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

------------------

From: Eric Biggers <redacted>

commit 192cabd6a296cbc57b3d8c05c4c89d87fc102506 upstream.

digsig_verify() requests a user key, then accesses its payload.
However, a revoked key has a NULL payload, and we failed to check for
this.  request_key() *does* skip revoked keys, but there is still a
window where the key can be revoked before we acquire its semaphore.

Fix it by checking for a NULL payload, treating it like a key which was
already revoked at the time it was requested.

Fixes: 051dbb918c7f ("crypto: digital signature verification support")
Reviewed-by: James Morris <redacted>
Cc: Dmitry Kasatkin <redacted>
Signed-off-by: Eric Biggers <redacted>
Signed-off-by: David Howells <dhowells@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
 lib/digsig.c |    6 ++++++
 1 file changed, 6 insertions(+)

--- a/lib/digsig.c
+++ b/lib/digsig.c

@@ -87,6 +87,12 @@ static int digsig_verify_rsa(struct key
 	down_read(&key->sem);
 	ukp = user_key_payload_locked(key);
 
+	if (!ukp) {
+		/* key was revoked before we acquired its semaphore */
+		err = -EKEYREVOKED;
+		goto err1;
+	}
+
 	if (ukp->datalen < sizeof(*pkh))
 		goto err1;