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, f2fs, and 15btrfs filesystems. Like fscrypt, not too much filesystem-specific 16code is needed 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, fs-verity only provides integrity protection, i.e. 42detection 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 49A standard file hash could be used instead of fs-verity. However, 50this is inefficient if the file is large and only a small portion may 51be accessed. This is often the case for Android application package 52(APK) files, for example. These typically contain many translations, 53classes, and other resources that are infrequently or even never 54accessed on a particular device. It would be slow and wasteful to 55read and hash the entire file before starting the application. 56 57Unlike an ahead-of-time hash, fs-verity also re-verifies data each 58time it's paged in. This ensures that malicious disk firmware can't 59undetectably change the contents of the file at runtime. 60 61fs-verity does not replace or obsolete dm-verity. dm-verity should 62still be used on read-only filesystems. fs-verity is for files that 63must live on a read-write filesystem because they are independently 64updated and potentially user-installed, so dm-verity cannot be used. 65 66fs-verity does not mandate a particular scheme for authenticating its 67file hashes. (Similarly, dm-verity does not mandate a particular 68scheme for authenticating its block device root hashes.) Options for 69authenticating fs-verity file hashes include: 70 71- Trusted userspace code. Often, the userspace code that accesses 72 files can be trusted to authenticate them. Consider e.g. an 73 application that wants to authenticate data files before using them, 74 or an application loader that is part of the operating system (which 75 is already authenticated in a different way, such as by being loaded 76 from a read-only partition that uses dm-verity) and that wants to 77 authenticate applications before loading them. In these cases, this 78 trusted userspace code can authenticate a file's contents by 79 retrieving its fs-verity digest using `FS_IOC_MEASURE_VERITY`_, then 80 verifying a signature of it using any userspace cryptographic 81 library that supports digital signatures. 82 83- Integrity Measurement Architecture (IMA). IMA supports fs-verity 84 file digests as an alternative to its traditional full file digests. 85 "IMA appraisal" enforces that files contain a valid, matching 86 signature in their "security.ima" extended attribute, as controlled 87 by the IMA policy. For more information, see the IMA documentation. 88 89- Trusted userspace code in combination with `Built-in signature 90 verification`_. This approach should be used only with great care. 91 92User API 93======== 94 95FS_IOC_ENABLE_VERITY 96-------------------- 97 98The FS_IOC_ENABLE_VERITY ioctl enables fs-verity on a file. It takes 99in a pointer to a struct fsverity_enable_arg, defined as 100follows:: 101 102 struct fsverity_enable_arg { 103 __u32 version; 104 __u32 hash_algorithm; 105 __u32 block_size; 106 __u32 salt_size; 107 __u64 salt_ptr; 108 __u32 sig_size; 109 __u32 __reserved1; 110 __u64 sig_ptr; 111 __u64 __reserved2[11]; 112 }; 113 114This structure contains the parameters of the Merkle tree to build for 115the file. It must be initialized as follows: 116 117- ``version`` must be 1. 118- ``hash_algorithm`` must be the identifier for the hash algorithm to 119 use for the Merkle tree, such as FS_VERITY_HASH_ALG_SHA256. See 120 ``include/uapi/linux/fsverity.h`` for the list of possible values. 121- ``block_size`` is the Merkle tree block size, in bytes. In Linux 122 v6.3 and later, this can be any power of 2 between (inclusively) 123 1024 and the minimum of the system page size and the filesystem 124 block size. In earlier versions, the page size was the only allowed 125 value. 126- ``salt_size`` is the size of the salt in bytes, or 0 if no salt is 127 provided. The salt is a value that is prepended to every hashed 128 block; it can be used to personalize the hashing for a particular 129 file or device. Currently the maximum salt size is 32 bytes. 130- ``salt_ptr`` is the pointer to the salt, or NULL if no salt is 131 provided. 132- ``sig_size`` is the size of the builtin signature in bytes, or 0 if no 133 builtin signature is provided. Currently the builtin signature is 134 (somewhat arbitrarily) limited to 16128 bytes. 135- ``sig_ptr`` is the pointer to the builtin signature, or NULL if no 136 builtin signature is provided. A builtin signature is only needed 137 if the `Built-in signature verification`_ feature is being used. It 138 is not needed for IMA appraisal, and it is not needed if the file 139 signature is being handled entirely in userspace. 140- All reserved fields must be zeroed. 141 142FS_IOC_ENABLE_VERITY causes the filesystem to build a Merkle tree for 143the file and persist it to a filesystem-specific location associated 144with the file, then mark the file as a verity file. This ioctl may 145take a long time to execute on large files, and it is interruptible by 146fatal signals. 