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

[Eric Biggers <ebiggers@kernel.org>]

On Thu, Feb 09, 2023 at 09:09:03AM -0800, Suren Baghdasaryan wrote:
> On Thu, Feb 2, 2023 at 1:11 PM Suren Baghdasaryan <surenb@google.com> wrote:
> >
> > On Wed, Feb 1, 2023 at 8:56 PM Eric Biggers <ebiggers@kernel.org> 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@mail.gmail.com/#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.
> 
> Hi Eric,
> Do you still object to using wake_up_pollfree() for this case?
> Changing higher levels to make cgroup_file_release() be tied to fput()
> would be ideal but I think that would be a big change for this one
> case. If you agree I'll Ack this patch.
> Thanks,
> Suren.
> 

I haven't read the code closely in this case.  I'm just letting you know that
wake_up_pollfree() is very much a last-resort option for when the waitqueue
lifetime can't be fixed.  So if you want to use wake_up_pollfree(), you need to
explain why no other fix is possible.  For example maybe the UAPI depends on the
waitqueue having a nonstandard lifetime.

- Eric