[PATCH v4 net-next] tcp: avoid reusing FIN_WAIT2 when trying to find port in connect() process

[PATCH v4 net-next] tcp: avoid reusing FIN_WAIT2 when trying to find port in connect() process

[Jason Xing]

From: Jason Xing <kernelxing@tencent.com>

We found that one close-wait socket was reset by the other side
due to a new connection reusing the same port 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. In some cases, 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@tencent.com>
Signed-off-by: Jason Xing <kernelxing@tencent.com>
---
v4
Link: https://lore.kernel.org/all/20240821153325.3204-1-kerneljasonxing@gmail.com/
1. Move the test statement earlier. (Eric)

v3
Link: https://lore.kernel.org/all/20240815113745.6668-1-kerneljasonxing@gmail.com/
1. take the ipv6 case into consideration. (Eric)

v2
Link: https://lore.kernel.org/all/20240814035136.60796-1-kerneljasonxing@gmail.com/
1. change from fin_wait2 to timewait test statement, no functional
change (Kuniyuki)
---
 net/ipv4/tcp_ipv4.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/net/ipv4/tcp_ipv4.c b/net/ipv4/tcp_ipv4.c
index fd17f25ff288..9cdf6e7c44d9 100644
--- a/net/ipv4/tcp_ipv4.c
+++ b/net/ipv4/tcp_ipv4.c
@@ -118,6 +118,9 @@ int tcp_twsk_unique(struct sock *sk, struct sock *sktw, void *twp)
 	struct tcp_sock *tp = tcp_sk(sk);
 	int ts_recent_stamp;
 
+	if (tw->tw_substate == TCP_FIN_WAIT2)
+		reuse = 0;
+
 	if (reuse == 2) {
 		/* Still does not detect *everything* that goes through
 		 * lo, since we require a loopback src or dst address
-- 
2.37.3

Re: [PATCH v4 net-next] tcp: avoid reusing FIN_WAIT2 when trying to find port in connect() process

[Eric Dumazet]

On Fri, Aug 23, 2024 at 2:12 AM Jason Xing <kerneljasonxing@gmail.com> wrote:
>
> From: Jason Xing <kernelxing@tencent.com>
>
> We found that one close-wait socket was reset by the other side
> due to a new connection reusing the same port 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. In some cases, 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@tencent.com>
> Signed-off-by: Jason Xing <kernelxing@tencent.com>
> ---

Reviewed-by: Eric Dumazet <edumazet@google.com>

Re: [PATCH v4 net-next] tcp: avoid reusing FIN_WAIT2 when trying to find port in connect() process

[patchwork-bot+netdevbpf]

Hello:

This patch was applied to netdev/net-next.git (main)
by Jakub Kicinski <kuba@kernel.org>:

On Fri, 23 Aug 2024 08:11:52 +0800 you wrote:
> From: Jason Xing <kernelxing@tencent.com>
> 
> We found that one close-wait socket was reset by the other side
> due to a new connection reusing the same port 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 is the summary with links:
  - [v4,net-next] tcp: avoid reusing FIN_WAIT2 when trying to find port in connect() process
    https://git.kernel.org/netdev/net-next/c/0d9e5df4a257

You are awesome, thank you!
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