Patch "net: tcp: fix unexcepted socket die when snd_wnd is 0" has been added to the 5.15-stable tree

[Sasha Levin <sashal@xxxxxxxxxx>]

This is a note to let you know that I've just added the patch titled

    net: tcp: fix unexcepted socket die when snd_wnd is 0

to the 5.15-stable tree which can be found at:
    http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
     net-tcp-fix-unexcepted-socket-die-when-snd_wnd-is-0.patch
and it can be found in the queue-5.15 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@xxxxxxxxxxxxxxx> know about it.



commit 095e43e3f54acaa454d0fd26757d2ed9c9f80da3
Author: Menglong Dong <imagedong@xxxxxxxxxxx>
Date:   Fri Aug 11 10:55:29 2023 +0800

    net: tcp: fix unexcepted socket die when snd_wnd is 0
    
    [ 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@xxxxxxxxxxxxxx/
    Signed-off-by: Menglong Dong <imagedong@xxxxxxxxxxx>
    Reviewed-by: Eric Dumazet <edumazet@xxxxxxxxxx>
    Signed-off-by: David S. Miller <davem@xxxxxxxxxxxxx>
    Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>

diff --git a/net/ipv4/tcp_timer.c b/net/ipv4/tcp_timer.c
index a8592c187b321..40a354dcfec5a 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;
 		}