Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
Date:   Thu, 18 Jul 2019 12:17:40 +0200
From:   Christian Brauner <christian@brauner.io>
To:     "Joel Fernandes (Google)" <joel@joelfernandes.org>
Cc:     linux-kernel@vger.kernel.org,
        Suren Baghdasaryan <surenb@google.com>,
        kernel-team@android.com, Andrea Arcangeli <aarcange@redhat.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        "Eric W. Biederman" <ebiederm@xmission.com>,
        Oleg Nesterov <oleg@redhat.com>, Tejun Heo <tj@kernel.org>
Subject: Re: [PATCH RFC v1] pidfd: fix a race in setting exit_state for pidfd
 polling
Message-ID: <20190718101735.pbu6nji6mfwq4mxa@brauner.io>
References: <20190717172100.261204-1-joel@joelfernandes.org>
MIME-Version: 1.0
Content-Type: text/plain; charset=utf-8
Content-Disposition: inline
In-Reply-To: <20190717172100.261204-1-joel@joelfernandes.org>
User-Agent: NeoMutt/20180716
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

On Wed, Jul 17, 2019 at 01:21:00PM -0400, Joel Fernandes wrote:
> From: Suren Baghdasaryan <surenb@google.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