147 148FS_IOC_ENABLE_VERITY checks for write access to the inode. However, 149it must be executed on an O_RDONLY file descriptor and no processes 150can have the file open for writing. Attempts to open the file for 151writing while this ioctl is executing will fail with ETXTBSY. (This 152is necessary to guarantee that no writable file descriptors will exist 153after verity is enabled, and to guarantee that the file's contents are 154stable while the Merkle tree is being built over it.) 155 156On success, FS_IOC_ENABLE_VERITY returns 0, and the file becomes a 157verity file. On failure (including the case of interruption by a 158fatal signal), no changes are made to the file. 159 160FS_IOC_ENABLE_VERITY can fail with the following errors: 161 162- ``EACCES``: the process does not have write access to the file 163- ``EBADMSG``: the builtin signature is malformed 164- ``EBUSY``: this ioctl is already running on the file 165- ``EEXIST``: the file already has verity enabled 166- ``EFAULT``: the caller provided inaccessible memory 167- ``EFBIG``: the file is too large to enable verity on 168- ``EINTR``: the operation was interrupted by a fatal signal 169- ``EINVAL``: unsupported version, hash algorithm, or block size; or 170 reserved bits are set; or the file descriptor refers to neither a 171 regular file nor a directory. 172- ``EISDIR``: the file descriptor refers to a directory 173- ``EKEYREJECTED``: the builtin signature doesn't match the file 174- ``EMSGSIZE``: the salt or builtin signature is too long 175- ``ENOKEY``: the ".fs-verity" keyring doesn't contain the certificate 176 needed to verify the builtin signature 177- ``ENOPKG``: fs-verity recognizes the hash algorithm, but it's not 178 available in the kernel's crypto API as currently configured (e.g. 179 for SHA-512, missing CONFIG_CRYPTO_SHA512). 180- ``ENOTTY``: this type of filesystem does not implement fs-verity 181- ``EOPNOTSUPP``: the kernel was not configured with fs-verity 182 support; or the filesystem superblock has not had the 'verity' 183 feature enabled on it; or the filesystem does not support fs-verity 184 on this file. (See `Filesystem support`_.) 185- ``EPERM``: the file is append-only; or, a builtin signature is 186 required and one was not provided. 187- ``EROFS``: the filesystem is read-only 188- ``ETXTBSY``: someone has the file open for writing. This can be the 189 caller's file descriptor, another open file descriptor, or the file 190 reference held by a writable memory map. 191 192FS_IOC_MEASURE_VERITY 193--------------------- 194 195The FS_IOC_MEASURE_VERITY ioctl retrieves the digest of a verity file. 196The fs-verity file digest is a cryptographic digest that identifies 197the file contents that are being enforced on reads; it is computed via 198a Merkle tree and is different from a traditional full-file digest. 199 200This ioctl takes in a pointer to a variable-length structure:: 201 202 struct fsverity_digest { 203 __u16 digest_algorithm; 204 __u16 digest_size; /* input/output */ 205 __u8 digest[]; 206 }; 207 208``digest_size`` is an input/output field. On input, it must be 209initialized to the number of bytes allocated for the variable-length 210``digest`` field. 211 212On success, 0 is returned and the kernel fills in the structure as 213follows: 214 215- ``digest_algorithm`` will be the hash algorithm used for the file 216 digest. It will match ``fsverity_enable_arg::hash_algorithm``. 217- ``digest_size`` will be the size of the digest in bytes, e.g. 32 218 for SHA-256. (This can be redundant with ``digest_algorithm``.) 219- ``digest`` will be the actual bytes of the digest. 220 221FS_IOC_MEASURE_VERITY is guaranteed to execute in constant time, 222regardless of the size of the file. 223 224FS_IOC_MEASURE_VERITY can fail with the following errors: 225 226- ``EFAULT``: the caller provided inaccessible memory 227- ``ENODATA``: the file is not a verity file 228- ``ENOTTY``: this type of filesystem does not implement fs-verity 229- ``EOPNOTSUPP``: the kernel was not configured with fs-verity 230 support, or the filesystem superblock has not had the 'verity' 231 feature enabled on it. (See `Filesystem support`_.) 232- ``EOVERFLOW``: the digest is longer than the specified 233 ``digest_size`` bytes. Try providing a larger buffer. 234 235FS_IOC_READ_VERITY_METADATA 236--------------------------- 237 238The FS_IOC_READ_VERITY_METADATA ioctl reads verity metadata from a 239verity file. This ioctl is available since Linux v5.12. 240 241This ioctl allows writing a server program that takes a verity file 242and serves it to a client program, such that the client can do its own 243fs-verity compatible verification of the file. This only makes sense 244if the client doesn't trust the server and if the server needs to 245provide the storage for the client. 246 247This is a fairly specialized use case, and most fs-verity users won't 248need this ioctl. 