Re: [PATCH RFC v1] pidfd: fix a race in setting exit_state for pidfd polling

[Suren Baghdasaryan]

On Thu, Jul 18, 2019 at 3:17 AM Christian Brauner <christian@xxxxxxxxxx> wrote:
>
> On Wed, Jul 17, 2019 at 01:21:00PM -0400, Joel Fernandes wrote:
> > From: Suren Baghdasaryan <surenb@xxxxxxxxxx>
> >
> > 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