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
Signed-off-by: Eric Biggers <ebiggers@xxxxxxxxxx>
---
fs/verity/signature.c | 24 ++++++++++++++++++++----
1 file changed, 20 insertions(+), 4 deletions(-)
diff --git a/fs/verity/signature.c b/fs/verity/signature.c
index b95acae64eac6..f6668d92d8151 100644
--- a/fs/verity/signature.c
+++ b/fs/verity/signature.c
@@ -70,10 +70,26 @@ int fsverity_verify_signature(const struct fsverity_info *vi,
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,
- VERIFYING_UNSPECIFIED_SIGNATURE,
- NULL, NULL);
+ 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.
+ */
+ err = -ENOKEY;
+ } else {
+ err = verify_pkcs7_signature(d,
+ sizeof(*d) + hash_alg->digest_size,
+ signature, sig_size,
+ fsverity_keyring,
+ VERIFYING_UNSPECIFIED_SIGNATURE,
+ NULL, NULL);
+ }
kfree(d);
if (err) {
base-commit: 456ae5fe9b448f44ebe98b391a3bae9c75df465e
--
2.41.0
