netdev - [RFC net v1] net/tcp: fix permanent FIN-WAIT-1 state with continuous zero window packets

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Message-ID: <20250919173016.3454395-1-wokezhong@tencent.com>
Date: Sat, 20 Sep 2025 01:30:15 +0800
From: HaiYang Zhong <wokezhong@...il.com>
To: edumazet@...gle.com,
	ncardwell@...gle.com,
	kuniyu@...gle.com,
	davem@...emloft.net,
	dsahern@...nel.org,
	kuba@...nel.org,
	pabeni@...hat.com,
	horms@...nel.org
Cc: netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	wokezhong@...cent.com
Subject: [RFC net v1] net/tcp: fix permanent FIN-WAIT-1 state with continuous zero window packets

When a TCP connection is in FIN-WAIT-1 state with the FIN packet blocked in
the send buffer, and the peer continuously sends zero-window advertisements,
the current implementation reset the zero-window probe timer while maintaining
the current `icsk->icsk_backoff`, causing the connection to remain permanently
in FIN-WAIT-1 state.

Reproduce conditions:
1. Peer's receive window is full and actively sending continuous zero window
   advertisements.
2. Local FIN packet is blocked in send buffer due to peer's zero-window.
3. Local socket has been closed (entered orphan state).

The root cause lies in the tcp_ack_probe() function: when receiving a zero-window ACK,
- It reset the probe timer while keeping the current `icsk->icsk_backoff`.
- This would result in the condition `icsk->icsk_backoff >= max_probes` false.
- Orphaned socket cannot be set to close.

This patch modifies the tcp_ack_probe() logic: when the socket is dead,
upon receiving a zero-window packet, instead of resetting the probe timer,
we maintain the current timer, ensuring the probe interval grows according
to 'icsk->icsk_backoff', thus causing the zero-window probe timer to eventually
timeout and close the socket.

Signed-off-by: HaiYang Zhong <wokezhong@...cent.com>
---
 net/ipv4/tcp_input.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
index 71b76e98371a..22fc82cb6b73 100644
--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c
@@ -3440,6 +3440,8 @@ static void tcp_ack_probe(struct sock *sk)
 	} else {
 		unsigned long when = tcp_probe0_when(sk, tcp_rto_max(sk));

+		if (sock_flag(sk, SOCK_DEAD) && icsk->icsk_backoff != 0)
+			return;
 		when = tcp_clamp_probe0_to_user_timeout(sk, when);
 		tcp_reset_xmit_timer(sk, ICSK_TIME_PROBE0, when, true);
 	}
-- 
2.43.7