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_READ_VERITY_METADATA 221--------------------------- 222 223The FS_IOC_READ_VERITY_METADATA ioctl reads verity metadata from a 224verity file. This ioctl is available since Linux v5.12. 225 226This ioctl allows writing a server program that takes a verity file 227and serves it to a client program, such that the client can do its own 228fs-verity compatible verification of the file. This only makes sense 229if the client doesn't trust the server and if the server needs to 230provide the storage for the client. 231 232This is a fairly specialized use case, and most fs-verity users won't 233need this ioctl. 234 235This ioctl takes in a pointer to the following structure:: 236 237 #define FS_VERITY_METADATA_TYPE_MERKLE_TREE 1 238 #define FS_VERITY_METADATA_TYPE_DESCRIPTOR 2 239 #define FS_VERITY_METADATA_TYPE_SIGNATURE 3 240 241 struct fsverity_read_metadata_arg { 242 __u64 metadata_type; 243 __u64 offset; 244 __u64 length; 245 __u64 buf_ptr; 246 __u64 __reserved; 247 }; 248 249``metadata_type`` specifies the type of metadata to read: 250 251- ``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. 256 257- ``FS_VERITY_METADATA_TYPE_DESCRIPTOR`` reads the fs-verity 258 descriptor. See `fs-verity descriptor`_. 259 260- ``FS_VERITY_METADATA_TYPE_SIGNATURE`` reads the signature which was 261 passed to FS_IOC_ENABLE_VERITY, if any. See `Built-in signature 262 verification`_. 263 264The semantics are similar to those of ``pread()``. ``offset`` 265specifies the offset in bytes into the metadata item to read from, and 266``length`` specifies the maximum number of bytes to read from the 267metadata item. ``buf_ptr`` is the pointer to the buffer to read into, 268cast to a 64-bit integer. ``__reserved`` must be 0. On success, the 269number of bytes read is returned. 0 is returned at the end of the 270metadata item. The returned length may be less than ``length``, for 271example if the ioctl is interrupted. 272 273The metadata returned by FS_IOC_READ_VERITY_METADATA isn't guaranteed 274to 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 276implement fs-verity compatible verification anyway (though absent a 277malicious disk, the metadata will indeed match). E.g. to implement 278this ioctl, the filesystem is allowed to just read the Merkle tree 279blocks from disk without actually verifying the path to the root node. 280 281FS_IOC_READ_VERITY_METADATA can fail with the following errors: 282 283- ``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`` 286 overflowed 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`_.) 295 296FS_IOC_GETFLAGS 297--------------- 298 299The existing ioctl FS_IOC_GETFLAGS (which isn't specific to fs-verity) 300can also be used to check whether a file has fs-verity enabled or not. 301To do so, check for FS_VERITY_FL (0x00100000) in the returned flags. 302 303The verity flag is not settable via FS_IOC_SETFLAGS. You must use 304FS_IOC_ENABLE_VERITY instead, since parameters must be provided. 305 306statx 307----- 308 309Since Linux v5.5, the statx() system call sets STATX_ATTR_VERITY if 310the file has fs-verity enabled. This can perform better than 311FS_IOC_GETFLAGS and FS_IOC_MEASURE_VERITY because it doesn't require 312opening the file, and opening verity files can be expensive. 313 314Accessing verity files 315====================== 316 317Applications can transparently access a verity file just like a 318non-verity one, with the following exceptions: 319 320- 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. 326 327- 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. 329 330- DAX (Direct Access) is not supported on verity files, because this 331 would circumvent the data verification. 332 333- Reads of data that doesn't match the verity Merkle tree will fail 334 with EIO (for read()) or SIGBUS (for mmap() reads). 335 336- 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`_. 339 340Direct access to the Merkle tree is not supported. Therefore, if a 341verity file is copied, or is backed up and restored, then it will lose 342its "verity"-ness. fs-verity is primarily meant for files like 343executables that are managed by a package manager. 344 345File digest computation 346======================= 347 348This section describes how fs-verity hashes the file contents using a 349Merkle tree to produce the digest which cryptographically identifies 350the file contents. This algorithm is the same for all filesystems 351that support fs-verity. 352 353Userspace only needs to be aware of this algorithm if it needs to 354compute fs-verity file digests itself, e.g. in order to sign files. 