netfilter: SYNPROXY: skip non-tcp packet in {ipv4, ipv6}_synproxy_hook

In function {ipv4,ipv6}_synproxy_hook we expect a normal tcp packet, but
the real server maybe reply an icmp error packet related to the exist
tcp conntrack, so we will access wrong tcp data.

Fix it by checking for the protocol field and only process tcp traffic.
Signed-off-by: Lin Zhang <xiaolou4617@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

net/ipv4/netfilter/ipt_SYNPROXY.c

@@ -330,7 +330,8 @@ static unsigned int ipv4_synproxy_hook(void *priv,
 	if (synproxy == NULL)
 		return NF_ACCEPT;
 
-	if (nf_is_loopback_packet(skb))
+	if (nf_is_loopback_packet(skb) ||
+	    ip_hdr(skb)->protocol != IPPROTO_TCP)
 		return NF_ACCEPT;
 
 	thoff = ip_hdrlen(skb);

net/ipv6/netfilter/ip6t_SYNPROXY.c

@@ -353,7 +353,7 @@ static unsigned int ipv6_synproxy_hook(void *priv,
 	nexthdr = ipv6_hdr(skb)->nexthdr;
 	thoff = ipv6_skip_exthdr(skb, sizeof(struct ipv6hdr), &nexthdr,
 				 &frag_off);
-	if (thoff < 0)
+	if (thoff < 0 || nexthdr != IPPROTO_TCP)
 		return NF_ACCEPT;
 
 	th = skb_header_pointer(skb, thoff, sizeof(_th), &_th);