netdev - [PATCH v2 net] tcp: fix retrans timestamp on passive Fast Open

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Message-Id: <20190513173205.212181-1-ycheng@google.com>
Date:   Mon, 13 May 2019 10:32:05 -0700
From:   Yuchung Cheng <ycheng@...gle.com>
To:     davem@...emloft.net
Cc:     netdev@...r.kernel.org, ncardwell@...gle.com,
        Yuchung Cheng <ycheng@...gle.com>
Subject: [PATCH v2 net] tcp: fix retrans timestamp on passive Fast Open

Commit c7d13c8faa74 ("tcp: properly track retry time on
passive Fast Open") sets the start of SYNACK retransmission
time on passive Fast Open in "retrans_stamp". However the
timestamp is not reset upon the handshake has completed. As a
result, future data packet retransmission may not update it in
tcp_retransmit_skb(). This may lead to socket aborting earlier
unexpectedly by retransmits_timed_out() since retrans_stamp remains
the SYNACK rtx time.

This bug only manifests on passive TFO sender that a) suffered
SYNACK timeout and then b) stalls on very first loss recovery. Any
successful loss recovery would reset the timestamp to avoid this
issue.

Fixes: c7d13c8faa74 ("tcp: properly track retry time on passive Fast Open")
Signed-off-by: Yuchung Cheng <ycheng@...gle.com>
Signed-off-by: Neal Cardwell <ncardwell@...gle.com>
---
 net/ipv4/tcp_input.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
index 20f6fac5882e..cf69f50855ea 100644
--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c
@@ -6024,6 +6024,9 @@ static int tcp_rcv_synsent_state_process(struct sock *sk, struct sk_buff *skb,
 static void tcp_rcv_synrecv_state_fastopen(struct sock *sk)
 {
 	tcp_try_undo_loss(sk, false);
+
+	/* Reset rtx states to prevent spurious retransmits_timed_out() */
+	tcp_sk(sk)->retrans_stamp = 0;
 	inet_csk(sk)->icsk_retransmits = 0;

 	/* Once we leave TCP_SYN_RECV or TCP_FIN_WAIT_1,
-- 
2.21.0.1020.gf2820cf01a-goog