355 356.. _fsverity_merkle_tree: 357 358Merkle tree 359----------- 360 361The file contents is divided into blocks, where the block size is 362configurable but is usually 4096 bytes. The end of the last block is 363zero-padded if needed. Each block is then hashed, producing the first 364level of hashes. Then, the hashes in this first level are grouped 365into 'blocksize'-byte blocks (zero-padding the ends as needed) and 366these blocks are hashed, producing the second level of hashes. This 367proceeds up the tree until only a single block remains. The hash of 368this block is the "Merkle tree root hash". 369 370If the file fits in one block and is nonempty, then the "Merkle tree 371root hash" is simply the hash of the single data block. If the file 372is empty, then the "Merkle tree root hash" is all zeroes. 373 374The "blocks" here are not necessarily the same as "filesystem blocks". 375 376If a salt was specified, then it's zero-padded to the closest multiple 377of the input size of the hash algorithm's compression function, e.g. 37864 bytes for SHA-256 or 128 bytes for SHA-512. The padded salt is 379prepended to every data or Merkle tree block that is hashed. 380 381The purpose of the block padding is to cause every hash to be taken 382over the same amount of data, which simplifies the implementation and 383keeps open more possibilities for hardware acceleration. The purpose 384of the salt padding is to make the salting "free" when the salted hash 385state is precomputed, then imported for each hash. 386 387Example: in the recommended configuration of SHA-256 and 4K blocks, 388128 hash values fit in each block. Thus, each level of the Merkle 389tree is approximately 128 times smaller than the previous, and for 390large files the Merkle tree's size converges to approximately 1/127 of 391the original file size. However, for small files, the padding is 392significant, making the space overhead proportionally more. 393 394.. _fsverity_descriptor: 395 396fs-verity descriptor 397-------------------- 398 399By itself, the Merkle tree root hash is ambiguous. For example, it 400can't a distinguish a large file from a small second file whose data 401is exactly the top-level hash block of the first file. Ambiguities 402also arise from the convention of padding to the next block boundary. 403 404To solve this problem, the fs-verity file digest is actually computed 405as a hash of the following structure, which contains the Merkle tree 406root hash as well as other fields such as the file size:: 407 408 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 */ 418 }; 419 420Built-in signature verification 421=============================== 422 423With CONFIG_FS_VERITY_BUILTIN_SIGNATURES=y, fs-verity supports putting 424a portion of an authentication policy (see `Use cases`_) in the 425kernel. Specifically, it adds support for: 426 4271. 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. 432 4332. `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. 439 4403. 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). 443 444fs-verity file digests must be signed in the following format, which 445is similar to the structure used by `FS_IOC_MEASURE_VERITY`_:: 446 447 struct fsverity_formatted_digest { 448 char magic[8]; /* must be "FSVerity" */ 449 __le16 digest_algorithm; 450 __le16 digest_size; 451 __u8 digest[]; 452 }; 453 454fs-verity's built-in signature verification support is meant as a 455relatively simple mechanism that can be used to provide some level of 456authenticity protection for verity files, as an alternative to doing 457the signature verification in userspace or using IMA-appraisal. 458However, with this mechanism, userspace programs still need to check 459that the verity bit is set, and there is no protection against verity 460files being swapped around. 461 462Filesystem support 463================== 464 465fs-verity is currently supported by the ext4 and f2fs filesystems. 466The CONFIG_FS_VERITY kconfig option must be enabled to use fs-verity 467on either filesystem. 468 469``include/linux/fsverity.h`` declares the interface between the 470``fs/verity/`` support layer and filesystems. Briefly, filesystems 471must provide an ``fsverity_operations`` structure that provides 472methods to read and write the verity metadata to a filesystem-specific 473location, 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 476pages have been read into the pagecache. (See `Verifying data`_.) 477 478ext4 479---- 480 481ext4 supports fs-verity since Linux v5.4 and e2fsprogs v1.45.2. 