Re: [PATCH] sched/psi: fix use-after-free in ep_remove_wait_queue()

[Suren Baghdasaryan]

On Wed, Feb 1, 2023 at 8:56 PM Eric Biggers <ebiggers@xxxxxxxxxx> wrote:
>
> On Wed, Feb 01, 2023 at 07:00:23PM -0800, Munehisa Kamata wrote:
> > diff --git a/kernel/sched/psi.c b/kernel/sched/psi.c
> > index 8ac8b81bfee6..6e66c15f6450 100644
> > --- a/kernel/sched/psi.c
> > +++ b/kernel/sched/psi.c
> > @@ -1343,10 +1343,11 @@ void psi_trigger_destroy(struct psi_trigger *t)
> >
> >       group = t->group;
> >       /*
> > -      * Wakeup waiters to stop polling. Can happen if cgroup is deleted
> > -      * from under a polling process.
> > +      * Wakeup waiters to stop polling and clear the queue to prevent it from
> > +      * being accessed later. Can happen if cgroup is deleted from under a
> > +      * polling process otherwise.
> >        */
> > -     wake_up_interruptible(&t->event_wait);
> > +     wake_up_pollfree(&t->event_wait);
> >
> >       mutex_lock(&group->trigger_lock);
>
> wake_up_pollfree() should only be used in extremely rare cases.  Why can't the
> lifetime of the waitqueue be fixed instead?

waitqueue lifetime in this case is linked to cgroup_file_release(),
which seems appropriate to me here. Unfortunately
cgroup_file_release() is not directly linked to the file's lifetime.
For more details see:
https://lore.kernel.org/all/CAJuCfpFZ3B4530TgsSHqp5F_gwfrDujwRYewKReJru==MdEHQg@xxxxxxxxxxxxxx/#t
.
So, if we want to fix the lifetime of the waitqueue, we would have to
tie cgroup_file_release() to the fput() somehow. IOW, the fix would
have to be done at the cgroups or higher (kernfs?) layer.
Thanks,
Suren.

>
> - Eric