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 .. SPDX-License-Identifier: GPL-2.0
 
 .. _fsverity:
 
 =======================================================
 fs-verity: read-only file-based authenticity protection
 =======================================================
 
 Introduction
 ============
 
 fs-verity (``fs/verity/``) is a support layer that filesystems can
 hook into to support transparent integrity and authenticity protection
 of read-only files.  Currently, it is supported by the ext4, f2fs, and
 btrfs filesystems.  Like fscrypt, not too much filesystem-specific
 code is needed to support fs-verity.
 
 fs-verity is similar to `dm-verity
 <https://www.kernel.org/doc/Documentation/device-mapper/verity.txt>`_
 but works on files rather than block devices.  On regular files on
 filesystems supporting fs-verity, userspace can execute an ioctl that
 causes the filesystem to build a Merkle tree for the file and persist
 it to a filesystem-specific location associated with the file.
 
 After this, the file is made readonly, and all reads from the file are
 automatically verified against the file's Merkle tree.  Reads of any
 corrupted data, including mmap reads, will fail.
 
 Userspace can use another ioctl to retrieve the root hash (actually
 the "fs-verity file digest", which is a hash that includes the Merkle
 tree root hash) that fs-verity is enforcing for the file.  This ioctl
 executes in constant time, regardless of the file size.
 
 fs-verity is essentially a way to hash a file in constant time,
 subject to the caveat that reads which would violate the hash will
 fail at runtime.
 
 Use cases
 =========
 
 By itself, the base fs-verity feature only provides integrity
 protection, i.e. detection of accidental (non-malicious) corruption.
 
 However, because fs-verity makes retrieving the file hash extremely
 efficient, it's primarily meant to be used as a tool to support
 authentication (detection of malicious modifications) or auditing
 (logging file hashes before use).
 
 Trusted userspace code (e.g. operating system code running on a
 read-only partition that is itself authenticated by dm-verity) can
 authenticate the contents of an fs-verity file by using the
 `FS_IOC_MEASURE_VERITY`_ ioctl to retrieve its hash, then verifying a
 digital signature of it.
 
 A standard file hash could be used instead of fs-verity.  However,
 this is inefficient if the file is large and only a small portion may
 be accessed.  This is often the case for Android application package
 (APK) files, for example.  These typically contain many translations,
 classes, and other resources that are infrequently or even never
 accessed on a particular device.  It would be slow and wasteful to
 read and hash the entire file before starting the application.
 
 Unlike an ahead-of-time hash, fs-verity also re-verifies data each
 time it's paged in.  This ensures that malicious disk firmware can't
 undetectably change the contents of the file at runtime.
 
 fs-verity does not replace or obsolete dm-verity.  dm-verity should
 still be used on read-only filesystems.  fs-verity is for files that
 must live on a read-write filesystem because they are independently
 updated and potentially user-installed, so dm-verity cannot be used.
 
 The base fs-verity feature is a hashing mechanism only; actually
 authenticating the files may be done by:
 
 * Userspace-only
 
 * Builtin signature verification + userspace policy
 
   fs-verity optionally supports a simple signature verification
   mechanism where users can configure the kernel to require that
   all fs-verity files be signed by a key loaded into a keyring;
   see `Built-in signature verification`_.
 
 * Integrity Measurement Architecture (IMA)
 
   IMA supports including fs-verity file digests and signatures in the
   IMA measurement list and verifying fs-verity based file signatures
   stored as security.ima xattrs, based on policy.
 
 
 User API
 ========
 
 FS_IOC_ENABLE_VERITY
 --------------------
 
 The FS_IOC_ENABLE_VERITY ioctl enables fs-verity on a file.  It takes
 in a pointer to a struct fsverity_enable_arg, defined as
 follows::
 
     struct fsverity_enable_arg {
             __u32 version;
             __u32 hash_algorithm;
             __u32 block_size;
             __u32 salt_size;
             __u64 salt_ptr;
             __u32 sig_size;
             __u32 __reserved1;
             __u64 sig_ptr;
             __u64 __reserved2[11];
     };
 
 This structure contains the parameters of the Merkle tree to build for
 the file, and optionally contains a signature.  It must be initialized
 as follows:
 
