TCP: check min TTL on received ICMP packets

This adds RFC5082 checks for TTL on received ICMP packets.
It adds some security against spoofed ICMP packets
disrupting GTSM protected sessions.
Signed-off-by: Stephen Hemminger <shemminger@vyatta.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/tcp_ipv4.c

@@ -370,6 +370,11 @@ void tcp_v4_err(struct sk_buff *icmp_skb, u32 info)
 	if (sk->sk_state == TCP_CLOSE)
 		goto out;
 
+	if (unlikely(iph->ttl < inet_sk(sk)->min_ttl)) {
+		NET_INC_STATS_BH(net, LINUX_MIB_TCPMINTTLDROP);
+		goto out;
+	}
+
 	icsk = inet_csk(sk);
 	tp = tcp_sk(sk);
 	seq = ntohl(th->seq);