From: Eric Biggers <ebiggers@xxxxxxxxxx>
If an fsverity builtin signature is given for a file but the
".fs-verity" keyring is empty, there's no real reason to run the PKCS#7
parser. Skip this to avoid the PKCS#7 attack surface when builtin
signature support is configured into the kernel but is not being used.
This is a hardening improvement, not a fix per se, but I've added
Fixes and Cc stable to get it out to more users.
Fixes: 432434c9f8e1 ("fs-verity: support builtin file signatures")
Cc: stable@xxxxxxxxxxxxxxx
Reviewed-by: Jarkko Sakkinen <jarkko@xxxxxxxxxx>
Signed-off-by: Eric Biggers <ebiggers@xxxxxxxxxx>
---
v3: improve the error message slightly
v2: check keyring and return early before allocating formatted digest
fs/verity/signature.c | 16 ++++++++++++++++
1 file changed, 16 insertions(+)
diff --git a/fs/verity/signature.c b/fs/verity/signature.c
index b95acae64eac6..90c07573dd77b 100644
--- a/fs/verity/signature.c
+++ b/fs/verity/signature.c
@@ -55,20 +55,36 @@ int fsverity_verify_signature(const struct fsverity_info *vi,
if (sig_size == 0) {
if (fsverity_require_signatures) {
fsverity_err(inode,
"require_signatures=1, rejecting unsigned file!");
return -EPERM;
}
return 0;
}
+ if (fsverity_keyring->keys.nr_leaves_on_tree == 0) {
+ /*
+ * The ".fs-verity" keyring is empty, due to builtin signatures
+ * being supported by the kernel but not actually being used.
+ * In this case, verify_pkcs7_signature() would always return an
+ * error, usually ENOKEY. It could also be EBADMSG if the
+ * PKCS#7 is malformed, but that isn't very important to
+ * distinguish. So, just skip to ENOKEY to avoid the attack
+ * surface of the PKCS#7 parser, which would otherwise be
+ * reachable by any task able to execute FS_IOC_ENABLE_VERITY.
+ */
+ fsverity_err(inode,
+ "fs-verity keyring is empty, rejecting signed file!");
+ return -ENOKEY;
+ }
+
d = kzalloc(sizeof(*d) + hash_alg->digest_size, GFP_KERNEL);
if (!d)
return -ENOMEM;
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->file_digest, hash_alg->digest_size);
err = verify_pkcs7_signature(d, sizeof(*d) + hash_alg->digest_size,
signature, sig_size, fsverity_keyring,
base-commit: 456ae5fe9b448f44ebe98b391a3bae9c75df465e
--
2.41.0
