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

[Kuniyuki Iwashima <kuniyu@amazon.com>]

From: Jason Xing <kerneljasonxing@gmail.com>
Date: Wed, 14 Aug 2024 11:51:36 +0800
> From: Jason Xing <kernelxing@tencent.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@tencent.com>
> Signed-off-by: Jason Xing <kernelxing@tencent.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) {

I prefer comparing explicitly like

  inet_twsk(sk2)->tw_substate == TCP_TIME_WAIT