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

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Message-ID: <CAL+tcoALxw0Pi+c1imyPHJu=ahaY5bU-adWPfCC4s4MGCQNTmA@mail.gmail.com>
Date: Thu, 15 Aug 2024 12:29:22 +0800
From: Jason Xing <kerneljasonxing@...il.com>
To: Kuniyuki Iwashima <kuniyu@...zon.com>
Cc: davem@...emloft.net, dsahern@...nel.org, edumazet@...gle.com, 
	jadedong@...cent.com, kernelxing@...cent.com, kuba@...nel.org, 
	ncardwell@...gle.com, netdev@...r.kernel.org, pabeni@...hat.com
Subject: Re: [PATCH net-next] tcp: avoid reusing FIN_WAIT2 when trying to find
 port in connect() process

Hello Kuniyuki,

On Thu, Aug 15, 2024 at 3:39 AM Kuniyuki Iwashima <kuniyu@...zon.com> wrote:
>
> From: Jason Xing <kerneljasonxing@...il.com>
> Date: Wed, 14 Aug 2024 11:51:36 +0800
> > 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) {
>
> I prefer comparing explicitly like
>
>   inet_twsk(sk2)->tw_substate == TCP_TIME_WAIT

Thanks, I will adjust in the v2 patch soon.

Thanks,
Jason