249 250This ioctl takes in a pointer to the following structure:: 251 252 #define FS_VERITY_METADATA_TYPE_MERKLE_TREE 1 253 #define FS_VERITY_METADATA_TYPE_DESCRIPTOR 2 254 #define FS_VERITY_METADATA_TYPE_SIGNATURE 3 255 256 struct fsverity_read_metadata_arg { 257 __u64 metadata_type; 258 __u64 offset; 259 __u64 length; 260 __u64 buf_ptr; 261 __u64 __reserved; 262 }; 263 264``metadata_type`` specifies the type of metadata to read: 265 266- ``FS_VERITY_METADATA_TYPE_MERKLE_TREE`` reads the blocks of the 267 Merkle tree. The blocks are returned in order from the root level 268 to the leaf level. Within each level, the blocks are returned in 269 the same order that their hashes are themselves hashed. 270 See `Merkle tree`_ for more information. 271 272- ``FS_VERITY_METADATA_TYPE_DESCRIPTOR`` reads the fs-verity 273 descriptor. See `fs-verity descriptor`_. 274 275- ``FS_VERITY_METADATA_TYPE_SIGNATURE`` reads the builtin signature 276 which was passed to FS_IOC_ENABLE_VERITY, if any. See `Built-in 277 signature verification`_. 278 279The semantics are similar to those of ``pread()``. ``offset`` 280specifies the offset in bytes into the metadata item to read from, and 281``length`` specifies the maximum number of bytes to read from the 282metadata item. ``buf_ptr`` is the pointer to the buffer to read into, 283cast to a 64-bit integer. ``__reserved`` must be 0. On success, the 284number of bytes read is returned. 0 is returned at the end of the 285metadata item. The returned length may be less than ``length``, for 286example if the ioctl is interrupted. 287 288The metadata returned by FS_IOC_READ_VERITY_METADATA isn't guaranteed 289to be authenticated against the file digest that would be returned by 290`FS_IOC_MEASURE_VERITY`_, as the metadata is expected to be used to 291implement fs-verity compatible verification anyway (though absent a 292malicious disk, the metadata will indeed match). E.g. to implement 293this ioctl, the filesystem is allowed to just read the Merkle tree 294blocks from disk without actually verifying the path to the root node. 295 296FS_IOC_READ_VERITY_METADATA can fail with the following errors: 297 298- ``EFAULT``: the caller provided inaccessible memory 299- ``EINTR``: the ioctl was interrupted before any data was read 300- ``EINVAL``: reserved fields were set, or ``offset + length`` 301 overflowed 302- ``ENODATA``: the file is not a verity file, or 303 FS_VERITY_METADATA_TYPE_SIGNATURE was requested but the file doesn't 304 have a builtin signature 305- ``ENOTTY``: this type of filesystem does not implement fs-verity, or 306 this ioctl is not yet implemented on it 307- ``EOPNOTSUPP``: the kernel was not configured with fs-verity 308 support, or the filesystem superblock has not had the 'verity' 309 feature enabled on it. (See `Filesystem support`_.) 310 311FS_IOC_GETFLAGS 312--------------- 313 314The existing ioctl FS_IOC_GETFLAGS (which isn't specific to fs-verity) 315can also be used to check whether a file has fs-verity enabled or not. 316To do so, check for FS_VERITY_FL (0x00100000) in the returned flags. 317 318The verity flag is not settable via FS_IOC_SETFLAGS. You must use 319FS_IOC_ENABLE_VERITY instead, since parameters must be provided. 320 321statx 322----- 323 324Since Linux v5.5, the statx() system call sets STATX_ATTR_VERITY if 325the file has fs-verity enabled. This can perform better than 326FS_IOC_GETFLAGS and FS_IOC_MEASURE_VERITY because it doesn't require 327opening the file, and opening verity files can be expensive. 328 329Accessing verity files 330====================== 331 332Applications can transparently access a verity file just like a 333non-verity one, with the following exceptions: 334 335- Verity files are readonly. They cannot be opened for writing or 336 truncate()d, even if the file mode bits allow it. Attempts to do 337 one of these things will fail with EPERM. However, changes to 338 metadata such as owner, mode, timestamps, and xattrs are still 339 allowed, since these are not measured by fs-verity. Verity files 340 can also still be renamed, deleted, and linked to. 341 342- Direct I/O is not supported on verity files. Attempts to use direct 343 I/O on such files will fall back to buffered I/O. 344 345- DAX (Direct Access) is not supported on verity files, because this 346 would circumvent the data verification. 347 348- Reads of data that doesn't match the verity Merkle tree will fail 349 with EIO (for read()) or SIGBUS (for mmap() reads). 350 351- If the sysctl "fs.verity.require_signatures" is set to 1 and the 352 file is not signed by a key in the ".fs-verity" keyring, then 353 opening the file will fail. See `Built-in signature verification`_. 354 355Direct access to the Merkle tree is not supported. Therefore, if a 356verity file is copied, or is backed up and restored, then it will lose 357its "verity"-ness. fs-verity is primarily meant for files like 358executables that are managed by a package manager. 359 360File digest computation 361======================= 362 363This section describes how fs-verity hashes the file contents using a 364Merkle tree to produce the digest which cryptographically identifies 365the file contents. This algorithm is the same for all filesystems 366that support fs-verity. 367 368Userspace only needs to be aware of this algorithm if it needs to 369compute fs-verity file digests itself, e.g. in order to sign files. 