@@ -27,9 +27,9 @@ 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.
+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
@@ -177,9 +177,10 @@ FS_IOC_ENABLE_VERITY can fail with the following errors:
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::
@@ -197,7 +198,7 @@ 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. 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.
@@ -257,25 +258,24 @@ non-verity one, with the following exceptions:
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:
@@ -325,9 +325,9 @@ 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::
+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 */
@@ -359,18 +359,18 @@ kernel. Specifically, it adds support for:
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" */
@@ -421,8 +421,8 @@ 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.
+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
@@ -592,8 +592,8 @@ weren't already directly answered in other parts of this document.
: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?
@@ -398,9 +398,9 @@ int fsverity_ioctl_enable(struct file *filp, const void __user *uarg)
* 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);
@@ -67,19 +67,19 @@ struct merkle_tree_params {
* 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 */
@@ -101,7 +101,7 @@ struct fsverity_descriptor {
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
@@ -1,6 +1,6 @@
// 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
*/
@@ -10,12 +10,12 @@
#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
@@ -51,7 +51,7 @@ int fsverity_ioctl_measure(struct file *filp, void __user *_uarg)
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;
@@ -124,18 +124,18 @@ int fsverity_init_merkle_tree_params(struct merkle_tree_params *params,
}
/*
- * 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,
- struct fsverity_descriptor *desc,
- u8 *measurement)
+static int compute_file_digest(struct fsverity_hash_alg *hash_alg,
+ struct fsverity_descriptor *desc,
+ 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;
@@ -199,15 +199,15 @@ struct fsverity_info *fsverity_create_info(const struct inode *inode,
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:
@@ -354,7 +354,7 @@ int __init fsverity_init_info_cache(void)
{
fsverity_info_cachep = KMEM_CACHE_USERCOPY(fsverity_info,
SLAB_RECLAIM_ACCOUNT,
- measurement);
+ file_digest);
if (!fsverity_info_cachep)
return -ENOMEM;
return 0;
@@ -32,8 +32,8 @@ static struct key *fsverity_keyring;
* @desc: the file's fsverity_descriptor
* @desc_size: size of @desc
*
- * If the file's fs-verity descriptor includes a signature of the file
- * measurement, verify it against the certificates in the fs-verity keyring.
+ * If the file's fs-verity descriptor includes a signature of the file digest,
+ * verify it against the certificates in the fs-verity keyring.
*
* Return: 0 on success (signature valid or not required); -errno on failure
*/
@@ -67,7 +67,7 @@ int fsverity_verify_signature(const struct fsverity_info *vi,
memcpy(d->magic, "FSVerity", 8);
d->digest_algorithm = cpu_to_le16(hash_alg - fsverity_hash_algs);
d->digest_size = cpu_to_le16(hash_alg->digest_size);
- memcpy(d->digest, vi->measurement, hash_alg->digest_size);
+ memcpy(d->digest, vi->file_digest, hash_alg->digest_size);
err = verify_pkcs7_signature(d, sizeof(*d) + hash_alg->digest_size,
desc->signature, sig_size,
@@ -90,8 +90,8 @@ int fsverity_verify_signature(const struct fsverity_info *vi,
return err;
}
- pr_debug("Valid signature for file measurement %s:%*phN\n",
- hash_alg->name, hash_alg->digest_size, vi->measurement);
+ pr_debug("Valid signature for file digest %s:%*phN\n",
+ hash_alg->name, hash_alg->digest_size, vi->file_digest);
return 0;
}