x25: Prevent skb overreads when checking call user data

x25_find_listener does not check that the amount of call user data given
in the skb is big enough in per-socket comparisons, hence buffer
overreads may occur.  Fix this by adding a check.
Signed-off-by: Matthew Daley <mattjd@gmail.com>
Cc: Eric Dumazet <eric.dumazet@gmail.com>
Cc: Andrew Hendry <andrew.hendry@gmail.com>
Cc: stable <stable@kernel.org>
Acked-by: Andrew Hendry <andrew.hendry@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/x25/af_x25.c

@@ -295,7 +295,8 @@ static struct sock *x25_find_listener(struct x25_address *addr,
 			 * Found a listening socket, now check the incoming
 			 * call user data vs this sockets call user data
 			 */
-			if(skb->len > 0 && x25_sk(s)->cudmatchlength > 0) {
+			if (x25_sk(s)->cudmatchlength > 0 &&
+				skb->len >= x25_sk(s)->cudmatchlength) {
 				if((memcmp(x25_sk(s)->calluserdata.cuddata,
 					skb->data,
 					x25_sk(s)->cudmatchlength)) == 0) {