tcp: clear tp->retrans_stamp in tcp_rcv_fastopen_synack()

Some applications were reporting ETIMEDOUT errors on apparently
good looking flows, according to packet dumps.

We were able to root cause the issue to an accidental setting
of tp->retrans_stamp in the following scenario:

- client sends TFO SYN with data.
- server has TFO disabled, ACKs only SYN but not payload.
- client receives SYNACK covering only SYN.
- tcp_ack() eats SYN and sets tp->retrans_stamp to 0.
- tcp_rcv_fastopen_synack() calls tcp_xmit_retransmit_queue()
  to retransmit TFO payload w/o SYN, sets tp->retrans_stamp to "now",
  but we are not in any loss recovery state.
- TFO payload is ACKed.
- we are not in any loss recovery state, and don't see any dupacks,
  so we don't get to any code path that clears tp->retrans_stamp.
- tp->retrans_stamp stays non-zero for the lifetime of the connection.
- after first RTO, tcp_clamp_rto_to_user_timeout() clamps second RTO
  to 1 jiffy due to bogus tp->retrans_stamp.
- on clamped RTO with non-zero icsk_retransmits, retransmits_timed_out()
  sets start_ts from tp->retrans_stamp from TFO payload retransmit
  hours/days ago, and computes bogus long elapsed time for loss recovery,
  and suffers ETIMEDOUT early.

Fixes: a7abf3cd ("tcp: consider using standard rtx logic in tcp_rcv_fastopen_synack()")
CC: stable@vger.kernel.org
Co-developed-by: Neal Cardwell <ncardwell@google.com>
Signed-off-by: Neal Cardwell <ncardwell@google.com>
Co-developed-by: Yuchung Cheng <ycheng@google.com>
Signed-off-by: Yuchung Cheng <ycheng@google.com>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Link: https://lore.kernel.org/r/20240614130615.396837-1-edumazet@google.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>

net/ipv4/tcp_input.c

@@ -6296,6 +6296,7 @@ static bool tcp_rcv_fastopen_synack(struct sock *sk, struct sk_buff *synack,
 		skb_rbtree_walk_from(data)
 			 tcp_mark_skb_lost(sk, data);
 		tcp_xmit_retransmit_queue(sk);
+		tp->retrans_stamp = 0;
 		NET_INC_STATS(sock_net(sk),
 				LINUX_MIB_TCPFASTOPENACTIVEFAIL);
 		return true;