ipv6: add tos reflection in TCP reset and ack

Currently, ipv6 stack does not do any TOS reflection. To make the
behavior consistent with v4 stack, this commit adds TOS reflection in
tcp_v6_reqsk_send_ack() and tcp_v6_send_reset(). We clear the lower
2-bit ECN value of the received TOS in compliance with RFC 3168 6.1.5
robustness principles.

Signed-off-by: Wei Wang <weiwan@google.com>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv6/tcp_ipv6.c

@@ -959,8 +959,8 @@ static void tcp_v6_send_response(const struct sock *sk, struct sk_buff *skb, u32
 	dst = ip6_dst_lookup_flow(sock_net(ctl_sk), ctl_sk, &fl6, NULL);
 	if (!IS_ERR(dst)) {
 		skb_dst_set(buff, dst);
-		ip6_xmit(ctl_sk, buff, &fl6, fl6.flowi6_mark, NULL, tclass,
-			 priority);
+		ip6_xmit(ctl_sk, buff, &fl6, fl6.flowi6_mark, NULL,
+			 tclass & ~INET_ECN_MASK, priority);
 		TCP_INC_STATS(net, TCP_MIB_OUTSEGS);
 		if (rst)
 			TCP_INC_STATS(net, TCP_MIB_OUTRSTS);
@@ -1068,8 +1068,8 @@ static void tcp_v6_send_reset(const struct sock *sk, struct sk_buff *skb)
 			label = ip6_flowlabel(ipv6h);
 	}
 
-	tcp_v6_send_response(sk, skb, seq, ack_seq, 0, 0, 0, oif, key, 1, 0,
-			     label, priority);
+	tcp_v6_send_response(sk, skb, seq, ack_seq, 0, 0, 0, oif, key, 1,
+			     ipv6_get_dsfield(ipv6h), label, priority);
 
 #ifdef CONFIG_TCP_MD5SIG
 out:
@@ -1122,7 +1122,7 @@ static void tcp_v6_reqsk_send_ack(const struct sock *sk, struct sk_buff *skb,
 			tcp_time_stamp_raw() + tcp_rsk(req)->ts_off,
 			req->ts_recent, sk->sk_bound_dev_if,
 			tcp_v6_md5_do_lookup(sk, &ipv6_hdr(skb)->saddr, l3index),
-			0, 0, sk->sk_priority);
+			ipv6_get_dsfield(ipv6_hdr(skb)), 0, sk->sk_priority);
 }