 - ``version`` must be 1.
 - ``hash_algorithm`` must be the identifier for the hash algorithm to
   use for the Merkle tree, such as FS_VERITY_HASH_ALG_SHA256.  See
   ``include/uapi/linux/fsverity.h`` for the list of possible values.
 - ``block_size`` must be the Merkle tree block size.  Currently, this
   must be equal to the system page size, which is usually 4096 bytes.
   Other sizes may be supported in the future.  This value is not
   necessarily the same as the filesystem block size.
 - ``salt_size`` is the size of the salt in bytes, or 0 if no salt is
   provided.  The salt is a value that is prepended to every hashed
   block; it can be used to personalize the hashing for a particular
   file or device.  Currently the maximum salt size is 32 bytes.
 - ``salt_ptr`` is the pointer to the salt, or NULL if no salt is
   provided.
 - ``sig_size`` is the size of the signature in bytes, or 0 if no
   signature is provided.  Currently the signature is (somewhat
   arbitrarily) limited to 16128 bytes.  See `Built-in signature
   verification`_ for more information.
 - ``sig_ptr``  is the pointer to the signature, or NULL if no
   signature is provided.
 - All reserved fields must be zeroed.
 
 FS_IOC_ENABLE_VERITY causes the filesystem to build a Merkle tree for
 the file and persist it to a filesystem-specific location associated
 with the file, then mark the file as a verity file.  This ioctl may
 take a long time to execute on large files, and it is interruptible by
 fatal signals.
 
 FS_IOC_ENABLE_VERITY checks for write access to the inode.  However,
 it must be executed on an O_RDONLY file descriptor and no processes
 can have the file open for writing.  Attempts to open the file for
 writing while this ioctl is executing will fail with ETXTBSY.  (This
 is necessary to guarantee that no writable file descriptors will exist
 after verity is enabled, and to guarantee that the file's contents are
 stable while the Merkle tree is being built over it.)
 
 On success, FS_IOC_ENABLE_VERITY returns 0, and the file becomes a
 verity file.  On failure (including the case of interruption by a
 fatal signal), no changes are made to the file.
 
 FS_IOC_ENABLE_VERITY can fail with the following errors:
 
 - ``EACCES``: the process does not have write access to the file
 - ``EBADMSG``: the signature is malformed
 - ``EBUSY``: this ioctl is already running on the file
 - ``EEXIST``: the file already has verity enabled
 - ``EFAULT``: the caller provided inaccessible memory
 - ``EINTR``: the operation was interrupted by a fatal signal
 - ``EINVAL``: unsupported version, hash algorithm, or block size; or
   reserved bits are set; or the file descriptor refers to neither a
   regular file nor a directory.
 - ``EISDIR``: the file descriptor refers to a directory
 - ``EKEYREJECTED``: the signature doesn't match the file
 - ``EMSGSIZE``: the salt or signature is too long
 - ``ENOKEY``: the fs-verity keyring doesn't contain the certificate
   needed to verify the signature
 - ``ENOPKG``: fs-verity recognizes the hash algorithm, but it's not
   available in the kernel's crypto API as currently configured (e.g.
   for SHA-512, missing CONFIG_CRYPTO_SHA512).
 - ``ENOTTY``: this type of filesystem does not implement fs-verity
 - ``EOPNOTSUPP``: the kernel was not configured with fs-verity
   support; or the filesystem superblock has not had the 'verity'
   feature enabled on it; or the filesystem does not support fs-verity
   on this file.  (See `Filesystem support`_.)
 - ``EPERM``: the file is append-only; or, a signature is required and
   one was not provided.
 - ``EROFS``: the filesystem is read-only
 - ``ETXTBSY``: someone has the file open for writing.  This can be the
   caller's file descriptor, another open file descriptor, or the file
   reference held by a writable memory map.
 
 FS_IOC_MEASURE_VERITY
 ---------------------
 
 The FS_IOC_MEASURE_VERITY ioctl retrieves the digest of a verity file.
 The fs-verity file digest is a cryptographic digest that identifies
 the file contents that are being enforced on reads; it is computed via
 a Merkle tree and is different from a traditional full-file digest.
 
 This ioctl takes in a pointer to a variable-length structure::
 
     struct fsverity_digest {
             __u16 digest_algorithm;
             __u16 digest_size; /* input/output */
             __u8 digest[];
     };
 
 ``digest_size`` is an input/output field.  On input, it must be
 initialized to the number of bytes allocated for the variable-length
 ``digest`` field.
 
 On success, 0 is returned and the kernel fills in the structure as
 follows:
 
 - ``digest_algorithm`` will be the hash algorithm used for the file
   digest.  It will match ``fsverity_enable_arg::hash_algorithm``.
 - ``digest_size`` will be the size of the digest in bytes, e.g. 32
   for SHA-256.  (This can be redundant with ``digest_algorithm``.)
 - ``digest`` will be the actual bytes of the digest.
 
