Message-ID: <CAL+tcoB7F3Aviygmxc_DhfLRQN8c=cdn-_1QrXhEWFpyeAQRDw@mail.gmail.com>
Date: Mon, 5 Aug 2024 19:12:53 +0800
From: Jason Xing <kerneljasonxing@...il.com>
To: Xueming Feng <kuro@...oa.me>
Cc: "David S . Miller" <davem@...emloft.net>, netdev@...r.kernel.org, 
	Eric Dumazet <edumazet@...gle.com>, Lorenzo Colitti <lorenzo@...gle.com>, 
	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

On Mon, Aug 5, 2024 at 6:34 PM Xueming Feng <kuro@...oa.me> wrote:
>
> On Mon, Aug 5, 2024 at 4:04 PM Jason Xing <kerneljasonxing@...il.com> wrote:
> >
> > On Mon, Aug 5, 2024 at 3:23 PM Eric Dumazet <edumazet@...gle.com> wrote:
> > >
> > > On Mon, Aug 5, 2024 at 6:52 AM Jason Xing <kerneljasonxing@...il.com> wrote:
> > > >
> > > > On Sat, Aug 3, 2024 at 11:48 PM Jason Xing <kerneljasonxing@...il.com> wrote:
> > > > >
> > > > > 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 ?
>
> My bad, I wasn't sure about the intend of the original commit that did not
> handle orphan sockets at the time of blaming, should have asked.
>
> > > > > >
> > > > > > 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.
> > > > >
> > > >
> > > > How about this one which can prevent double calling
> > > > inet_csk_destroy_sock() when we call destroy and close nearly at the
> > > > same time under that circumstance:
> > > >
> > > > diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c
> > > > index e03a342c9162..d5d3b21cc824 100644
> > > > --- a/net/ipv4/tcp.c
> > > > +++ b/net/ipv4/tcp.c
> > > > @@ -4646,7 +4646,7 @@ int tcp_abort(struct sock *sk, int err)
> > > >         local_bh_disable();
> > > >         bh_lock_sock(sk);
> > > >
> > > > -       if (!sock_flag(sk, SOCK_DEAD)) {
> > > > +       if (sk->sk_state != TCP_CLOSE) {
> > > >                 if (tcp_need_reset(sk->sk_state))
> > > >                         tcp_send_active_reset(sk, GFP_ATOMIC,
> > > >                                               SK_RST_REASON_NOT_SPECIFIED);
> > > >
> > > > Each time we call inet_csk_destroy_sock(), we must make sure we've
> > > > already set the state to TCP_CLOSE. Based on this, I think we can use
> > > > this as an indicator to avoid calling twice to destroy the socket.
> > >
> > > I do not think this will work.
> > >
> > > With this patch, a listener socket will not get an error notification.
> >
> > Oh, you're right.
> >
> > I think we can add this particular case in the if or if-else statement
> > to handle.
> >
> > Thanks,
> > Jason
>
> Summarizing above conversation, I've made a v2-ish patch, which should
> handles the double abort, and removes redundent tcp_write_queue_purge.
> Please take a look, in the meanwhile, I will see if I can make a test
> for tcp_abort. If this looks good, I will make a formal v2 patch.
>
> Any advice is welcomed. (Including on how to use this mail thread, I don't
> have much experience with it.)
>
> Signed-off-by: Xueming Feng <kuro@...oa.me>
> ---
>  net/ipv4/tcp.c | 15 ++++++++-------
>  1 file changed, 8 insertions(+), 7 deletions(-)
>
> diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c
> index e03a342c9162..039a9c9301b7 100644
> --- a/net/ipv4/tcp.c
> +++ b/net/ipv4/tcp.c
> @@ -4614,6 +4614,10 @@ int tcp_abort(struct sock *sk, int err)
>  {
>         int state = inet_sk_state_load(sk);
>
> +       /* Avoid force-closing the same socket twice. */
> +       if (state == TCP_CLOSE) {
> +               return 0;
> +       }

No need to add brackets here.

And I don't think it's a correct position to test and return because
we need to take care of the race condition when accessing lock_sock()
by tcp_abort() and __tcp_close(). What if tcp_abort() just passes the
check and then tcp_close() rapidly grabs the socket lock and sets the
state to TCP_CLOSE?

Thanks,
Jason

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