releases/3.18.78/lib-digsig-fix-dereference-of-null-user_key_payload.patch

From 192cabd6a296cbc57b3d8c05c4c89d87fc102506 Mon Sep 17 00:00:00 2001
From: Eric Biggers <ebiggers@google.com>
Date: Mon, 9 Oct 2017 12:43:20 -0700
Subject: lib/digsig: fix dereference of NULL user_key_payload

From: Eric Biggers <ebiggers@google.com>

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 <james.l.morris@oracle.com>
Cc: Dmitry Kasatkin <dmitry.kasatkin@intel.com>
Signed-off-by: Eric Biggers <ebiggers@google.com>
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
@@ -86,6 +86,12 @@ static int digsig_verify_rsa(struct key
 	down_read(&key->sem);
 	ukp = key->payload.data;
 
+	if (!ukp) {
+		/* key was revoked before we acquired its semaphore */
+		err = -EKEYREVOKED;
+		goto err1;
+	}
+
 	if (ukp->datalen < sizeof(*pkh))
 		goto err1;