tcp: setup timestamp offset when write_seq already set

Found that when randomized tcp offsets are enabled (by default)
TCP client can still start new connections without them. Later,
if server does active close and re-uses sockets in TIME-WAIT
state, new SYN from client can be rejected on PAWS check inside
tcp_timewait_state_process(), because either tw_ts_recent or
rcv_tsval doesn't really have an offset set.

Here is how to reproduce it with LTP netstress tool:
netstress -R 1 &
netstress -H 127.0.0.1 -lr 1000000 -a1

[...]
< S seq 1956977072 win 43690 TS val 295618 ecr 459956970
> . ack 1956911535 win 342 TS val 459967184 ecr 1547117608
< R seq 1956911535 win 0 length 0
+1. < S seq 1956977072 win 43690 TS val 296640 ecr 459956970
> S. seq 657450664 ack 1956977073 win 43690 TS val 459968205 ecr 296640

Fixes: 95a22caee396 ("tcp: randomize tcp timestamp offsets for each connection")
Signed-off-by: Alexey Kodanev <[e-mail]>
Acked-by: Eric Dumazet <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

net/ipv4/tcp_ipv4.c

···
 	struct flowi4 *fl4;
 	struct rtable *rt;
 	int err;
+	u32 seq;
 	struct ip_options_rcu *inet_opt;
 	struct inet_timewait_death_row *tcp_death_row = &sock_net(sk)->ipv4.tcp_death_row;
 
···
 	sk_setup_caps(sk, &rt->dst);
 	rt = NULL;
 
-	if (!tp->write_seq && likely(!tp->repair))
-		tp->write_seq = secure_tcp_sequence_number(inet->inet_saddr,
-							   inet->inet_daddr,
-							   inet->inet_sport,
-							   usin->sin_port,
-							   &tp->tsoffset);
+	if (likely(!tp->repair)) {
+		seq = secure_tcp_sequence_number(inet->inet_saddr,
+						 inet->inet_daddr,
+						 inet->inet_sport,
+						 usin->sin_port,
+						 &tp->tsoffset);
+		if (!tp->write_seq)
+			tp->write_seq = seq;
+	}
 
 	inet->inet_id = tp->write_seq ^ jiffies;
 

net/ipv6/tcp_ipv6.c

···
 	struct flowi6 fl6;
 	struct dst_entry *dst;
 	int addr_type;
+	u32 seq;
 	int err;
 	struct inet_timewait_death_row *tcp_death_row = &sock_net(sk)->ipv4.tcp_death_row;
 
···
 
 	sk_set_txhash(sk);
 
-	if (!tp->write_seq && likely(!tp->repair))
-		tp->write_seq = secure_tcpv6_sequence_number(np->saddr.s6_addr32,
-							     sk->sk_v6_daddr.s6_addr32,
-							     inet->inet_sport,
-							     inet->inet_dport,
-							     &tp->tsoffset);
+	if (likely(!tp->repair)) {
+		seq = secure_tcpv6_sequence_number(np->saddr.s6_addr32,
+						   sk->sk_v6_daddr.s6_addr32,
+						   inet->inet_sport,
+						   inet->inet_dport,
+						   &tp->tsoffset);
+		if (!tp->write_seq)
+			tp->write_seq = seq;
+	}
 
 	if (tcp_fastopen_defer_connect(sk, &err))
 		return err;