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From:   Suren Baghdasaryan <surenb@google.com>
Date:   Thu, 18 Jul 2019 09:05:22 -0700
Message-ID: <CAJuCfpHOPfLMEjP3gqEEOwqj8bv2GnBPVL5rurjokL4X4WVugg@mail.gmail.com>
Subject: Re: [PATCH RFC v1] pidfd: fix a race in setting exit_state for pidfd polling
To:     Christian Brauner <christian@brauner.io>
Cc:     "Joel Fernandes (Google)" <joel@joelfernandes.org>,
        LKML <linux-kernel@vger.kernel.org>,
        kernel-team <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>
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On Thu, Jul 18, 2019 at 3:17 AM Christian Brauner <christian@brauner.io> wrote:
>
> 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. :)

Definitely. I'll work with Joel on making it upstreamable and posting
as a separate patch.

>
> Christian