linux-kernel - Re: [PATCH RFC v1] pidfd: fix a race in setting exit

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Date:   Thu, 18 Jul 2019 12:17:40 +0200
From:   Christian Brauner <christian@...uner.io>
To:     "Joel Fernandes (Google)" <joel@...lfernandes.org>
Cc:     linux-kernel@...r.kernel.org,
        Suren Baghdasaryan <surenb@...gle.com>,
        kernel-team@...roid.com, Andrea Arcangeli <aarcange@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        Oleg Nesterov <oleg@...hat.com>, Tejun Heo <tj@...nel.org>
Subject: Re: [PATCH RFC v1] pidfd: fix a race in setting exit_state for pidfd
 polling

On Wed, Jul 17, 2019 at 01:21:00PM -0400, Joel Fernandes wrote:
> From: Suren Baghdasaryan <surenb@...gle.com>
> 
> There is a race between reading task->exit_state in pidfd_poll and writing
> it after do_notify_parent calls do_notify_pidfd. Expected sequence of
> events is:
> 
> CPU 0                            CPU 1
> ------------------------------------------------
> exit_notify
>   do_notify_parent
>     do_notify_pidfd
>   tsk->exit_state = EXIT_DEAD
>                                   pidfd_poll
>                                      if (tsk->exit_state)
> 
> However nothing prevents the following sequence:
> 
> CPU 0                            CPU 1
> ------------------------------------------------
> exit_notify
>   do_notify_parent
>     do_notify_pidfd
>                                    pidfd_poll
>                                       if (tsk->exit_state)
>   tsk->exit_state = EXIT_DEAD
> 
> This causes a polling task to wait forever, since poll blocks because
> exit_state is 0 and the waiting task is not notified again. A stress
> test continuously doing pidfd poll and process exits uncovered this bug,

Btw, if that stress test is in any way upstreamable I'd like to put this
into for-next as well. :)

Christian