370 371.. _fsverity_merkle_tree: 372 373Merkle tree 374----------- 375 376The file contents is divided into blocks, where the block size is 377configurable but is usually 4096 bytes. The end of the last block is 378zero-padded if needed. Each block is then hashed, producing the first 379level of hashes. Then, the hashes in this first level are grouped 380into 'blocksize'-byte blocks (zero-padding the ends as needed) and 381these blocks are hashed, producing the second level of hashes. This 382proceeds up the tree until only a single block remains. The hash of 383this block is the "Merkle tree root hash". 384 385If the file fits in one block and is nonempty, then the "Merkle tree 386root hash" is simply the hash of the single data block. If the file 387is empty, then the "Merkle tree root hash" is all zeroes. 388 389The "blocks" here are not necessarily the same as "filesystem blocks". 390 391If a salt was specified, then it's zero-padded to the closest multiple 392of the input size of the hash algorithm's compression function, e.g. 39364 bytes for SHA-256 or 128 bytes for SHA-512. The padded salt is 394prepended to every data or Merkle tree block that is hashed. 395 396The purpose of the block padding is to cause every hash to be taken 397over the same amount of data, which simplifies the implementation and 398keeps open more possibilities for hardware acceleration. The purpose 399of the salt padding is to make the salting "free" when the salted hash 400state is precomputed, then imported for each hash. 401 402Example: in the recommended configuration of SHA-256 and 4K blocks, 403128 hash values fit in each block. Thus, each level of the Merkle 404tree is approximately 128 times smaller than the previous, and for 405large files the Merkle tree's size converges to approximately 1/127 of 406the original file size. However, for small files, the padding is 407significant, making the space overhead proportionally more. 408 409.. _fsverity_descriptor: 410 411fs-verity descriptor 412-------------------- 413 414By itself, the Merkle tree root hash is ambiguous. For example, it 415can't a distinguish a large file from a small second file whose data 416is exactly the top-level hash block of the first file. Ambiguities 417also arise from the convention of padding to the next block boundary. 418 419To solve this problem, the fs-verity file digest is actually computed 420as a hash of the following structure, which contains the Merkle tree 421root hash as well as other fields such as the file size:: 422 423 struct fsverity_descriptor { 424 __u8 version; /* must be 1 */ 425 __u8 hash_algorithm; /* Merkle tree hash algorithm */ 426 __u8 log_blocksize; /* log2 of size of data and tree blocks */ 427 __u8 salt_size; /* size of salt in bytes; 0 if none */ 428 __le32 __reserved_0x04; /* must be 0 */ 429 __le64 data_size; /* size of file the Merkle tree is built over */ 430 __u8 root_hash[64]; /* Merkle tree root hash */ 431 __u8 salt[32]; /* salt prepended to each hashed block */ 432 __u8 __reserved[144]; /* must be 0's */ 433 }; 434 435Built-in signature verification 436=============================== 437 438CONFIG_FS_VERITY_BUILTIN_SIGNATURES=y adds supports for in-kernel 439verification of fs-verity builtin signatures. 440 441**IMPORTANT**! Please take great care before using this feature. 442It is not the only way to do signatures with fs-verity, and the 443alternatives (such as userspace signature verification, and IMA 444appraisal) can be much better. It's also easy to fall into a trap 445of thinking this feature solves more problems than it actually does. 446 447Enabling this option adds the following: 448 4491. At boot time, the kernel creates a keyring named ".fs-verity". The 450 root user can add trusted X.509 certificates to this keyring using 451 the add_key() system call. 452 4532. `FS_IOC_ENABLE_VERITY`_ accepts a pointer to a PKCS#7 formatted 454 detached signature in DER format of the file's fs-verity digest. 455 On success, the ioctl persists the signature alongside the Merkle 456 tree. Then, any time the file is opened, the kernel verifies the 457 file's actual digest against this signature, using the certificates 458 in the ".fs-verity" keyring. 459 4603. A new sysctl "fs.verity.require_signatures" is made available. 461 When set to 1, the kernel requires that all verity files have a 462 correctly signed digest as described in (2). 463 464The data that the signature as described in (2) must be a signature of 465is the fs-verity file digest in the following format:: 466 467 struct fsverity_formatted_digest { 468 char magic[8]; /* must be "FSVerity" */ 469 __le16 digest_algorithm; 470 __le16 digest_size; 471 __u8 digest[]; 472 }; 473 474That's it. It should be emphasized again that fs-verity builtin 475signatures are not the only way to do signatures with fs-verity. See 476`Use cases`_ for an overview of ways in which fs-verity can be used. 477fs-verity builtin signatures have some major limitations that should 478be carefully considered before using them: 479 480- Builtin signature verification does *not* make the kernel enforce 481 that any files actually have fs-verity enabled. Thus, it is not a 482 complete authentication policy. Currently, if it is used, the only 483 way to complete the authentication policy is for trusted userspace 484 code to explicitly check whether files have fs-verity enabled with a 485 signature before they are accessed. (With 486 fs.verity.require_signatures=1, just checking whether fs-verity is 487 enabled suffices.) But, in this case the trusted userspace code 488 could just store the signature alongside the file and verify it 489 itself using a cryptographic library, instead of using this feature. 