tcp: fix possible NULL dereference in tcp_vX_send_reset()

After commit ca777eff ("tcp: remove dst refcount false sharing for
prequeue mode") we have to relax check against skb dst in
tcp_v[46]_send_reset() if prequeue dropped the dst.

If a socket is provided, a full lookup was done to find this socket,
so the dst test can be skipped.

Bugzilla: https://bugzilla.kernel.org/show_bug.cgi?id=88191Reported-by: NJaša Bartelj <jasa.bartelj@gmail.com>
Signed-off-by: NEric Dumazet <edumazet@google.com>
Reported-by: NDaniel Borkmann <dborkman@redhat.com>
Fixes: ca777eff ("tcp: remove dst refcount false sharing for prequeue mode")
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ipv4/tcp_ipv4.c

@@ -598,7 +598,10 @@ static void tcp_v4_send_reset(struct sock *sk, struct sk_buff *skb)
 	if (th->rst)
 		return;
 
-	if (skb_rtable(skb)->rt_type != RTN_LOCAL)
+	/* If sk not NULL, it means we did a successful lookup and incoming
+	 * route had to be correct. prequeue might have dropped our dst.
+	 */
+	if (!sk && skb_rtable(skb)->rt_type != RTN_LOCAL)
 		return;
 
 	/* Swap the send and the receive. */

net/ipv6/tcp_ipv6.c

@@ -903,7 +903,10 @@ static void tcp_v6_send_reset(struct sock *sk, struct sk_buff *skb)
 	if (th->rst)
 		return;
 
-	if (!ipv6_unicast_destination(skb))
+	/* If sk not NULL, it means we did a successful lookup and incoming
+	 * route had to be correct. prequeue might have dropped our dst.
+	 */
+	if (!sk && !ipv6_unicast_destination(skb))
 		return;
 
 #ifdef CONFIG_TCP_MD5SIG