 FS_IOC_MEASURE_VERITY is guaranteed to execute in constant time,
 regardless of the size of the file.
 
 FS_IOC_MEASURE_VERITY can fail with the following errors:
 
 - ``EFAULT``: the caller provided inaccessible memory
 - ``ENODATA``: the file is not a verity file
 - ``ENOTTY``: this type of filesystem does not implement fs-verity
 - ``EOPNOTSUPP``: the kernel was not configured with fs-verity
   support, or the filesystem superblock has not had the 'verity'
   feature enabled on it.  (See `Filesystem support`_.)
 - ``EOVERFLOW``: the digest is longer than the specified
   ``digest_size`` bytes.  Try providing a larger buffer.
 
 FS_IOC_READ_VERITY_METADATA
 ---------------------------
 
 The FS_IOC_READ_VERITY_METADATA ioctl reads verity metadata from a
 verity file.  This ioctl is available since Linux v5.12.
 
 This ioctl allows writing a server program that takes a verity file
 and serves it to a client program, such that the client can do its own
 fs-verity compatible verification of the file.  This only makes sense
 if the client doesn't trust the server and if the server needs to
 provide the storage for the client.
 
 This is a fairly specialized use case, and most fs-verity users won't
 need this ioctl.
 
 This ioctl takes in a pointer to the following structure::
 
    #define FS_VERITY_METADATA_TYPE_MERKLE_TREE     1
    #define FS_VERITY_METADATA_TYPE_DESCRIPTOR      2
    #define FS_VERITY_METADATA_TYPE_SIGNATURE       3
 
    struct fsverity_read_metadata_arg {
            __u64 metadata_type;
            __u64 offset;
            __u64 length;
            __u64 buf_ptr;
            __u64 __reserved;
    };
 
 ``metadata_type`` specifies the type of metadata to read:
 
 - ``FS_VERITY_METADATA_TYPE_MERKLE_TREE`` reads the blocks of the
   Merkle tree.  The blocks are returned in order from the root level
   to the leaf level.  Within each level, the blocks are returned in
   the same order that their hashes are themselves hashed.
   See `Merkle tree`_ for more information.
 
 - ``FS_VERITY_METADATA_TYPE_DESCRIPTOR`` reads the fs-verity
   descriptor.  See `fs-verity descriptor`_.
 
 - ``FS_VERITY_METADATA_TYPE_SIGNATURE`` reads the signature which was
   passed to FS_IOC_ENABLE_VERITY, if any.  See `Built-in signature
   verification`_.
 
 The semantics are similar to those of ``pread()``.  ``offset``
 specifies the offset in bytes into the metadata item to read from, and
 ``length`` specifies the maximum number of bytes to read from the
 metadata item.  ``buf_ptr`` is the pointer to the buffer to read into,
 cast to a 64-bit integer.  ``__reserved`` must be 0.  On success, the
 number of bytes read is returned.  0 is returned at the end of the
 metadata item.  The returned length may be less than ``length``, for
 example if the ioctl is interrupted.
 
 The metadata returned by FS_IOC_READ_VERITY_METADATA isn't guaranteed
 to be authenticated against the file digest that would be returned by
 `FS_IOC_MEASURE_VERITY`_, as the metadata is expected to be used to
 implement fs-verity compatible verification anyway (though absent a
 malicious disk, the metadata will indeed match).  E.g. to implement
 this ioctl, the filesystem is allowed to just read the Merkle tree
 blocks from disk without actually verifying the path to the root node.
 
 FS_IOC_READ_VERITY_METADATA can fail with the following errors:
 
 - ``EFAULT``: the caller provided inaccessible memory
 - ``EINTR``: the ioctl was interrupted before any data was read
 - ``EINVAL``: reserved fields were set, or ``offset + length``
   overflowed
 - ``ENODATA``: the file is not a verity file, or
   FS_VERITY_METADATA_TYPE_SIGNATURE was requested but the file doesn't
   have a built-in signature
 - ``ENOTTY``: this type of filesystem does not implement fs-verity, or
   this ioctl is not yet implemented on it
 - ``EOPNOTSUPP``: the kernel was not configured with fs-verity
   support, or the filesystem superblock has not had the 'verity'
   feature enabled on it.  (See `Filesystem support`_.)
 
 FS_IOC_GETFLAGS
 ---------------
 
 The existing ioctl FS_IOC_GETFLAGS (which isn't specific to fs-verity)
 can also be used to check whether a file has fs-verity enabled or not.
 To do so, check for FS_VERITY_FL (0x00100000) in the returned flags.
 