490 491- A file's builtin signature can only be set at the same time that 492 fs-verity is being enabled on the file. Changing or deleting the 493 builtin signature later requires re-creating the file. 494 495- Builtin signature verification uses the same set of public keys for 496 all fs-verity enabled files on the system. Different keys cannot be 497 trusted for different files; each key is all or nothing. 498 499- The sysctl fs.verity.require_signatures applies system-wide. 500 Setting it to 1 only works when all users of fs-verity on the system 501 agree that it should be set to 1. This limitation can prevent 502 fs-verity from being used in cases where it would be helpful. 503 504- Builtin signature verification can only use signature algorithms 505 that are supported by the kernel. For example, the kernel does not 506 yet support Ed25519, even though this is often the signature 507 algorithm that is recommended for new cryptographic designs. 508 509- fs-verity builtin signatures are in PKCS#7 format, and the public 510 keys are in X.509 format. These formats are commonly used, 511 including by some other kernel features (which is why the fs-verity 512 builtin signatures use them), and are very feature rich. 513 Unfortunately, history has shown that code that parses and handles 514 these formats (which are from the 1990s and are based on ASN.1) 515 often has vulnerabilities as a result of their complexity. This 516 complexity is not inherent to the cryptography itself. 517 518 fs-verity users who do not need advanced features of X.509 and 519 PKCS#7 should strongly consider using simpler formats, such as plain 520 Ed25519 keys and signatures, and verifying signatures in userspace. 521 522 fs-verity users who choose to use X.509 and PKCS#7 anyway should 523 still consider that verifying those signatures in userspace is more 524 flexible (for other reasons mentioned earlier in this document) and 525 eliminates the need to enable CONFIG_FS_VERITY_BUILTIN_SIGNATURES 526 and its associated increase in kernel attack surface. In some cases 527 it can even be necessary, since advanced X.509 and PKCS#7 features 528 do not always work as intended with the kernel. For example, the 529 kernel does not check X.509 certificate validity times. 530 531 Note: IMA appraisal, which supports fs-verity, does not use PKCS#7 532 for its signatures, so it partially avoids the issues discussed 533 here. IMA appraisal does use X.509. 534 535Filesystem support 536================== 537 538fs-verity is supported by several filesystems, described below. The 539CONFIG_FS_VERITY kconfig option must be enabled to use fs-verity on 540any of these filesystems. 541 542``include/linux/fsverity.h`` declares the interface between the 543``fs/verity/`` support layer and filesystems. Briefly, filesystems 544must provide an ``fsverity_operations`` structure that provides 545methods to read and write the verity metadata to a filesystem-specific 546location, including the Merkle tree blocks and 547``fsverity_descriptor``. Filesystems must also call functions in 548``fs/verity/`` at certain times, such as when a file is opened or when 549pages have been read into the pagecache. (See `Verifying data`_.) 550 551ext4 552---- 553 554ext4 supports fs-verity since Linux v5.4 and e2fsprogs v1.45.2. 555 556To create verity files on an ext4 filesystem, the filesystem must have 557been formatted with ``-O verity`` or had ``tune2fs -O verity`` run on 558it. "verity" is an RO_COMPAT filesystem feature, so once set, old 559kernels will only be able to mount the filesystem readonly, and old 560versions of e2fsck will be unable to check the filesystem. 561 562Originally, an ext4 filesystem with the "verity" feature could only be 563mounted when its block size was equal to the system page size 564(typically 4096 bytes). In Linux v6.3, this limitation was removed. 565 566ext4 sets the EXT4_VERITY_FL on-disk inode flag on verity files. It 567can only be set by `FS_IOC_ENABLE_VERITY`_, and it cannot be cleared. 568 569ext4 also supports encryption, which can be used simultaneously with 570fs-verity. In this case, the plaintext data is verified rather than 571the ciphertext. This is necessary in order to make the fs-verity file 572digest meaningful, since every file is encrypted differently. 