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From:   Suren Baghdasaryan <surenb@google.com>
Date:   Thu, 9 Feb 2023 09:09:03 -0800
Message-ID: <CAJuCfpH4aAAfEJeFzZSGsifhFNCpzZ17MEzXtxhZqoX04jrWbA@mail.gmail.com>
Subject: Re: [PATCH] sched/psi: fix use-after-free in ep_remove_wait_queue()
To:     Eric Biggers <ebiggers@kernel.org>
Cc:     Munehisa Kamata <kamatam@amazon.com>, hannes@cmpxchg.org,
        hdanton@sina.com, linux-kernel@vger.kernel.org, linux-mm@kvack.org,
        mengcc@amazon.com, stable@vger.kernel.org
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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.

> Thanks,
> Suren.
>
> >
> > - Eric