 The verity flag is not settable via FS_IOC_SETFLAGS.  You must use
 FS_IOC_ENABLE_VERITY instead, since parameters must be provided.
 
 statx
 -----
 
 Since Linux v5.5, the statx() system call sets STATX_ATTR_VERITY if
 the file has fs-verity enabled.  This can perform better than
 FS_IOC_GETFLAGS and FS_IOC_MEASURE_VERITY because it doesn't require
 opening the file, and opening verity files can be expensive.
 
 Accessing verity files
 ======================
 
 Applications can transparently access a verity file just like a
 non-verity one, with the following exceptions:
 
 - Verity files are readonly.  They cannot be opened for writing or
   truncate()d, even if the file mode bits allow it.  Attempts to do
   one of these things will fail with EPERM.  However, changes to
   metadata such as owner, mode, timestamps, and xattrs are still
   allowed, since these are not measured by fs-verity.  Verity files
   can also still be renamed, deleted, and linked to.
 
 - Direct I/O is not supported on verity files.  Attempts to use direct
   I/O on such files will fall back to buffered I/O.
 
 - DAX (Direct Access) is not supported on verity files, because this
   would circumvent the data verification.
 
 - Reads of data that doesn't match the verity Merkle tree will fail
   with EIO (for read()) or SIGBUS (for mmap() reads).
 
 - If the sysctl "fs.verity.require_signatures" is set to 1 and the
   file is not signed by a key in the fs-verity keyring, then opening
   the file will fail.  See `Built-in signature verification`_.
 
 Direct access to the Merkle tree is not supported.  Therefore, if a
 verity file is copied, or is backed up and restored, then it will lose
 its "verity"-ness.  fs-verity is primarily meant for files like
 executables that are managed by a package manager.
 
 File digest computation
 =======================
 
 This section describes how fs-verity hashes the file contents using a
 Merkle tree to produce the digest which cryptographically identifies
 the file contents.  This algorithm is the same for all filesystems
 that support fs-verity.
 
 Userspace only needs to be aware of this algorithm if it needs to
 compute fs-verity file digests itself, e.g. in order to sign files.
 
 .. _fsverity_merkle_tree:
 
 Merkle tree
 -----------
 
 The file contents is divided into blocks, where the block size is
 configurable but is usually 4096 bytes.  The end of the last block is
 zero-padded if needed.  Each block is then hashed, producing the first
 level of hashes.  Then, the hashes in this first level are grouped
 into 'blocksize'-byte blocks (zero-padding the ends as needed) and
 these blocks are hashed, producing the second level of hashes.  This
 proceeds up the tree until only a single block remains.  The hash of
 this block is the "Merkle tree root hash".
 
 If the file fits in one block and is nonempty, then the "Merkle tree
 root hash" is simply the hash of the single data block.  If the file
 is empty, then the "Merkle tree root hash" is all zeroes.
 
 The "blocks" here are not necessarily the same as "filesystem blocks".
 
 If a salt was specified, then it's zero-padded to the closest multiple
 of the input size of the hash algorithm's compression function, e.g.
 64 bytes for SHA-256 or 128 bytes for SHA-512.  The padded salt is
 prepended to every data or Merkle tree block that is hashed.
 
 The purpose of the block padding is to cause every hash to be taken
 over the same amount of data, which simplifies the implementation and
 keeps open more possibilities for hardware acceleration.  The purpose
 of the salt padding is to make the salting "free" when the salted hash
 state is precomputed, then imported for each hash.
 
 Example: in the recommended configuration of SHA-256 and 4K blocks,
 128 hash values fit in each block.  Thus, each level of the Merkle
 tree is approximately 128 times smaller than the previous, and for
 large files the Merkle tree's size converges to approximately 1/127 of
 the original file size.  However, for small files, the padding is
 significant, making the space overhead proportionally more.
 
 .. _fsverity_descriptor:
 
 fs-verity descriptor
 --------------------
 
 By itself, the Merkle tree root hash is ambiguous.  For example, it
 can't a distinguish a large file from a small second file whose data
 is exactly the top-level hash block of the first file.  Ambiguities
 also arise from the convention of padding to the next block boundary.
 