573 574ext4 stores the verity metadata (Merkle tree and fsverity_descriptor) 575past the end of the file, starting at the first 64K boundary beyond 576i_size. This approach works because (a) verity files are readonly, 577and (b) pages fully beyond i_size aren't visible to userspace but can 578be read/written internally by ext4 with only some relatively small 579changes to ext4. This approach avoids having to depend on the 580EA_INODE feature and on rearchitecturing ext4's xattr support to 581support paging multi-gigabyte xattrs into memory, and to support 582encrypting xattrs. Note that the verity metadata *must* be encrypted 583when the file is, since it contains hashes of the plaintext data. 584 585ext4 only allows verity on extent-based files. 586 587f2fs 588---- 589 590f2fs supports fs-verity since Linux v5.4 and f2fs-tools v1.11.0. 591 592To create verity files on an f2fs filesystem, the filesystem must have 593been formatted with ``-O verity``. 594 595f2fs sets the FADVISE_VERITY_BIT on-disk inode flag on verity files. 596It can only be set by `FS_IOC_ENABLE_VERITY`_, and it cannot be 597cleared. 598 599Like ext4, f2fs stores the verity metadata (Merkle tree and 600fsverity_descriptor) past the end of the file, starting at the first 60164K boundary beyond i_size. See explanation for ext4 above. 602Moreover, f2fs supports at most 4096 bytes of xattr entries per inode 603which usually wouldn't be enough for even a single Merkle tree block. 604 605f2fs doesn't support enabling verity on files that currently have 606atomic or volatile writes pending. 607 608btrfs 609----- 610 611btrfs supports fs-verity since Linux v5.15. Verity-enabled inodes are 612marked with a RO_COMPAT inode flag, and the verity metadata is stored 613in separate btree items. 614 615Implementation details 616====================== 617 618Verifying data 619-------------- 620 621fs-verity ensures that all reads of a verity file's data are verified, 622regardless of which syscall is used to do the read (e.g. mmap(), 623read(), pread()) and regardless of whether it's the first read or a 624later read (unless the later read can return cached data that was 625already verified). Below, we describe how filesystems implement this. 626 627Pagecache 628~~~~~~~~~ 629 630For filesystems using Linux's pagecache, the ``->read_folio()`` and 631``->readahead()`` methods must be modified to verify folios before 632they are marked Uptodate. Merely hooking ``->read_iter()`` would be 633insufficient, since ``->read_iter()`` is not used for memory maps. 634 635Therefore, fs/verity/ provides the function fsverity_verify_blocks() 636which verifies data that has been read into the pagecache of a verity 637inode. The containing folio must still be locked and not Uptodate, so 638it's not yet readable by userspace. As needed to do the verification, 639fsverity_verify_blocks() will call back into the filesystem to read 640hash blocks via fsverity_operations::read_merkle_tree_page(). 641 642fsverity_verify_blocks() returns false if verification failed; in this 643case, the filesystem must not set the folio Uptodate. Following this, 644as per the usual Linux pagecache behavior, attempts by userspace to 645read() from the part of the file containing the folio will fail with 646EIO, and accesses to the folio within a memory map will raise SIGBUS. 647 648In principle, verifying a data block requires verifying the entire 649path in the Merkle tree from the data block to the root hash. 650However, for efficiency the filesystem may cache the hash blocks. 651Therefore, fsverity_verify_blocks() only ascends the tree reading hash 652blocks until an already-verified hash block is seen. It then verifies 653the path to that block. 654 655This optimization, which is also used by dm-verity, results in 656excellent sequential read performance. This is because usually (e.g. 657127 in 128 times for 4K blocks and SHA-256) the hash block from the 658bottom level of the tree will already be cached and checked from 659reading a previous data block. However, random reads perform worse. 660 661Block device based filesystems 662~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 663 664Block device based filesystems (e.g. ext4 and f2fs) in Linux also use 665the pagecache, so the above subsection applies too. However, they 666also usually read many data blocks from a file at once, grouped into a 667structure called a "bio". To make it easier for these types of 668filesystems to support fs-verity, fs/verity/ also provides a function 669fsverity_verify_bio() which verifies all data blocks in a bio. 670 671ext4 and f2fs also support encryption. If a verity file is also 672encrypted, the data must be decrypted before being verified. To 673support this, these filesystems allocate a "post-read context" for 674each bio and store it in ``->bi_private``:: 675 676 struct bio_post_read_ctx { 677 struct bio *bio; 678 struct work_struct work; 679 unsigned int cur_step; 680 unsigned int enabled_steps; 681 }; 682 683``enabled_steps`` is a bitmask that specifies whether decryption, 684verity, or both is enabled. After the bio completes, for each needed 685postprocessing step the filesystem enqueues the bio_post_read_ctx on a 686workqueue, and then the workqueue work does the decryption or 687verification. Finally, folios where no decryption or verity error 688occurred are marked Uptodate, and the folios are unlocked. 