482 483To create verity files on an ext4 filesystem, the filesystem must have 484been formatted with ``-O verity`` or had ``tune2fs -O verity`` run on 485it. "verity" is an RO_COMPAT filesystem feature, so once set, old 486kernels will only be able to mount the filesystem readonly, and old 487versions of e2fsck will be unable to check the filesystem. Moreover, 488currently ext4 only supports mounting a filesystem with the "verity" 489feature when its block size is equal to PAGE_SIZE (often 4096 bytes). 490 491ext4 sets the EXT4_VERITY_FL on-disk inode flag on verity files. It 492can only be set by `FS_IOC_ENABLE_VERITY`_, and it cannot be cleared. 493 494ext4 also supports encryption, which can be used simultaneously with 495fs-verity. In this case, the plaintext data is verified rather than 496the ciphertext. This is necessary in order to make the fs-verity file 497digest meaningful, since every file is encrypted differently. 498 499ext4 stores the verity metadata (Merkle tree and fsverity_descriptor) 500past the end of the file, starting at the first 64K boundary beyond 501i_size. This approach works because (a) verity files are readonly, 502and (b) pages fully beyond i_size aren't visible to userspace but can 503be read/written internally by ext4 with only some relatively small 504changes to ext4. This approach avoids having to depend on the 505EA_INODE feature and on rearchitecturing ext4's xattr support to 506support paging multi-gigabyte xattrs into memory, and to support 507encrypting xattrs. Note that the verity metadata *must* be encrypted 508when the file is, since it contains hashes of the plaintext data. 509 510Currently, ext4 verity only supports the case where the Merkle tree 511block size, filesystem block size, and page size are all the same. It 512also only supports extent-based files. 513 514f2fs 515---- 516 517f2fs supports fs-verity since Linux v5.4 and f2fs-tools v1.11.0. 518 519To create verity files on an f2fs filesystem, the filesystem must have 520been formatted with ``-O verity``. 521 522f2fs sets the FADVISE_VERITY_BIT on-disk inode flag on verity files. 523It can only be set by `FS_IOC_ENABLE_VERITY`_, and it cannot be 524cleared. 525 526Like ext4, f2fs stores the verity metadata (Merkle tree and 527fsverity_descriptor) past the end of the file, starting at the first 52864K boundary beyond i_size. See explanation for ext4 above. 529Moreover, f2fs supports at most 4096 bytes of xattr entries per inode 530which wouldn't be enough for even a single Merkle tree block. 531 532Currently, f2fs verity only supports a Merkle tree block size of 4096. 533Also, f2fs doesn't support enabling verity on files that currently 534have atomic or volatile writes pending. 535 536Implementation details 537====================== 538 539Verifying data 540-------------- 541 542fs-verity ensures that all reads of a verity file's data are verified, 543regardless of which syscall is used to do the read (e.g. mmap(), 544read(), pread()) and regardless of whether it's the first read or a 545later read (unless the later read can return cached data that was 546already verified). Below, we describe how filesystems implement this. 547 548Pagecache 549~~~~~~~~~ 550 551For filesystems using Linux's pagecache, the ``->readpage()`` and 552``->readahead()`` methods must be modified to verify pages before they 553are marked Uptodate. Merely hooking ``->read_iter()`` would be 554insufficient, since ``->read_iter()`` is not used for memory maps. 555 556Therefore, fs/verity/ provides a function fsverity_verify_page() which 557verifies a page that has been read into the pagecache of a verity 558inode, but is still locked and not Uptodate, so it's not yet readable 559by userspace. As needed to do the verification, 560fsverity_verify_page() will call back into the filesystem to read 561Merkle tree pages via fsverity_operations::read_merkle_tree_page(). 562 563fsverity_verify_page() returns false if verification failed; in this 564case, the filesystem must not set the page Uptodate. Following this, 565as per the usual Linux pagecache behavior, attempts by userspace to 566read() from the part of the file containing the page will fail with 567EIO, and accesses to the page within a memory map will raise SIGBUS. 568 569fsverity_verify_page() currently only supports the case where the 570Merkle tree block size is equal to PAGE_SIZE (often 4096 bytes). 