 To solve this problem, the fs-verity file digest is actually computed
 as a hash of the following structure, which contains the Merkle tree
 root hash as well as other fields such as the file size::
 
     struct fsverity_descriptor {
             __u8 version;           /* must be 1 */
             __u8 hash_algorithm;    /* Merkle tree hash algorithm */
             __u8 log_blocksize;     /* log2 of size of data and tree blocks */
             __u8 salt_size;         /* size of salt in bytes; 0 if none */
             __le32 __reserved_0x04; /* must be 0 */
             __le64 data_size;       /* size of file the Merkle tree is built over */
             __u8 root_hash[64];     /* Merkle tree root hash */
             __u8 salt[32];          /* salt prepended to each hashed block */
             __u8 __reserved[144];   /* must be 0's */
     };
 
 Built-in signature verification
 ===============================
 
 With CONFIG_FS_VERITY_BUILTIN_SIGNATURES=y, fs-verity supports putting
 a portion of an authentication policy (see `Use cases`_) in the
 kernel.  Specifically, it adds support for:
 
 1. At fs-verity module initialization time, a keyring ".fs-verity" is
    created.  The root user can add trusted X.509 certificates to this
    keyring using the add_key() system call, then (when done)
    optionally use keyctl_restrict_keyring() to prevent additional
    certificates from being added.
 
 2. `FS_IOC_ENABLE_VERITY`_ accepts a pointer to a PKCS#7 formatted
    detached signature in DER format of the file's fs-verity digest.
    On success, this signature is persisted alongside the Merkle tree.
    Then, any time the file is opened, the kernel will verify the
    file's actual digest against this signature, using the certificates
    in the ".fs-verity" keyring.
 
 3. A new sysctl "fs.verity.require_signatures" is made available.
    When set to 1, the kernel requires that all verity files have a
    correctly signed digest as described in (2).
 
 fs-verity file digests must be signed in the following format, which
 is similar to the structure used by `FS_IOC_MEASURE_VERITY`_::
 
     struct fsverity_formatted_digest {
             char magic[8];                  /* must be "FSVerity" */
             __le16 digest_algorithm;
             __le16 digest_size;
             __u8 digest[];
     };
 
 fs-verity's built-in signature verification support is meant as a
 relatively simple mechanism that can be used to provide some level of
 authenticity protection for verity files, as an alternative to doing
 the signature verification in userspace or using IMA-appraisal.
 However, with this mechanism, userspace programs still need to check
 that the verity bit is set, and there is no protection against verity
 files being swapped around.
 
 Filesystem support
 ==================
 
 fs-verity is supported by several filesystems, described below.  The
 CONFIG_FS_VERITY kconfig option must be enabled to use fs-verity on
 any of these filesystems.
 
 ``include/linux/fsverity.h`` declares the interface between the
 ``fs/verity/`` support layer and filesystems.  Briefly, filesystems
 must provide an ``fsverity_operations`` structure that provides
 methods to read and write the verity metadata to a filesystem-specific
 location, including the Merkle tree blocks and
 ``fsverity_descriptor``.  Filesystems must also call functions in
 ``fs/verity/`` at certain times, such as when a file is opened or when
 pages have been read into the pagecache.  (See `Verifying data`_.)
 
 ext4
 ----
 
 ext4 supports fs-verity since Linux v5.4 and e2fsprogs v1.45.2.
 
 To create verity files on an ext4 filesystem, the filesystem must have
 been formatted with ``-O verity`` or had ``tune2fs -O verity`` run on
 it.  "verity" is an RO_COMPAT filesystem feature, so once set, old
 kernels will only be able to mount the filesystem readonly, and old
 versions of e2fsck will be unable to check the filesystem.  Moreover,
 currently ext4 only supports mounting a filesystem with the "verity"
 feature when its block size is equal to PAGE_SIZE (often 4096 bytes).
 
 ext4 sets the EXT4_VERITY_FL on-disk inode flag on verity files.  It
 can only be set by `FS_IOC_ENABLE_VERITY`_, and it cannot be cleared.
 
 ext4 also supports encryption, which can be used simultaneously with
 fs-verity.  In this case, the plaintext data is verified rather than
 the ciphertext.  This is necessary in order to make the fs-verity file
 digest meaningful, since every file is encrypted differently.
 
 ext4 stores the verity metadata (Merkle tree and fsverity_descriptor)
 past the end of the file, starting at the first 64K boundary beyond
 i_size.  This approach works because (a) verity files are readonly,
 and (b) pages fully beyond i_size aren't visible to userspace but can
 be read/written internally by ext4 with only some relatively small
 changes to ext4.  This approach avoids having to depend on the
 EA_INODE feature and on rearchitecturing ext4's xattr support to
 support paging multi-gigabyte xattrs into memory, and to support
 encrypting xattrs.  Note that the verity metadata *must* be encrypted
 when the file is, since it contains hashes of the plaintext data.
 