689 690On many filesystems, files can contain holes. Normally, 691``->readahead()`` simply zeroes hole blocks and considers the 692corresponding data to be up-to-date; no bios are issued. To prevent 693this case from bypassing fs-verity, filesystems use 694fsverity_verify_blocks() to verify hole blocks. 695 696Filesystems also disable direct I/O on verity files, since otherwise 697direct I/O would bypass fs-verity. 698 699Userspace utility 700================= 701 702This document focuses on the kernel, but a userspace utility for 703fs-verity can be found at: 704 705 https://git.kernel.org/pub/scm/fs/fsverity/fsverity-utils.git 706 707See the README.md file in the fsverity-utils source tree for details, 708including examples of setting up fs-verity protected files. 709 710Tests 711===== 712 713To test fs-verity, use xfstests. For example, using `kvm-xfstests 714<https://github.com/tytso/xfstests-bld/blob/master/Documentation/kvm-quickstart.md>`_:: 715 716 kvm-xfstests -c ext4,f2fs,btrfs -g verity 717 718FAQ 719=== 720 721This section answers frequently asked questions about fs-verity that 722weren't already directly answered in other parts of this document. 723 724:Q: Why isn't fs-verity part of IMA? 725:A: fs-verity and IMA (Integrity Measurement Architecture) have 726 different focuses. fs-verity is a filesystem-level mechanism for 727 hashing individual files using a Merkle tree. In contrast, IMA 728 specifies a system-wide policy that specifies which files are 729 hashed and what to do with those hashes, such as log them, 730 authenticate them, or add them to a measurement list. 731 732 IMA supports the fs-verity hashing mechanism as an alternative 733 to full file hashes, for those who want the performance and 734 security benefits of the Merkle tree based hash. However, it 735 doesn't make sense to force all uses of fs-verity to be through 736 IMA. fs-verity already meets many users' needs even as a 737 standalone filesystem feature, and it's testable like other 738 filesystem features e.g. with xfstests. 739 740:Q: Isn't fs-verity useless because the attacker can just modify the 741 hashes in the Merkle tree, which is stored on-disk? 742:A: To verify the authenticity of an fs-verity file you must verify 743 the authenticity of the "fs-verity file digest", which 744 incorporates the root hash of the Merkle tree. See `Use cases`_. 745 746:Q: Isn't fs-verity useless because the attacker can just replace a 747 verity file with a non-verity one? 748:A: See `Use cases`_. In the initial use case, it's really trusted 749 userspace code that authenticates the files; fs-verity is just a 750 tool to do this job efficiently and securely. The trusted 751 userspace code will consider non-verity files to be inauthentic. 752 753:Q: Why does the Merkle tree need to be stored on-disk? Couldn't you 754 store just the root hash? 755:A: If the Merkle tree wasn't stored on-disk, then you'd have to 756 compute the entire tree when the file is first accessed, even if 757 just one byte is being read. This is a fundamental consequence of 758 how Merkle tree hashing works. To verify a leaf node, you need to 759 verify the whole path to the root hash, including the root node 760 (the thing which the root hash is a hash of). But if the root 761 node isn't stored on-disk, you have to compute it by hashing its 762 children, and so on until you've actually hashed the entire file. 763 764 That defeats most of the point of doing a Merkle tree-based hash, 765 since if you have to hash the whole file ahead of time anyway, 766 then you could simply do sha256(file) instead. That would be much 767 simpler, and a bit faster too. 768 769 It's true that an in-memory Merkle tree could still provide the 770 advantage of verification on every read rather than just on the 771 first read. However, it would be inefficient because every time a 772 hash page gets evicted (you can't pin the entire Merkle tree into 773 memory, since it may be very large), in order to restore it you 774 again need to hash everything below it in the tree. This again 775 defeats most of the point of doing a Merkle tree-based hash, since 776 a single block read could trigger re-hashing gigabytes of data. 777 778:Q: But couldn't you store just the leaf nodes and compute the rest? 779:A: See previous answer; this really just moves up one level, since 780 one could alternatively interpret the data blocks as being the 781 leaf nodes of the Merkle tree. It's true that the tree can be 782 computed much faster if the leaf level is stored rather than just 783 the data, but that's only because each level is less than 1% the 784 size of the level below (assuming the recommended settings of 785 SHA-256 and 4K blocks). For the exact same reason, by storing 786 "just the leaf nodes" you'd already be storing over 99% of the 787 tree, so you might as well simply store the whole tree. 788 789:Q: Can the Merkle tree be built ahead of time, e.g. distributed as 790 part of a package that is installed to many computers? 