fs-verity: rename "file measurement" to "file digest"

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.

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

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.

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::

follows:

- ``digest_algorithm`` will be the hash algorithm used for the file

  measurement.  It will match ``fsverity_enable_arg::hash_algorithm``.

  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.

  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`_.

  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 measurement computation

============================

File digest 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.

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 the file measurement itself, e.g. in order to sign the file.

compute fs-verity file digests itself, e.g. in order to sign files.

.. _fsverity_merkle_tree:

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::

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 */

   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.

   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 measurement against this signature, using the

   certificates in the ".fs-verity" keyring.

   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 file measurement as described in (2).

   correctly signed digest 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`_::

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" */

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.

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

: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`_.

    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?

	 * Some pages of the file may have been evicted from pagecache after

	 * being used in the Merkle tree construction, then read into pagecache

	 * again by another process reading from the file concurrently.  Since

	 * these pages didn't undergo verification against the file measurement

	 * which fs-verity now claims to be enforcing, we have to wipe the

	 * pagecache to ensure that all future reads are verified.

	 * these pages didn't undergo verification against the file digest which

	 * fs-verity now claims to be enforcing, we have to wipe the pagecache

	 * to ensure that all future reads are verified.

	 */

	filemap_write_and_wait(inode->i_mapping);

	invalidate_inode_pages2(inode->i_mapping);

 * When a verity file is first opened, an instance of this struct is allocated

 * and stored in ->i_verity_info; it remains until the inode is evicted.  It

 * caches information about the Merkle tree that's needed to efficiently verify

 * data read from the file.  It also caches the file measurement.  The Merkle

 * tree pages themselves are not cached here, but the filesystem may cache them.

 * data read from the file.  It also caches the file digest.  The Merkle tree

 * pages themselves are not cached here, but the filesystem may cache them.

 */

struct fsverity_info {

	struct merkle_tree_params tree_params;

	u8 root_hash[FS_VERITY_MAX_DIGEST_SIZE];

	u8 measurement[FS_VERITY_MAX_DIGEST_SIZE];

	u8 file_digest[FS_VERITY_MAX_DIGEST_SIZE];

	const struct inode *inode;

};

/*

 * Merkle tree properties.  The file measurement is the hash of this structure

 * excluding the signature and with the sig_size field set to 0.

 * Merkle tree properties.  The fs-verity file digest is the hash of this

 * structure excluding the signature and with the sig_size field set to 0.

 */

struct fsverity_descriptor {

	__u8 version;		/* must be 1 */

					 sizeof(struct fsverity_descriptor))

/*

 * Format in which verity file measurements are signed in built-in signatures.

 * Format in which fs-verity file digests are signed in built-in signatures.

 * This is the same as 'struct fsverity_digest', except here some magic bytes

 * are prepended to provide some context about what is being signed in case the

 * same key is used for non-fsverity purposes, and here the fields have fixed

// SPDX-License-Identifier: GPL-2.0

/*

 * Ioctl to get a verity file's measurement

 * Ioctl to get a verity file's digest

 *

 * Copyright 2019 Google LLC

 */

#include <linux/uaccess.h>

/**

 * fsverity_ioctl_measure() - get a verity file's measurement

 * @filp: file to get measurement of

 * fsverity_ioctl_measure() - get a verity file's digest

 * @filp: file to get digest of

 * @_uarg: user pointer to fsverity_digest

 *

 * Retrieve the file measurement that the kernel is enforcing for reads from a

 * verity file.  See the "FS_IOC_MEASURE_VERITY" section of

 * Retrieve the file digest that the kernel is enforcing for reads from a verity

 * file.  See the "FS_IOC_MEASURE_VERITY" section of

 * Documentation/filesystems/fsverity.rst for the documentation.

 *

 * Return: 0 on success, -errno on failure

	if (copy_to_user(uarg, &arg, sizeof(arg)))

		return -EFAULT;

	if (copy_to_user(uarg->digest, vi->measurement, hash_alg->digest_size))

	if (copy_to_user(uarg->digest, vi->file_digest, hash_alg->digest_size))

		return -EFAULT;

	return 0;

}

/*

 * Compute the file measurement by hashing the fsverity_descriptor excluding the

 * Compute the file digest by hashing the fsverity_descriptor excluding the

 * signature and with the sig_size field set to 0.

 */

static int compute_file_measurement(struct fsverity_hash_alg *hash_alg,

static int compute_file_digest(struct fsverity_hash_alg *hash_alg,

			       struct fsverity_descriptor *desc,

				    u8 *measurement)

			       u8 *file_digest)

{

	__le32 sig_size = desc->sig_size;

	int err;

	desc->sig_size = 0;

	err = fsverity_hash_buffer(hash_alg, desc, sizeof(*desc), measurement);

	err = fsverity_hash_buffer(hash_alg, desc, sizeof(*desc), file_digest);

	desc->sig_size = sig_size;

	return err;

	memcpy(vi->root_hash, desc->root_hash, vi->tree_params.digest_size);

	err = compute_file_measurement(vi->tree_params.hash_alg, desc,

				       vi->measurement);

	err = compute_file_digest(vi->tree_params.hash_alg, desc,

				  vi->file_digest);

	if (err) {

		fsverity_err(inode, "Error %d computing file measurement", err);

		fsverity_err(inode, "Error %d computing file digest", err);

		goto out;

	}

	pr_debug("Computed file measurement: %s:%*phN\n",

	pr_debug("Computed file digest: %s:%*phN\n",

		 vi->tree_params.hash_alg->name,

		 vi->tree_params.digest_size, vi->measurement);

		 vi->tree_params.digest_size, vi->file_digest);

	err = fsverity_verify_signature(vi, desc, desc_size);

out:

{

	fsverity_info_cachep = KMEM_CACHE_USERCOPY(fsverity_info,

						   SLAB_RECLAIM_ACCOUNT,

						   measurement);

						   file_digest);

	if (!fsverity_info_cachep)

		return -ENOMEM;

	return 0;