 Currently, ext4 verity only supports the case where the Merkle tree
 block size, filesystem block size, and page size are all the same.  It
 also only supports extent-based files.
 
 f2fs
 ----
 
 f2fs supports fs-verity since Linux v5.4 and f2fs-tools v1.11.0.
 
 To create verity files on an f2fs filesystem, the filesystem must have
 been formatted with ``-O verity``.
 
 f2fs sets the FADVISE_VERITY_BIT on-disk inode flag on verity files.
 It can only be set by `FS_IOC_ENABLE_VERITY`_, and it cannot be
 cleared.
 
 Like ext4, f2fs stores the verity metadata (Merkle tree and
 fsverity_descriptor) past the end of the file, starting at the first
 64K boundary beyond i_size.  See explanation for ext4 above.
 Moreover, f2fs supports at most 4096 bytes of xattr entries per inode
 which wouldn't be enough for even a single Merkle tree block.
 
 Currently, f2fs verity only supports a Merkle tree block size of 4096.
 Also, f2fs doesn't support enabling verity on files that currently
 have atomic or volatile writes pending.
 
 btrfs
 -----
 
 btrfs supports fs-verity since Linux v5.15.  Verity-enabled inodes are
 marked with a RO_COMPAT inode flag, and the verity metadata is stored
 in separate btree items.
 
 Implementation details
 ======================
 
 Verifying data
 --------------
 
 fs-verity ensures that all reads of a verity file's data are verified,
 regardless of which syscall is used to do the read (e.g. mmap(),
 read(), pread()) and regardless of whether it's the first read or a
 later read (unless the later read can return cached data that was
 already verified).  Below, we describe how filesystems implement this.
 