791:A: This isn't currently supported. It was part of the original 792 design, but was removed to simplify the kernel UAPI and because it 793 wasn't a critical use case. Files are usually installed once and 794 used many times, and cryptographic hashing is somewhat fast on 795 most modern processors. 796 797:Q: Why doesn't fs-verity support writes? 798:A: Write support would be very difficult and would require a 799 completely different design, so it's well outside the scope of 800 fs-verity. Write support would require: 801 802 - A way to maintain consistency between the data and hashes, 803 including all levels of hashes, since corruption after a crash 804 (especially of potentially the entire file!) is unacceptable. 805 The main options for solving this are data journalling, 806 copy-on-write, and log-structured volume. But it's very hard to 807 retrofit existing filesystems with new consistency mechanisms. 808 Data journalling is available on ext4, but is very slow. 809 810 - Rebuilding the Merkle tree after every write, which would be 811 extremely inefficient. Alternatively, a different authenticated 812 dictionary structure such as an "authenticated skiplist" could 813 be used. However, this would be far more complex. 814 815 Compare it to dm-verity vs. dm-integrity. dm-verity is very 816 simple: the kernel just verifies read-only data against a 817 read-only Merkle tree. In contrast, dm-integrity supports writes 818 but is slow, is much more complex, and doesn't actually support 819 full-device authentication since it authenticates each sector 820 independently, i.e. there is no "root hash". It doesn't really 821 make sense for the same device-mapper target to support these two 822 very different cases; the same applies to fs-verity. 823 824:Q: Since verity files are immutable, why isn't the immutable bit set? 825:A: The existing "immutable" bit (FS_IMMUTABLE_FL) already has a 826 specific set of semantics which not only make the file contents 827 read-only, but also prevent the file from being deleted, renamed, 828 linked to, or having its owner or mode changed. These extra 829 properties are unwanted for fs-verity, so reusing the immutable 830 bit isn't appropriate. 831 832:Q: Why does the API use ioctls instead of setxattr() and getxattr()? 833:A: Abusing the xattr interface for basically arbitrary syscalls is 834 heavily frowned upon by most of the Linux filesystem developers. 835 An xattr should really just be an xattr on-disk, not an API to 836 e.g. magically trigger construction of a Merkle tree. 837 838:Q: Does fs-verity support remote filesystems? 839:A: So far all filesystems that have implemented fs-verity support are 840 local filesystems, but in principle any filesystem that can store 841 per-file verity metadata can support fs-verity, regardless of 842 whether it's local or remote. Some filesystems may have fewer 843 options of where to store the verity metadata; one possibility is 844 to store it past the end of the file and "hide" it from userspace 845 by manipulating i_size. The data verification functions provided 846 by ``fs/verity/`` also assume that the filesystem uses the Linux 847 pagecache, but both local and remote filesystems normally do so. 848 849:Q: Why is anything filesystem-specific at all? Shouldn't fs-verity 850 be implemented entirely at the VFS level? 851:A: There are many reasons why this is not possible or would be very 852 difficult, including the following: 853 854 - To prevent bypassing verification, folios must not be marked 855 Uptodate until they've been verified. Currently, each 856 filesystem is responsible for marking folios Uptodate via 857 ``->readahead()``. Therefore, currently it's not possible for 858 the VFS to do the verification on its own. Changing this would 859 require significant changes to the VFS and all filesystems. 860 861 - It would require defining a filesystem-independent way to store 862 the verity metadata. Extended attributes don't work for this 863 because (a) the Merkle tree may be gigabytes, but many 864 filesystems assume that all xattrs fit into a single 4K 865 filesystem block, and (b) ext4 and f2fs encryption doesn't 866 encrypt xattrs, yet the Merkle tree *must* be encrypted when the 867 file contents are, because it stores hashes of the plaintext 868 file contents. 869 870 So the verity metadata would have to be stored in an actual 871 file. Using a separate file would be very ugly, since the 872 metadata is fundamentally part of the file to be protected, and 873 it could cause problems where users could delete the real file 874 but not the metadata file or vice versa. On the other hand, 875 having it be in the same file would break applications unless 876 filesystems' notion of i_size were divorced from the VFS's, 877 which would be complex and require changes to all filesystems. 878 879 - It's desirable that FS_IOC_ENABLE_VERITY uses the filesystem's 880 transaction mechanism so that either the file ends up with 881 verity enabled, or no changes were made. Allowing intermediate 882 states to occur after a crash may cause problems. 883