gro: Fix use after free in tcp_gro_receive

After calling skb_gro_receive skb->len can no longer be relied
on since if the skb was merged using frags, then its pages will
have been removed and the length reduced.

This caused tcp_gro_receive to prematurely end merging which
resulted in suboptimal performance with ixgbe.

The fix is to store skb->len on the stack.
Reported-by: Mark Wagner <mwagner@redhat.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/tcp.c

@@ -2511,6 +2511,7 @@ struct sk_buff **tcp_gro_receive(struct sk_buff **head, struct sk_buff *skb)
 	struct sk_buff *p;
 	struct tcphdr *th;
 	struct tcphdr *th2;
+	unsigned int len;
 	unsigned int thlen;
 	unsigned int flags;
 	unsigned int mss = 1;
@@ -2531,6 +2532,7 @@ struct sk_buff **tcp_gro_receive(struct sk_buff **head, struct sk_buff *skb)
 
 	skb_gro_pull(skb, thlen);
 
+	len = skb_gro_len(skb);
 	flags = tcp_flag_word(th);
 
 	for (; (p = *head); head = &p->next) {
@@ -2561,7 +2563,7 @@ found:
 
 	mss = skb_shinfo(p)->gso_size;
 
-	flush |= (skb_gro_len(skb) > mss) | !skb_gro_len(skb);
+	flush |= (len > mss) | !len;
 	flush |= (ntohl(th2->seq) + skb_gro_len(p)) ^ ntohl(th->seq);
 
 	if (flush || skb_gro_receive(head, skb)) {
@@ -2574,7 +2576,7 @@ found:
 	tcp_flag_word(th2) |= flags & (TCP_FLAG_FIN | TCP_FLAG_PSH);
 
 out_check_final:
-	flush = skb_gro_len(skb) < mss;
+	flush = len < mss;
 	flush |= flags & (TCP_FLAG_URG | TCP_FLAG_PSH | TCP_FLAG_RST |
 			  TCP_FLAG_SYN | TCP_FLAG_FIN);