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From: Luiz Augusto von Dentz <luiz.dentz@gmail.com>
To: Desmond Cheong Zhi Xi <desmondcheongzx@gmail.com>
Cc: Jiacheng Xu <578001344xu@gmail.com>,
	"linux-bluetooth@vger.kernel.org"
	<linux-bluetooth@vger.kernel.org>,
	Jakub Kicinski <kuba@kernel.org>,
	"open list:NETWORKING [GENERAL]" <netdev@vger.kernel.org>,
	Eric Dumazet <edumazet@google.com>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	Marcel Holtmann <marcel@holtmann.org>,
	Johan Hedberg <johan.hedberg@gmail.com>,
	David Miller <davem@davemloft.net>,
	Paolo Abeni <pabeni@redhat.com>
Subject: Re: possible deadlock in rfcomm_sk_state_change
Date: Tue, 30 Aug 2022 10:41:19 -0700	[thread overview]
Message-ID: <CABBYNZLZv_Y6E-rFc3kKFk+PqwNkWAzneAw=cUTEY4yW-cTs1Q@mail.gmail.com> (raw)
In-Reply-To: <f53dfd70-f8b3-8401-3f5a-d738b2f242e1@gmail.com>

Hi Desmond,

On Mon, Aug 29, 2022 at 11:48 PM Desmond Cheong Zhi Xi
<desmondcheongzx@gmail.com> wrote:
>
> +cc Bluetooth and Networking maintainers
>
> Hi Jiacheng,
>
> On 28/8/22 04:03, Jiacheng Xu wrote:
> > Hi,
> >
> > I believe the deadlock is more than possible but actually real.
> > I got a poc that could stably trigger the deadlock.
> >
> > poc: https://drive.google.com/file/d/1PjqvMtHsrrGM1MIRGKl_zJGR-teAMMQy/view?usp=sharing
> >
> > Description/Root cause:
> > In rfcomm_sock_shutdown(), lock_sock() is called when releasing and
> > shutting down socket.
> > However, lock_sock() has to be called once more when the sk_state is
> > changed because the
> > lock is not always held when rfcomm_sk_state_change() is called. One
> > such call stack is:
> >
> >    rfcomm_sock_shutdown():
> >      lock_sock();
> >      __rfcomm_sock_close():
> >        rfcomm_dlc_close():
> >          __rfcomm_dlc_close():
> >            rfcomm_dlc_lock();
> >            rfcomm_sk_state_change():
> >              lock_sock();
> >
> > Besides the recursive deadlock, there is also an
> > issue of a lock hierarchy inversion between rfcomm_dlc_lock() and
> > lock_sock() if the socket is locked in rfcomm_sk_state_change().
>
>
> Thanks for the poc and for following the trail all the way to the root
> cause - this was a known issue and I didn't realize the patch wasn't
> applied.
>
> >  > Reference:
> https://lore.kernel.org/all/20211004180734.434511-1-desmondcheongzx@gmail.com/
> >
>
> Fwiw, I tested the patch again with syzbot. It still applies cleanly to
> the head of bluetooth-next and seems to address the root cause.
>
> Any thoughts from the maintainers on this issue and the proposed fix?

We probably need to introduce a test to rfcomm-tester to reproduce
this sort of problem, I also would like to avoid introducing a work
just to trigger a state change since we don't have such problem on the
likes of L2CAP socket so perhaps we need to rework the code a little
bit to avoid the locking problems.

> Best,
> Desmond



-- 
Luiz Augusto von Dentz

  reply	other threads:[~2022-08-30 17:45 UTC|newest]

Thread overview: 5+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2022-08-27 16:19 possible deadlock in rfcomm_sk_state_change Jiacheng Xu
2022-08-28 11:03 ` Jiacheng Xu
2022-08-30  6:48   ` Desmond Cheong Zhi Xi
2022-08-30 17:41     ` Luiz Augusto von Dentz [this message]
2022-08-30 19:23       ` Luiz Augusto von Dentz

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