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

[Joel Fernandes <joel@joelfernandes.org>]

On Wed, Jul 17, 2019 at 07:55:57PM +0200, Christian Brauner 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,
> > and the below patch fixes it.
> > 
> > To fix this, we set tsk->exit_state before calling do_notify_pidfd.
> > 
> > Cc: kernel-team@android.com
> > Signed-off-by: Suren Baghdasaryan <surenb@google.com>
> > Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
> 
> That means in such a situation other users will see EXIT_ZOMBIE where
> they didn't see that before until after the parent failed to get
> notified.
> 
> That's a rather subtle internal change. I was worried about
> __ptrace_detach() since it explicitly checks for EXIT_ZOMBIE but it
> seems to me that this is fine since we hold write_lock_irq(&tasklist_lock);
> at the point when we do set p->exit_signal.

Right.

> Acked-by: Christian Brauner <christian@brauner.io>

Thanks.

> Once Oleg confirms that I'm right not to worty I'll pick this up.

Ok.