tcp: avoid unconditional congestion window undo on SYN retransmit

Previously if an active TCP open has SYN timeout, it always undo the
cwnd upon receiving the SYNACK. This is because tcp_clean_rtx_queue
would reset tp->retrans_stamp when SYN is acked, which fools then
tcp_try_undo_loss and tcp_packet_delayed. Addressing this issue is
required to properly support undo for spurious SYN timeout.

Fixing this is tricky -- for active TCP open tp->retrans_stamp
records the time when the handshake starts, not the first
retransmission time as the name may suggest. The simplest fix is
for tcp_packet_delayed to ensure it is valid before comparing with
other timestamp.

One side effect of this change is active TCP Fast Open that incurred
SYN timeout. Upon receiving a SYN-ACK that only acknowledged
the SYN, it would immediately retransmit unacknowledged data in
tcp_ack() because the data is marked lost after SYN timeout. But
the retransmission would have an incorrect ack sequence number since
rcv_nxt has not been updated yet tcp_rcv_synsent_state_process(), the
retransmission needs to properly handed by tcp_rcv_fastopen_synack()
like before.

Signed-off-by: Yuchung Cheng <ycheng@google.com>
Signed-off-by: Neal Cardwell <ncardwell@google.com>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/tcp_input.c

@@ -2252,7 +2252,7 @@ static bool tcp_skb_spurious_retrans(const struct tcp_sock *tp,
  */
 static inline bool tcp_packet_delayed(const struct tcp_sock *tp)
 {
-	return !tp->retrans_stamp ||
+	return tp->retrans_stamp &&
 	       tcp_tsopt_ecr_before(tp, tp->retrans_stamp);
 }
 
@@ -3521,7 +3521,7 @@ static void tcp_xmit_recovery(struct sock *sk, int rexmit)
 {
 	struct tcp_sock *tp = tcp_sk(sk);
 
-	if (rexmit == REXMIT_NONE)
+	if (rexmit == REXMIT_NONE || sk->sk_state == TCP_SYN_SENT)
 		return;
 
 	if (unlikely(rexmit == 2)) {