netdev - [PATCH net-next] tcp: avoid reusing FIN_WAIT2 when trying to find port in connect() process

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Message-Id: <20240814035136.60796-1-kerneljasonxing@gmail.com>
Date: Wed, 14 Aug 2024 11:51:36 +0800
From: Jason Xing <kerneljasonxing@...il.com>
To: davem@...emloft.net,
	edumazet@...gle.com,
	kuba@...nel.org,
	pabeni@...hat.com,
	dsahern@...nel.org,
	ncardwell@...gle.com
Cc: netdev@...r.kernel.org,
	Jason Xing <kernelxing@...cent.com>,
	Jade Dong <jadedong@...cent.com>
Subject: [PATCH net-next] tcp: avoid reusing FIN_WAIT2 when trying to find port in connect() process

From: Jason Xing <kernelxing@...cent.com>

We found that one close-wait socket was reset by the other side
which is beyond our expectation, so we have to investigate the
underlying reason. The following experiment is conducted in the
test environment. We limit the port range from 40000 to 40010
and delay the time to close() after receiving a fin from the
active close side, which can help us easily reproduce like what
happened in production.

Here are three connections captured by tcpdump:
127.0.0.1.40002 > 127.0.0.1.9999: Flags [S], seq 2965525191
127.0.0.1.9999 > 127.0.0.1.40002: Flags [S.], seq 2769915070
127.0.0.1.40002 > 127.0.0.1.9999: Flags [.], ack 1
127.0.0.1.40002 > 127.0.0.1.9999: Flags [F.], seq 1, ack 1
// a few seconds later, within 60 seconds
127.0.0.1.40002 > 127.0.0.1.9999: Flags [S], seq 2965590730
127.0.0.1.9999 > 127.0.0.1.40002: Flags [.], ack 2
127.0.0.1.40002 > 127.0.0.1.9999: Flags [R], seq 2965525193
// later, very quickly
127.0.0.1.40002 > 127.0.0.1.9999: Flags [S], seq 2965590730
127.0.0.1.9999 > 127.0.0.1.40002: Flags [S.], seq 3120990805
127.0.0.1.40002 > 127.0.0.1.9999: Flags [.], ack 1

As we can see, the first flow is reset because:
1) client starts a new connection, I mean, the second one
2) client tries to find a suitable port which is a timewait socket
   (its state is timewait, substate is fin_wait2)
3) client occupies that timewait port to send a SYN
4) server finds a corresponding close-wait socket in ehash table,
   then replies with a challenge ack
5) client sends an RST to terminate this old close-wait socket.

I don't think the port selection algo can choose a FIN_WAIT2 socket
when we turn on tcp_tw_reuse because on the server side there
remain unread data. If one side haven't call close() yet, we should
not consider it as expendable and treat it at will.

Even though, sometimes, the server isn't able to call close() as soon
as possible like what we expect, it can not be terminated easily,
especially due to a second unrelated connection happening.

After this patch, we can see the expected failure if we start a
connection when all the ports are occupied in fin_wait2 state:
"Ncat: Cannot assign requested address."

Reported-by: Jade Dong <jadedong@...cent.com>
Signed-off-by: Jason Xing <kernelxing@...cent.com>
---
 net/ipv4/inet_hashtables.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/net/ipv4/inet_hashtables.c b/net/ipv4/inet_hashtables.c
index 9bfcfd016e18..6115ee0c5d90 100644
--- a/net/ipv4/inet_hashtables.c
+++ b/net/ipv4/inet_hashtables.c
@@ -563,7 +563,8 @@ static int __inet_check_established(struct inet_timewait_death_row *death_row,
 			continue;

 		if (likely(inet_match(net, sk2, acookie, ports, dif, sdif))) {
-			if (sk2->sk_state == TCP_TIME_WAIT) {
+			if (sk2->sk_state == TCP_TIME_WAIT &&
+			    inet_twsk(sk2)->tw_substate != TCP_FIN_WAIT2) {
 				tw = inet_twsk(sk2);
 				if (sk->sk_protocol == IPPROTO_TCP &&
 				    tcp_twsk_unique(sk, sk2, twp))
-- 
2.37.3