 Pagecache
 ~~~~~~~~~
 
 For filesystems using Linux's pagecache, the ``->read_folio()`` and
 ``->readahead()`` methods must be modified to verify pages before they
 are marked Uptodate.  Merely hooking ``->read_iter()`` would be
 insufficient, since ``->read_iter()`` is not used for memory maps.
 
 Therefore, fs/verity/ provides a function fsverity_verify_page() which
 verifies a page that has been read into the pagecache of a verity
 inode, but is still locked and not Uptodate, so it's not yet readable
 by userspace.  As needed to do the verification,
 fsverity_verify_page() will call back into the filesystem to read
 Merkle tree pages via fsverity_operations::read_merkle_tree_page().
 
 fsverity_verify_page() returns false if verification failed; in this
 case, the filesystem must not set the page Uptodate.  Following this,
 as per the usual Linux pagecache behavior, attempts by userspace to
 read() from the part of the file containing the page will fail with
 EIO, and accesses to the page within a memory map will raise SIGBUS.
 
 fsverity_verify_page() currently only supports the case where the
 Merkle tree block size is equal to PAGE_SIZE (often 4096 bytes).
 
 In principle, fsverity_verify_page() verifies the entire path in the
 Merkle tree from the data page to the root hash.  However, for
 efficiency the filesystem may cache the hash pages.  Therefore,
 fsverity_verify_page() only ascends the tree reading hash pages until
 an already-verified hash page is seen, as indicated by the PageChecked
 bit being set.  It then verifies the path to that page.
 
 This optimization, which is also used by dm-verity, results in
 excellent sequential read performance.  This is because usually (e.g.
 127 in 128 times for 4K blocks and SHA-256) the hash page from the
 bottom level of the tree will already be cached and checked from
 reading a previous data page.  However, random reads perform worse.
 
 Block device based filesystems
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
 Block device based filesystems (e.g. ext4 and f2fs) in Linux also use
 the pagecache, so the above subsection applies too.  However, they
 also usually read many pages from a file at once, grouped into a
 structure called a "bio".  To make it easier for these types of
 filesystems to support fs-verity, fs/verity/ also provides a function
 fsverity_verify_bio() which verifies all pages in a bio.
 
 ext4 and f2fs also support encryption.  If a verity file is also
 encrypted, the pages must be decrypted before being verified.  To
 support this, these filesystems allocate a "post-read context" for
 each bio and store it in ``->bi_private``::
 
     struct bio_post_read_ctx {
            struct bio *bio;
            struct work_struct work;
            unsigned int cur_step;
            unsigned int enabled_steps;
     };
 
 ``enabled_steps`` is a bitmask that specifies whether decryption,
 verity, or both is enabled.  After the bio completes, for each needed
 postprocessing step the filesystem enqueues the bio_post_read_ctx on a
 workqueue, and then the workqueue work does the decryption or
 verification.  Finally, pages where no decryption or verity error
 occurred are marked Uptodate, and the pages are unlocked.
 
 On many filesystems, files can contain holes.  Normally,
 ``->readahead()`` simply zeroes holes and sets the corresponding pages
 Uptodate; no bios are issued.  To prevent this case from bypassing
 fs-verity, these filesystems use fsverity_verify_page() to verify hole
 pages.
 
 Filesystems also disable direct I/O on verity files, since otherwise
 direct I/O would bypass fs-verity.
 
 Userspace utility
 =================
 
 This document focuses on the kernel, but a userspace utility for
 fs-verity can be found at:
 
         https://git.kernel.org/pub/scm/linux/kernel/git/ebiggers/fsverity-utils.git
 
 See the README.md file in the fsverity-utils source tree for details,
 including examples of setting up fs-verity protected files.
 
 Tests
 =====
 
 To test fs-verity, use xfstests.  For example, using `kvm-xfstests
 <https://github.com/tytso/xfstests-bld/blob/master/Documentation/kvm-quickstart.md>`_::
 
     kvm-xfstests -c ext4,f2fs,btrfs -g verity
 
 FAQ
 ===
 
 This section answers frequently asked questions about fs-verity that
 weren't already directly answered in other parts of this document.
 
 :Q: Why isn't fs-verity part of IMA?
 :A: fs-verity and IMA (Integrity Measurement Architecture) have
     different focuses.  fs-verity is a filesystem-level mechanism for
     hashing individual files using a Merkle tree.  In contrast, IMA
     specifies a system-wide policy that specifies which files are
     hashed and what to do with those hashes, such as log them,
     authenticate them, or add them to a measurement list.
 
     IMA supports the fs-verity hashing mechanism as an alternative
     to full file hashes, for those who want the performance and
     security benefits of the Merkle tree based hash.  However, it
     doesn't make sense to force all uses of fs-verity to be through
     IMA.  fs-verity already meets many users' needs even as a
     standalone filesystem feature, and it's testable like other
     filesystem features e.g. with xfstests.
 
 :Q: Isn't fs-verity useless because the attacker can just modify the
     hashes in the Merkle tree, which is stored on-disk?
 :A: To verify the authenticity of an fs-verity file you must verify
     the authenticity of the "fs-verity file digest", which
     incorporates the root hash of the Merkle tree.  See `Use cases`_.
 
 :Q: Isn't fs-verity useless because the attacker can just replace a
     verity file with a non-verity one?
 :A: See `Use cases`_.  In the initial use case, it's really trusted
     userspace code that authenticates the files; fs-verity is just a
     tool to do this job efficiently and securely.  The trusted
     userspace code will consider non-verity files to be inauthentic.
 
 :Q: Why does the Merkle tree need to be stored on-disk?  Couldn't you
     store just the root hash?
 :A: If the Merkle tree wasn't stored on-disk, then you'd have to
     compute the entire tree when the file is first accessed, even if
     just one byte is being read.  This is a fundamental consequence of
     how Merkle tree hashing works.  To verify a leaf node, you need to
     verify the whole path to the root hash, including the root node
     (the thing which the root hash is a hash of).  But if the root
     node isn't stored on-disk, you have to compute it by hashing its
     children, and so on until you've actually hashed the entire file.
 
     That defeats most of the point of doing a Merkle tree-based hash,
     since if you have to hash the whole file ahead of time anyway,
