gso: Ensure that the packet is long enough

When we get a GSO packet from an untrusted source, we need to
ensure that it is sufficiently long so that we don't end up
crashing.

Based on discovery and patch by Ian Campbell.

Signed-off-by: Herbert Xu <[e-mail]>
Tested-by: Ian Campbell <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

net/ipv4/tcp.c

···
 	unsigned int seq;
 	__be32 delta;
 	unsigned int oldlen;
-	unsigned int len;
+	unsigned int mss;
 
 	if (!pskb_may_pull(skb, sizeof(*th)))
 		goto out;
···
 	oldlen = (u16)~skb->len;
 	__skb_pull(skb, thlen);
 
+	mss = skb_shinfo(skb)->gso_size;
+	if (unlikely(skb->len <= mss))
+		goto out;
+
 	if (skb_gso_ok(skb, features | NETIF_F_GSO_ROBUST)) {
 		/* Packet is from an untrusted source, reset gso_segs. */
 		int type = skb_shinfo(skb)->gso_type;
-		int mss;
 
 		if (unlikely(type &
 			     ~(SKB_GSO_TCPV4 |
···
 			     !(type & (SKB_GSO_TCPV4 | SKB_GSO_TCPV6))))
 			goto out;
 
-		mss = skb_shinfo(skb)->gso_size;
 		skb_shinfo(skb)->gso_segs = DIV_ROUND_UP(skb->len, mss);
 
 		segs = NULL;
···
 	if (IS_ERR(segs))
 		goto out;
 
-	len = skb_shinfo(skb)->gso_size;
-	delta = htonl(oldlen + (thlen + len));
+	delta = htonl(oldlen + (thlen + mss));
 
 	skb = segs;
 	th = tcp_hdr(skb);
···
 			     csum_fold(csum_partial(skb_transport_header(skb),
 						    thlen, skb->csum));
 
-		seq += len;
+		seq += mss;
 		skb = skb->next;
 		th = tcp_hdr(skb);