ipv6: tcp: send consistent autoflowlabel in TIME_WAIT state

In case autoflowlabel is in action, skb_get_hash_flowi6()
derives a non zero skb->hash to the flowlabel.

If skb->hash is zero, a flow dissection is performed.

Since all TCP skbs sent from ESTABLISH state inherit their
skb->hash from sk->sk_txhash, we better keep a copy
of sk->sk_txhash into the TIME_WAIT socket.

After this patch, ACK or RST packets sent on behalf of
a TIME_WAIT socket have the flowlabel that was previously
used by the flow.
Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/inet_timewait_sock.h

@@ -70,6 +70,7 @@ struct inet_timewait_sock {
 				tw_flowlabel	: 20,
 				tw_pad		: 2,	/* 2 bits hole */
 				tw_tos		: 8;
+	u32			tw_txhash;
 	struct timer_list	tw_timer;
 	struct inet_bind_bucket	*tw_tb;
 };

net/ipv4/tcp_minisocks.c

@@ -283,6 +283,7 @@ void tcp_time_wait(struct sock *sk, int state, int timeo)
 			tw->tw_v6_rcv_saddr = sk->sk_v6_rcv_saddr;
 			tw->tw_tclass = np->tclass;
 			tw->tw_flowlabel = be32_to_cpu(np->flow_label & IPV6_FLOWLABEL_MASK);
+			tw->tw_txhash = sk->sk_txhash;
 			tw->tw_ipv6only = sk->sk_ipv6only;
 		}
 #endif

net/ipv6/tcp_ipv6.c

@@ -883,9 +883,16 @@ static void tcp_v6_send_response(const struct sock *sk, struct sk_buff *skb, u32
 		fl6.flowi6_oif = oif;
 	}
 
-	if (sk)
-		mark = (sk->sk_state == TCP_TIME_WAIT) ?
-			inet_twsk(sk)->tw_mark : sk->sk_mark;
+	if (sk) {
+		if (sk->sk_state == TCP_TIME_WAIT) {
+			mark = inet_twsk(sk)->tw_mark;
+			/* autoflowlabel relies on buff->hash */
+			skb_set_hash(buff, inet_twsk(sk)->tw_txhash,
+				     PKT_HASH_TYPE_L4);
+		} else {
+			mark = sk->sk_mark;
+		}
+	}
 	fl6.flowi6_mark = IP6_REPLY_MARK(net, skb->mark) ?: mark;
 	fl6.fl6_dport = t1->dest;
 	fl6.fl6_sport = t1->source;