Subject: [PATCH 26/32] tcp: better handle TCP_USER_TIMEOUT in SYN_SENT state

From: Eric Dumazet <[email-masked]>
Date: Thu, 26 Sep 2019 15:42:51 -0700

[ Upstream commit a66b10c05ee2d744189e9a2130394b070883d289 ]

Yuchung Cheng and Marek Majkowski independently reported a weird
behavior of TCP_USER_TIMEOUT option when used at connect() time.

When the TCP_USER_TIMEOUT is reached, tcp_write_timeout()
believes the flow should live, and the following condition
in tcp_clamp_rto_to_user_timeout() programs one jiffie timers :

    remaining = icsk->icsk_user_timeout - elapsed;
    if (remaining <= 0)
        return 1; /* user timeout has passed; fire ASAP */

This silly situation ends when the max syn rtx count is reached.

This patch makes sure we honor both TCP_SYNCNT and TCP_USER_TIMEOUT,
avoiding these spurious SYN packets.

Fixes: b701a99e431d ("tcp: Add tcp_clamp_rto_to_user_timeout() helper to improve accuracy")
Signed-off-by: Eric Dumazet <[email-masked]>
Reported-by: Yuchung Cheng <[email-masked]>
Reported-by: Marek Majkowski <[email-masked]>
Cc: Jon Maxwell <[email-masked]>
Link: https://marc.info/?l=linux-netdev&m=156940118307949&w=2
Acked-by: Jon Maxwell <[email-masked]>
Tested-by: Marek Majkowski <[email-masked]>
Signed-off-by: Marek Majkowski <[email-masked]>
Acked-by: Yuchung Cheng <[email-masked]>
Signed-off-by: David S. Miller <[email-masked]>
---
 net/ipv4/tcp_timer.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/net/ipv4/tcp_timer.c b/net/ipv4/tcp_timer.c
index c801cd37cc2a..3e8b38c73d8c 100644
--- a/net/ipv4/tcp_timer.c
+++ b/net/ipv4/tcp_timer.c
@@ -210,7 +210,7 @@ static int tcp_write_timeout(struct sock *sk)
 	struct inet_connection_sock *icsk = inet_csk(sk);
 	struct tcp_sock *tp = tcp_sk(sk);
 	struct net *net = sock_net(sk);
-	bool expired, do_reset;
+	bool expired = false, do_reset;
 	int retry_until;
 
 	if ((1 << sk->sk_state) & (TCPF_SYN_SENT | TCPF_SYN_RECV)) {
@@ -242,9 +242,10 @@ static int tcp_write_timeout(struct sock *sk)
 			if (tcp_out_of_resources(sk, do_reset))
 				return 1;
 		}
+	}
+	if (!expired)
 		expired = retransmits_timed_out(sk, retry_until,
 						icsk->icsk_user_timeout);
-	}
 	tcp_fastopen_active_detect_blackhole(sk, expired);
 
 	if (BPF_SOCK_OPS_TEST_FLAG(tp, BPF_SOCK_OPS_RTO_CB_FLAG))
-- 
2.20.1