fs-verity: add a documentation file

Add a documentation file for fs-verity, covering:

- Introduction
- Use cases
- User API
- FS_IOC_ENABLE_VERITY
- FS_IOC_MEASURE_VERITY
- FS_IOC_GETFLAGS
- Accessing verity files
- File measurement computation
- Merkle tree
- fs-verity descriptor
- Built-in signature verification
- Filesystem support
- ext4
- f2fs
- Implementation details
- Verifying data
- Pagecache
- Block device based filesystems
- Userspace utility
- Tests
- FAQ

Reviewed-by: Theodore Ts'o <[e-mail]>
Reviewed-by: Jaegeuk Kim <[e-mail]>
Signed-off-by: Eric Biggers <[e-mail]>

Documentation/filesystems/fsverity.rst

···
+.. 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 and f2fs
+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 "file measurement", which is a hash that includes the 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 is up to userspace.  However, to meet some
+users' needs, 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`_.  Support for fs-verity file hashes
+in IMA (Integrity Measurement Architecture) policies is also planned.
+
+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 :c:type:`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 measurement of a verity
+file.  The file measurement is a digest that cryptographically
+identifies the file contents that are being enforced on reads.
+
+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
+  measurement.  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_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.
+
+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's verity measurement 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 measurement computation
+============================
+
+This section describes how fs-verity hashes the file contents using a
+Merkle tree to produce the "file measurement" 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 the file measurement itself, e.g. in order to sign the file.
+
+.. _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 verity file measurement 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 sig_size;        /* 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 */
+    };
+
+Note that the ``sig_size`` field must be set to 0 for the purpose of
+computing the file measurement, even if a signature was provided (or
+will be provided) to `FS_IOC_ENABLE_VERITY`_.
+
+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 measurement.  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 measurement 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 file measurement as described in (2).
+
+File measurements must be signed in the following format, which is
+similar to the structure used by `FS_IOC_MEASURE_VERITY`_::
+
+    struct fsverity_signed_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 currently supported by the ext4 and f2fs filesystems.
+The CONFIG_FS_VERITY kconfig option must be enabled to use fs-verity
+on either filesystem.
+
+``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 TODO 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 file
+measurement 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 TODO 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.
+
+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 ``->readpage()`` and
+``->readpages()`` 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.
+
+Files on ext4 and f2fs may contain holes.  Normally, ``->readpages()``
+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.
+
+ext4 and f2fs disable direct I/O on verity files, since otherwise
+direct I/O would bypass fs-verity.  (They also do the same for
+encrypted files.)
+
+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 -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 is planned to support the fs-verity hashing mechanism as an
+    alternative to doing full file hashes, for people who want the
+    performance and security benefits of the Merkle tree based hash.
+    But it doesn't make sense to force all uses of fs-verity to be
+    through IMA.  As a standalone filesystem feature, fs-verity
+    already meets many users' needs, 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 "file measurement", which is basically 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 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: Only ext4 and f2fs support is implemented currently, 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
+      ``->readpages()``.  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.

Documentation/filesystems/index.rst

···
 
    journalling
    fscrypt
+   fsverity