x25: Prevent skb overreads when checking call user data

commit 7f81e25 upstream.

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 <[email protected]>
Cc: Eric Dumazet <[email protected]>
Cc: Andrew Hendry <[email protected]>
Acked-by: Andrew Hendry <[email protected]>
Signed-off-by: David S. Miller <[email protected]>
Signed-off-by: Greg Kroah-Hartman <[email protected]>

Author: Matthew Daley

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) {