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Message-ID: <20230211145246.GH4163@brightrain.aerifal.cx>
Date: Sat, 11 Feb 2023 09:52:47 -0500
From: Rich Felker <dalias@...c.org>
To: Alexey Izbyshev <izbyshev@...ras.ru>
Cc: musl@...ts.openwall.com
Subject: Re: [PATCH] mq_notify: fix close/recv race on failure path
On Sat, Feb 11, 2023 at 05:45:14PM +0300, Alexey Izbyshev wrote:
> On 2023-02-10 19:29, Rich Felker wrote:
> >On Wed, Dec 14, 2022 at 09:49:26AM +0300, Alexey Izbyshev wrote:
> >>On 2022-12-14 05:26, Rich Felker wrote:
> >>>On Wed, Nov 09, 2022 at 01:46:13PM +0300, Alexey Izbyshev wrote:
> >>>>In case of failure mq_notify closes the socket immediately after
> >>>>sending a cancellation request to the worker thread that is going to
> >>>>call or have already called recv on that socket. Even if we don't
> >>>>consider the kernel behavior when the only descriptor to an
> >>>>object that
> >>>>is being used in a system call is closed, if the socket descriptor is
> >>>>closed before the kernel looks at it, another thread could open a
> >>>>descriptor with the same value in the meantime, resulting in recv
> >>>>acting on a wrong object.
> >>>>
> >>>>Fix the race by moving pthread_cancel call before the barrier wait to
> >>>>guarantee that the cancellation flag is set before the worker thread
> >>>>enters recv.
> >>>>---
> >>>>Other ways to fix this:
> >>>>
> >>>>* Remove the racing close call from mq_notify and surround recv
> >>>> with pthread_cleanup_push/pop.
> >>>>
> >>>>* Make the worker thread joinable initially, join it before closing
> >>>> the socket on the failure path, and detach it on the happy path.
> >>>> This would also require disabling cancellation around join/detach
> >>>> to ensure that mq_notify itself is not cancelled in an inappropriate
> >>>> state.
> >>>
> >>>I'd put this aside for a while because of the pthread barrier
> >>>involvement I kinda didn't want to deal with. The fix you have sounds
> >>>like it works, but I think I'd rather pursue one of the other
> >>>approaches, probably the joinable thread one.
> >>>
> >>>At present, the implementation of barriers seems to be buggy (I need
> >>>to dig back up the post about that), and they're also a really
> >>>expensive synchronization tool that goes both directions where we
> >>>really only need one direction (notifying the caller we're done
> >>>consuming the args). I'd rather switch to a semaphore, which is the
> >>>lightest and most idiomatic (at least per present-day musl idioms) way
> >>>to do this.
> >>>
> >>This sounds good to me. The same approach can also be used in
> >>timer_create (assuming it's acceptable to add dependency on
> >>pthread_cancel to that code).
> >>
> >>>Using a joinable thread also lets us ensure we don't leave around
> >>>threads that are waiting to be scheduled just to exit on failure
> >>>return. Depending on scheduling attributes, this probably could be
> >>>bad.
> >>>
> >>I also prefer this approach, though mostly for aesthetic reasons (I
> >>haven't thought about the scheduling behavior). I didn't use it only
> >>because I felt it's a "logically larger" change than simply moving
> >>the pthread_barrier_wait call. And I wasn't aware that barriers are
> >>buggy in musl.
> >
> >Finally following up on this. How do the attached commits look?
> >
> The first and third patches add calls to sem_wait, pthread_join, and
> pthread_detach, which are cancellation points in musl, so
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Nice catch -- this is actually a bug. pthread_detach is not permitted
to be a cancellation point.
Rich
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