[PATCH v3] fsverity: skip PKCS#7 parser when keyring is empty

[Eric Biggers <ebiggers@kernel.org>]

From: Eric Biggers <ebiggers@google.com>

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@vger.kernel.org
Reviewed-by: Jarkko Sakkinen <jarkko@kernel.org>
Signed-off-by: Eric Biggers <ebiggers@google.com>
---

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