[PATCH 5.4 229/367] fsverity: skip PKCS#7 parser when keyring is empty

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5.4-stable review patch.  If anyone has any objections, please let me know.


From: Eric Biggers <ebiggers@xxxxxxxxxx>

commit 919dc320956ea353a7fb2d84265195ad5ef525ac upstream.

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>
Link: https://lore.kernel.org/r/20230820173237.2579-1-ebiggers@xxxxxxxxxx
Signed-off-by: Eric Biggers <ebiggers@xxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
 fs/verity/signature.c |   16 ++++++++++++++++
 1 file changed, 16 insertions(+)

--- a/fs/verity/signature.c
+++ b/fs/verity/signature.c
@@ -58,6 +58,22 @@ int fsverity_verify_signature(const stru
 		return -EBADMSG;
+	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;

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