bypassing fs-verity, these 541filesystems use fsverity_verify_page() to verify hole pages. 542 543ext4 and f2fs disable direct I/O on verity files, since otherwise 544direct I/O would bypass fs-verity. (They also do the same for 545encrypted files.) 546 547Userspace utility 548================= 549 550This document focuses on the kernel, but a userspace utility for 551fs-verity can be found at: 552 553 https://git.kernel.org/pub/scm/linux/kernel/git/ebiggers/fsverity-utils.git 554 555See the README.md file in the fsverity-utils source tree for details, 556including examples of setting up fs-verity protected files. 557 558Tests 559===== 560 561To test fs-verity, use xfstests. For example, using `kvm-xfstests 562<https://github.com/tytso/xfstests-bld/blob/master/Documentation/kvm-quickstart.md>`_:: 563 564 kvm-xfstests -c ext4,f2fs -g verity 565 566FAQ 567=== 568 569This section answers frequently asked questions about fs-verity that 570weren't already directly answered in other parts of this document. 571 572:Q: Why isn't fs-verity part of IMA? 573:A: fs-verity and IMA (Integrity Measurement Architecture) have 574 different focuses. fs-verity is a filesystem-level mechanism for 575 hashing individual files using a Merkle tree. In contrast, IMA 576 specifies a system-wide policy that specifies which files are 577 hashed and what to do with those hashes, such as log them, 578 authenticate them, or add them to a measurement list. 579 580 IMA is planned to support the fs-verity hashing mechanism as an 581 alternative to doing full file hashes, for people who want the 582 performance and security benefits of the Merkle tree based hash. 583 But it doesn't make sense to force all uses of fs-verity to be 584 through IMA. As a standalone filesystem feature, fs-verity 585 already meets many users' needs, and it's testable like other 586 filesystem features e.g. with xfstests. 587 588:Q: Isn't fs-verity useless because the attacker can just modify the 589 hashes in the Merkle tree, which is stored on-disk? 590:A: To verify the authenticity of an fs-verity file you must verify 591 the authenticity of the "fs-verity file digest", which 592 incorporates the root hash of the Merkle tree. See `Use cases`_. 593 594:Q: Isn't fs-verity useless because the attacker can just replace a 595 verity file with a non-verity one? 596:A: See `Use cases`_. In the initial use case, it's really trusted 597 userspace code that authenticates the files; fs-verity is just a 598 tool to do this job efficiently and securely. The trusted 599 userspace code will consider non-verity files to be inauthentic. 600 601:Q: Why does the Merkle tree need to be stored on-disk? Couldn't you 602 store just the root hash? 603:A: If the Merkle tree wasn't stored on-disk, then you'd have to 604 compute the entire tree when the file is first accessed, even if 605 just one byte is being read. This is a fundamental consequence of 606 how Merkle tree hashing works. To verify a leaf node, you need to 607 verify the whole path to the root hash, including the root node 608 (the thing which the root hash is a hash of). But if the root 609 node isn't stored on-disk, you have to compute it by hashing its 610 children, and so on until you've actually hashed the entire file. 611 612 That defeats most of the point of doing a Merkle tree-based hash, 613 since if you have to hash the whole file ahead of time anyway, 614 then you could simply do sha256(file) instead. That would be much 615 simpler, and a bit faster too. 616 617 It's true that an in-memory Merkle tree could still provide the 618 advantage of verification on every read rather than just on the 619 first read. However, it would be inefficient because every time a 620 hash page gets evicted (you can't pin the entire Merkle tree into 621 memory, since it may be very large), in order to restore it you 622 again need to hash everything below it in the tree. This again 623 defeats most of the point of doing a Merkle tree-based hash, since 624 a single block read could trigger re-hashing gigabytes of data. 625 626:Q: But couldn't you store just the leaf nodes and compute the rest? 627:A: See previous answer; this really just moves up one level, since 628 one could alternatively interpret the data blocks as being the 629 leaf nodes of the Merkle tree. It's true that the tree can be 630 computed much faster if the leaf level is stored rather than just 631 the data, but that's only because each level is less than 1% the 632 size of the level below (assuming the recommended settings of 633 SHA-256 and 4K blocks). For the exact same reason, by storing 634 "just the leaf nodes" you'd already be storing over 99% of the 635 tree, so you might as well simply store the whole tree. 636 637:Q: Can the Merkle tree be built ahead of time, e.g. distributed as 638 part of a package that is installed to many computers? 