[PATCH 5.10 100/406] net: tcp: fix unexcepted socket die when snd_wnd is 0

Greg Kroah-Hartman
2023-09-17T19:23:39Z

From: Menglong Dong <imagedong@tencent.com>

[ Upstream commit e89688e3e97868451a5d05b38a9d2633d6785cd4 ]

In tcp_retransmit_timer(), a window shrunk connection will be regarded
as timeout if 'tcp_jiffies32 - tp->rcv_tstamp > TCP_RTO_MAX'. This is not
right all the time.

The retransmits will become zero-window probes in tcp_retransmit_timer()
if the 'snd_wnd==0'. Therefore, the icsk->icsk_rto will come up to
TCP_RTO_MAX sooner or later.

However, the timer can be delayed and be triggered after 122877ms, not
TCP_RTO_MAX, as I tested.

Therefore, 'tcp_jiffies32 - tp->rcv_tstamp > TCP_RTO_MAX' is always true
once the RTO come up to TCP_RTO_MAX, and the socket will die.

Fix this by replacing the 'tcp_jiffies32' with '(u32)icsk->icsk_timeout',
which is exact the timestamp of the timeout.

However, "tp->rcv_tstamp" can restart from idle, then tp->rcv_tstamp
could already be a long time (minutes or hours) in the past even on the
first RTO. So we double check the timeout with the duration of the
retransmission.

Meanwhile, making "2 * TCP_RTO_MAX" as the timeout to avoid the socket
dying too soon.

Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Link: https://lore.kernel.org/netdev/CADxym3YyMiO+zMD4zj03YPM3FBi-1LHi6gSD2XT8pyAMM096pg@mail.gmail.com/
Signed-off-by: Menglong Dong <imagedong@tencent.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
net/ipv4/tcp_timer.c | 18 +++++++++++++++++-
1 file changed, 17 insertions(+), 1 deletion(-)

diff --git a/net/ipv4/tcp_timer.c b/net/ipv4/tcp_timer.c
index d2e07bb30164c..5c7e10939dd90 100644
--- a/net/ipv4/tcp_timer.c
+++ b/net/ipv4/tcp_timer.c
@@ -437,6 +437,22 @@ static void tcp_fastopen_synack_timer(struct sock *sk, struct request_sock *req)
TCP_TIMEOUT_INIT << req->num_timeout, TCP_RTO_MAX);
}

+static bool tcp_rtx_probe0_timed_out(const struct sock *sk,
+ const struct sk_buff *skb)
+{
+ const struct tcp_sock *tp = tcp_sk(sk);
+ const int timeout = TCP_RTO_MAX * 2;
+ u32 rcv_delta, rtx_delta;
+
+ rcv_delta = inet_csk(sk)->icsk_timeout - tp->rcv_tstamp;
+ if (rcv_delta <= timeout)
+ return false;
+
+ rtx_delta = (u32)msecs_to_jiffies(tcp_time_stamp(tp) -
+ (tp->retrans_stamp ?: tcp_skb_timestamp(skb)));
+
+ return rtx_delta > timeout;
+}

/**
* tcp_retransmit_timer() - The TCP retransmit timeout handler
@@ -502,7 +518,7 @@ void tcp_retransmit_timer(struct sock *sk)
tp->snd_una, tp->snd_nxt);
}
#endif
- if (tcp_jiffies32 - tp->rcv_tstamp > TCP_RTO_MAX) {
+ if (tcp_rtx_probe0_timed_out(sk, skb)) {
tcp_write_err(sk);
goto out;
}
--
2.40.1