linux-kernel - Re: [PATCH net] tcp: fix forever orphan socket caused by tcp

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Message-ID: <CAL+tcoAHBSDLTNobA1MJ2itLja1xnWwmejDioPBQJh83oma55Q@mail.gmail.com>
Date: Sat, 3 Aug 2024 23:48:03 +0800
From: Jason Xing <kerneljasonxing@...il.com>
To: Eric Dumazet <edumazet@...gle.com>
Cc: Xueming Feng <kuro@...oa.me>, Lorenzo Colitti <lorenzo@...gle.com>, 
	"David S . Miller" <davem@...emloft.net>, netdev@...r.kernel.org, 
	Neal Cardwell <ncardwell@...gle.com>, Yuchung Cheng <ycheng@...gle.com>, 
	Soheil Hassas Yeganeh <soheil@...gle.com>, David Ahern <dsahern@...nel.org>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH net] tcp: fix forever orphan socket caused by tcp_abort

Hello Eric,

On Thu, Aug 1, 2024 at 9:17 PM Eric Dumazet <edumazet@...gle.com> wrote:
>
> On Thu, Aug 1, 2024 at 1:17 PM Xueming Feng <kuro@...oa.me> wrote:
> >
> > We have some problem closing zero-window fin-wait-1 tcp sockets in our
> > environment. This patch come from the investigation.
> >
> > Previously tcp_abort only sends out reset and calls tcp_done when the
> > socket is not SOCK_DEAD aka. orphan. For orphan socket, it will only
> > purging the write queue, but not close the socket and left it to the
> > timer.
> >
> > While purging the write queue, tp->packets_out and sk->sk_write_queue
> > is cleared along the way. However tcp_retransmit_timer have early
> > return based on !tp->packets_out and tcp_probe_timer have early
> > return based on !sk->sk_write_queue.
> >
> > This caused ICSK_TIME_RETRANS and ICSK_TIME_PROBE0 not being resched
> > and socket not being killed by the timers. Converting a zero-windowed
> > orphan to a forever orphan.
> >
> > This patch removes the SOCK_DEAD check in tcp_abort, making it send
> > reset to peer and close the socket accordingly. Preventing the
> > timer-less orphan from happening.
> >
> > Fixes: e05836ac07c7 ("tcp: purge write queue upon aborting the connection")
> > Fixes: bffd168c3fc5 ("tcp: clear tp->packets_out when purging write queue")
> > Signed-off-by: Xueming Feng <kuro@...oa.me>
>
> This seems legit, but are you sure these two blamed commits added this bug ?
>
> Even before them, we should have called tcp_done() right away, instead
> of waiting for a (possibly long) timer to complete the job.
>
> This might be important when killing millions of sockets on a busy server.
>
> CC Lorenzo
>
> Lorenzo, do you recall why your patch was testing the SOCK_DEAD flag ?

I guess that one of possible reasons is to avoid double-free,
something like this, happening in inet_csk_destroy_sock().

Let me assume: if we call tcp_close() first under the memory pressure
which means tcp_check_oom() returns true and then it will call
inet_csk_destroy_sock() in __tcp_close(), later tcp_abort() will call
tcp_done() to free the sk again in the inet_csk_destroy_sock() when
not testing the SOCK_DEAD flag in tcp_abort.

Do you think the above case could happen?

Thanks,
Jason