x25: Validate incoming call user data lengths

X.25 call user data is being copied in its entirety from incoming messages
without consideration to the size of the destination buffers, leading to
possible buffer overflows. Validate incoming call user data lengths before
these copies are performed.

It appears this issue was noticed some time ago, however nothing seemed to
come of it: see http://www.spinics.net/lists/linux-x25/msg00043.html and
commit 8db09f26.
Signed-off-by: Matthew Daley <mattjd@gmail.com>
Acked-by: Eric Dumazet <eric.dumazet@gmail.com>
Tested-by: Andrew Hendry <andrew.hendry@gmail.com>
Cc: stable <stable@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/x25/af_x25.c

@@ -958,6 +958,12 @@ int x25_rx_call_request(struct sk_buff *skb, struct x25_neigh *nb,
 		goto out_clear_request;
 	skb_pull(skb,len);
 
+	/*
+	 *	Ensure that the amount of call user data is valid.
+	 */
+	if (skb->len > X25_MAX_CUD_LEN)
+		goto out_clear_request;
+
 	/*
 	 *	Find a listener for the particular address/cud pair.
 	 */

net/x25/x25_in.c

@@ -127,6 +127,9 @@ static int x25_state1_machine(struct sock *sk, struct sk_buff *skb, int frametyp
 		 *	Copy any Call User Data.
 		 */
 		if (skb->len > 0) {
+			if (skb->len > X25_MAX_CUD_LEN)
+				goto out_clear;
+
 			skb_copy_from_linear_data(skb,
 						  x25->calluserdata.cuddata,
 						  skb->len);