Messages in this thread | | | From | Suren Baghdasaryan <> | Date | Wed, 17 Jul 2019 11:09:59 -0700 | Subject | Re: [PATCH RFC v1] pidfd: fix a race in setting exit_state for pidfd polling |
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On Wed, Jul 17, 2019 at 10:56 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, > > 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.
I'm a little nervous about that myself even though in my stress testing this worked fine. I think the safest change would be to move do_notify_pidfd() out of do_notify_parent() and call it after tsk->exit_state is finalized. The downside of that is that there are 4 places we call do_notify_parent(), so instead of calling do_notify_pidfd() one time from do_notify_parent() we will be calling it 4 times now.
Also my original patch had memory barriers to ensure correct ordering of tsk->exit_state writes before reads. In this final version Joel removed them, so I suppose he found out they are not needed. Joel, please clarify. Thanks!
> 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. > > Acked-by: Christian Brauner <christian@brauner.io> > > Once Oleg confirms that I'm right not to worty I'll pick this up. > > Thanks! > Christian > > > > > --- > > kernel/exit.c | 8 +++++--- > > 1 file changed, 5 insertions(+), 3 deletions(-) > > > > diff --git a/kernel/exit.c b/kernel/exit.c > > index a75b6a7f458a..740ceacb4b76 100644 > > --- a/kernel/exit.c > > +++ b/kernel/exit.c > > @@ -720,6 +720,7 @@ static void exit_notify(struct task_struct *tsk, int group_dead) > > if (group_dead) > > kill_orphaned_pgrp(tsk->group_leader, NULL); > > > > + tsk->exit_state = EXIT_ZOMBIE; > > if (unlikely(tsk->ptrace)) { > > int sig = thread_group_leader(tsk) && > > thread_group_empty(tsk) && > > @@ -1156,10 +1157,11 @@ static int wait_task_zombie(struct wait_opts *wo, struct task_struct *p) > > ptrace_unlink(p); > > > > /* If parent wants a zombie, don't release it now */ > > - state = EXIT_ZOMBIE; > > + p->exit_state = EXIT_ZOMBIE; > > if (do_notify_parent(p, p->exit_signal)) > > - state = EXIT_DEAD; > > - p->exit_state = state; > > + p->exit_state = EXIT_DEAD; > > + > > + state = p->exit_state; > > write_unlock_irq(&tasklist_lock); > > } > > if (state == EXIT_DEAD) > > -- > > 2.22.0.657.g960e92d24f-goog > >
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