639:A: This isn't currently supported. It was part of the original 640 design, but was removed to simplify the kernel UAPI and because it 641 wasn't a critical use case. Files are usually installed once and 642 used many times, and cryptographic hashing is somewhat fast on 643 most modern processors. 644 645:Q: Why doesn't fs-verity support writes? 646:A: Write support would be very difficult and would require a 647 completely different design, so it's well outside the scope of 648 fs-verity. Write support would require: 649 650 - A way to maintain consistency between the data and hashes, 651 including all levels of hashes, since corruption after a crash 652 (especially of potentially the entire file!) is unacceptable. 653 The main options for solving this are data journalling, 654 copy-on-write, and log-structured volume. But it's very hard to 655 retrofit existing filesystems with new consistency mechanisms. 656 Data journalling is available on ext4, but is very slow. 657 658 - Rebuilding the Merkle tree after every write, which would be 659 extremely inefficient. Alternatively, a different authenticated 660 dictionary structure such as an "authenticated skiplist" could 661 be used. However, this would be far more complex. 662 663 Compare it to dm-verity vs. dm-integrity. dm-verity is very 664 simple: the kernel just verifies read-only data against a 665 read-only Merkle tree. In contrast, dm-integrity supports writes 666 but is slow, is much more complex, and doesn't actually support 667 full-device authentication since it authenticates each sector 668 independently, i.e. there is no "root hash". It doesn't really 669 make sense for the same device-mapper target to support these two 670 very different cases; the same applies to fs-verity. 671 672:Q: Since verity files are immutable, why isn't the immutable bit set? 673:A: The existing "immutable" bit (FS_IMMUTABLE_FL) already has a 674 specific set of semantics which not only make the file contents 675 read-only, but also prevent the file from being deleted, renamed, 676 linked to, or having its owner or mode changed. These extra 677 properties are unwanted for fs-verity, so reusing the immutable 678 bit isn't appropriate. 679 680:Q: Why does the API use ioctls instead of setxattr() and getxattr()? 681:A: Abusing the xattr interface for basically arbitrary syscalls is 682 heavily frowned upon by most of the Linux filesystem developers. 683 An xattr should really just be an xattr on-disk, not an API to 684 e.g. magically trigger construction of a Merkle tree. 685 686:Q: Does fs-verity support remote filesystems? 687:A: Only ext4 and f2fs support is implemented currently, but in 688 principle any filesystem that can store per-file verity metadata 689 can support fs-verity, regardless of whether it's local or remote. 690 Some filesystems may have fewer options of where to store the 691 verity metadata; one possibility is to store it past the end of 692 the file and "hide" it from userspace by manipulating i_size. The 693 data verification functions provided by ``fs/verity/`` also assume 694 that the filesystem uses the Linux pagecache, but both local and 695 remote filesystems normally do so. 696 697:Q: Why is anything filesystem-specific at all? Shouldn't fs-verity 698 be implemented entirely at the VFS level? 699:A: There are many reasons why this is not possible or would be very 700 difficult, including the following: 701 702 - To prevent bypassing verification, pages must not be marked 703 Uptodate until they've been verified. Currently, each 704 filesystem is responsible for marking pages Uptodate via 705 ``->readpages()``. Therefore, currently it's not possible for 706 the VFS to do the verification on its own. Changing this would 707 require significant changes to the VFS and all filesystems. 708 709 - It would require defining a filesystem-independent way to store 710 the verity metadata. Extended attributes don't work for this 711 because (a) the Merkle tree may be gigabytes, but many 712 filesystems assume that all xattrs fit into a single 4K 713 filesystem block, and (b) ext4 and f2fs encryption doesn't 714 encrypt xattrs, yet the Merkle tree *must* be encrypted when the 715 file contents are, because it stores hashes of the plaintext 716 file contents. 717 718 So the verity metadata would have to be stored in an actual 719 file. Using a separate file would be very ugly, since the 720 metadata is fundamentally part of the file to be protected, and 721 it could cause problems where users could delete the real file 722 but not the metadata file or vice versa. On the other hand, 723 having it be in the same file would break applications unless 724 filesystems' notion of i_size were divorced from the VFS's, 725 which would be complex and require changes to all filesystems. 726 727 - It's desirable that FS_IOC_ENABLE_VERITY uses the filesystem's 728 transaction mechanism so that either the file ends up with 729 verity enabled, or no changes were made. Allowing intermediate 730 states to occur after a crash may cause problems. 731