Re: NULL pointer dereference in pick_next_task_fair

[Ram Muthiah <rammuthiah@google.com>]

On Tue, Oct 29, 2019 at 4:50 AM 'Quentin Perret' via kernel-team
<kernel-team@android.com> wrote:
>
> On Tuesday 29 Oct 2019 at 12:34:11 (+0100), Peter Zijlstra wrote:
> > On Mon, Oct 28, 2019 at 05:46:03PM +0000, Quentin Perret wrote:
> > > The issue is very transient and relatively hard to reproduce.
> > >
> > > After digging a bit, the offending commit seems to be:
> > >
> > >     67692435c411 ("sched: Rework pick_next_task() slow-path")
> > >
> > > By 'offending' I mean that reverting it makes the issue go away. The
> > > issue comes from the fact that pick_next_entity() returns a NULL se in
> > > the 'simple' path of pick_next_task_fair(), which causes obvious
> > > problems in the subsequent call to set_next_entity().
> > >
> > > I'll dig more, but if anybody understands the issue in the meatime feel
> > > free to send me a patch to try out :)
> >
> > Can you please see if this makes any difference?
> >
> > ---
> >  kernel/sched/core.c | 6 ++++--
> >  kernel/sched/fair.c | 2 +-
> >  kernel/sched/idle.c | 3 +--
> >  3 files changed, 6 insertions(+), 5 deletions(-)
> >
> > diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> > index 7880f4f64d0e..abd2d4f80381 100644
> > --- a/kernel/sched/core.c
> > +++ b/kernel/sched/core.c
> > @@ -3922,8 +3922,10 @@ pick_next_task(struct rq *rq, struct task_struct *prev, struct rq_flags *rf)
> >                       goto restart;
> >
> >               /* Assumes fair_sched_class->next == idle_sched_class */
> > -             if (unlikely(!p))
> > -                     p = idle_sched_class.pick_next_task(rq, prev, rf);
> > +             if (unlikely(!p)) {
> > +                     prev->sched_class->put_prev_task(rq, prev, rf);
> > +                     p = idle_sched_class.pick_next_task(rq, NULL, NULL);
> > +             }
> >
> >               return p;
> >       }
> > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> > index 83ab35e2374f..2aad94bb7165 100644
> > --- a/kernel/sched/fair.c
> > +++ b/kernel/sched/fair.c
> > @@ -6820,7 +6820,7 @@ pick_next_task_fair(struct rq *rq, struct task_struct *prev, struct rq_flags *rf
> >  simple:
> >  #endif
> >       if (prev)
> > -             put_prev_task(rq, prev);
> > +             prev->sched_class->put_prev_task(rq, prev, rf);
> >
> >       do {
> >               se = pick_next_entity(cfs_rq, NULL);
> > diff --git a/kernel/sched/idle.c b/kernel/sched/idle.c
> > index 8dad5aa600ea..e8dfc84f375a 100644
> > --- a/kernel/sched/idle.c
> > +++ b/kernel/sched/idle.c
> > @@ -390,8 +390,7 @@ pick_next_task_idle(struct rq *rq, struct task_struct *prev, struct rq_flags *rf
> >  {
> >       struct task_struct *next = rq->idle;
> >
> > -     if (prev)
> > -             put_prev_task(rq, prev);
> > +     WARN_ON_ONCE(prev || rf);
> >
> >       set_next_task_idle(rq, next);
> >
> >
>
> Unfortunately the issue went into hiding this morning and I struggle to
> reproduce it (this is turning bordeline nightmare now TBH).
>
> I'll try the patch once my reproducer is fixed :/
>
> Thank you very much for the help,
> Quentin
>
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>

Quentin and I were able to create a setup which reproduces the issue.

Given this, I tried Peter's proposed fix and was still able to reproduce the
issue unfortunately. Current patch is located here -
https://android-review.googlesource.com/c/kernel/common/+/1153487

Our mitigation for this issue on the android-mainline branch has been to
revert 67692435c411 ("sched: Rework pick_next_task() slow-path").
https://android-review.googlesource.com/c/kernel/common/+/1152564

I'll spend some time detailing repro steps next. I should be able to
provide an update on those details early next week.

We appreciate the help so far.
Thanks,
Ram