     then you could simply do sha256(file) instead.  That would be much
     simpler, and a bit faster too.
 
     It's true that an in-memory Merkle tree could still provide the
     advantage of verification on every read rather than just on the
     first read.  However, it would be inefficient because every time a
     hash page gets evicted (you can't pin the entire Merkle tree into
     memory, since it may be very large), in order to restore it you
     again need to hash everything below it in the tree.  This again
     defeats most of the point of doing a Merkle tree-based hash, since
     a single block read could trigger re-hashing gigabytes of data.
 
 :Q: But couldn't you store just the leaf nodes and compute the rest?
 :A: See previous answer; this really just moves up one level, since
     one could alternatively interpret the data blocks as being the
     leaf nodes of the Merkle tree.  It's true that the tree can be
     computed much faster if the leaf level is stored rather than just
     the data, but that's only because each level is less than 1% the
     size of the level below (assuming the recommended settings of
     SHA-256 and 4K blocks).  For the exact same reason, by storing
     "just the leaf nodes" you'd already be storing over 99% of the
     tree, so you might as well simply store the whole tree.
 
 :Q: Can the Merkle tree be built ahead of time, e.g. distributed as
     part of a package that is installed to many computers?
 :A: This isn't currently supported.  It was part of the original
     design, but was removed to simplify the kernel UAPI and because it
     wasn't a critical use case.  Files are usually installed once and
     used many times, and cryptographic hashing is somewhat fast on
     most modern processors.
 
 :Q: Why doesn't fs-verity support writes?
 :A: Write support would be very difficult and would require a
     completely different design, so it's well outside the scope of
     fs-verity.  Write support would require:
 
     - A way to maintain consistency between the data and hashes,
       including all levels of hashes, since corruption after a crash
       (especially of potentially the entire file!) is unacceptable.
       The main options for solving this are data journalling,
       copy-on-write, and log-structured volume.  But it's very hard to
       retrofit existing filesystems with new consistency mechanisms.
       Data journalling is available on ext4, but is very slow.
 
     - Rebuilding the Merkle tree after every write, which would be
       extremely inefficient.  Alternatively, a different authenticated
       dictionary structure such as an "authenticated skiplist" could
       be used.  However, this would be far more complex.
 
     Compare it to dm-verity vs. dm-integrity.  dm-verity is very
     simple: the kernel just verifies read-only data against a
     read-only Merkle tree.  In contrast, dm-integrity supports writes
     but is slow, is much more complex, and doesn't actually support
     full-device authentication since it authenticates each sector
     independently, i.e. there is no "root hash".  It doesn't really
     make sense for the same device-mapper target to support these two
     very different cases; the same applies to fs-verity.
 
 :Q: Since verity files are immutable, why isn't the immutable bit set?
 :A: The existing "immutable" bit (FS_IMMUTABLE_FL) already has a
     specific set of semantics which not only make the file contents
     read-only, but also prevent the file from being deleted, renamed,
     linked to, or having its owner or mode changed.  These extra
     properties are unwanted for fs-verity, so reusing the immutable
     bit isn't appropriate.
 
 :Q: Why does the API use ioctls instead of setxattr() and getxattr()?
 :A: Abusing the xattr interface for basically arbitrary syscalls is
     heavily frowned upon by most of the Linux filesystem developers.
     An xattr should really just be an xattr on-disk, not an API to
     e.g. magically trigger construction of a Merkle tree.
 
 :Q: Does fs-verity support remote filesystems?
 :A: So far all filesystems that have implemented fs-verity support are
     local filesystems, but in principle any filesystem that can store
     per-file verity metadata can support fs-verity, regardless of
     whether it's local or remote.  Some filesystems may have fewer
     options of where to store the verity metadata; one possibility is
     to store it past the end of the file and "hide" it from userspace
     by manipulating i_size.  The data verification functions provided
     by ``fs/verity/`` also assume that the filesystem uses the Linux
     pagecache, but both local and remote filesystems normally do so.
 
 :Q: Why is anything filesystem-specific at all?  Shouldn't fs-verity
     be implemented entirely at the VFS level?
 :A: There are many reasons why this is not possible or would be very
     difficult, including the following:
 
     - To prevent bypassing verification, pages must not be marked
       Uptodate until they've been verified.  Currently, each
       filesystem is responsible for marking pages Uptodate via
       ``->readahead()``.  Therefore, currently it's not possible for
       the VFS to do the verification on its own.  Changing this would
       require significant changes to the VFS and all filesystems.
 
     - It would require defining a filesystem-independent way to store
       the verity metadata.  Extended attributes don't work for this
       because (a) the Merkle tree may be gigabytes, but many
       filesystems assume that all xattrs fit into a single 4K
       filesystem block, and (b) ext4 and f2fs encryption doesn't
       encrypt xattrs, yet the Merkle tree *must* be encrypted when the
       file contents are, because it stores hashes of the plaintext
       file contents.
 
       So the verity metadata would have to be stored in an actual
       file.  Using a separate file would be very ugly, since the
       metadata is fundamentally part of the file to be protected, and
       it could cause problems where users could delete the real file
       but not the metadata file or vice versa.  On the other hand,
       having it be in the same file would break applications unless
       filesystems' notion of i_size were divorced from the VFS's,
       which would be complex and require changes to all filesystems.
 
     - It's desirable that FS_IOC_ENABLE_VERITY uses the filesystem's
       transaction mechanism so that either the file ends up with
       verity enabled, or no changes were made.  Allowing intermediate
       states to occur after a crash may cause problems.

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