571 572In principle, fsverity_verify_page() verifies the entire path in the 573Merkle tree from the data page to the root hash. However, for 574efficiency the filesystem may cache the hash pages. Therefore, 575fsverity_verify_page() only ascends the tree reading hash pages until 576an already-verified hash page is seen, as indicated by the PageChecked 577bit being set. It then verifies the path to that page. 578 579This optimization, which is also used by dm-verity, results in 580excellent sequential read performance. This is because usually (e.g. 581127 in 128 times for 4K blocks and SHA-256) the hash page from the 582bottom level of the tree will already be cached and checked from 583reading a previous data page. However, random reads perform worse. 584 585Block device based filesystems 586~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 587 588Block device based filesystems (e.g. ext4 and f2fs) in Linux also use 589the pagecache, so the above subsection applies too. However, they 590also usually read many pages from a file at once, grouped into a 591structure called a "bio". To make it easier for these types of 592filesystems to support fs-verity, fs/verity/ also provides a function 593fsverity_verify_bio() which verifies all pages in a bio. 594 595ext4 and f2fs also support encryption. If a verity file is also 596encrypted, the pages must be decrypted before being verified. To 597support this, these filesystems allocate a "post-read context" for 598each bio and store it in ``->bi_private``:: 599 600 struct bio_post_read_ctx { 601 struct bio *bio; 602 struct work_struct work; 603 unsigned int cur_step; 604 unsigned int enabled_steps; 605 }; 606 607``enabled_steps`` is a bitmask that specifies whether decryption, 608verity, or both is enabled. After the bio completes, for each needed 609postprocessing step the filesystem enqueues the bio_post_read_ctx on a 610workqueue, and then the workqueue work does the decryption or 611verification. Finally, pages where no decryption or verity error 612occurred are marked Uptodate, and the pages are unlocked. 613 614Files on ext4 and f2fs may contain holes. Normally, ``->readahead()`` 615simply zeroes holes and sets the corresponding pages Uptodate; no bios 616are issued. To prevent this case from bypassing fs-verity, these 617filesystems use fsverity_verify_page() to verify hole pages. 618 619ext4 and f2fs disable direct I/O on verity files, since otherwise 620direct I/O would bypass fs-verity. (They also do the same for 621encrypted files.) 622 623Userspace utility 624================= 625 626This document focuses on the kernel, but a userspace utility for 627fs-verity can be found at: 628 629 https://git.kernel.org/pub/scm/linux/kernel/git/ebiggers/fsverity-utils.git 630 631See the README.md file in the fsverity-utils source tree for details, 632including examples of setting up fs-verity protected files. 633 634Tests 635===== 636 637To test fs-verity, use xfstests. For example, using `kvm-xfstests 638<https://github.com/tytso/xfstests-bld/blob/master/Documentation/kvm-quickstart.md>`_:: 639 640 kvm-xfstests -c ext4,f2fs -g verity 641 642FAQ 643=== 644 645This section answers frequently asked questions about fs-verity that 646weren't already directly answered in other parts of this document. 647 648: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. 655 656 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. 663 664: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`_. 669 670: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. 676 677: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. 687 688 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. 692 693 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. 701 702: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. 712 713: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. 720 721: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: 725 726 - 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. 733 734 - 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. 738 739 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. 747 748: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. 755 756: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. 761 762: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. 772 773: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: 777 778 - 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 ``->readahead()``. 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. 784 785 - 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 792 file contents. 793 794 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. 802 803 - 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. 807