Re: [PATCH] sched/fair: Prevent dead task groups from regaining cfs_rq's

[Vincent Guittot]

On Thu, 4 Nov 2021 at 16:13, Mathias Krause <minipli@xxxxxxxxxxxxxx> wrote:
>
> Am 04.11.21 um 09:50 schrieb Vincent Guittot:
> > On Wed, 3 Nov 2021 at 23:04, Benjamin Segall <bsegall@xxxxxxxxxx> wrote:
> >>
> >> Mathias Krause <minipli@xxxxxxxxxxxxxx> writes:
> >>
> >>> Kevin is reporting crashes which point to a use-after-free of a cfs_rq
> >>> in update_blocked_averages(). Initial debugging revealed that we've live
> >>> cfs_rq's (on_list=1) in an about to be kfree()'d task group in
> >>> free_fair_sched_group(). However, it was unclear how that can happen.
> >>> [...]
> >>> Fixes: a7b359fc6a37 ("sched/fair: Correctly insert cfs_rq's to list on unthrottle")
> >>> Cc: Odin Ugedal <odin@xxxxxxx>
> >>> Cc: Michal Koutný <mkoutny@xxxxxxxx>
> >>> Reported-by: Kevin Tanguy <kevin.tanguy@xxxxxxxxxxxx>
> >>> Suggested-by: Brad Spengler <spender@xxxxxxxxxxxxxx>
> >>> Signed-off-by: Mathias Krause <minipli@xxxxxxxxxxxxxx>
> >>> ---
> >>>  kernel/sched/core.c | 18 +++++++++++++++---
> >>>  1 file changed, 15 insertions(+), 3 deletions(-)
> >>>
> >>> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> >>> index 978460f891a1..60125a6c9d1b 100644
> >>> --- a/kernel/sched/core.c
> >>> +++ b/kernel/sched/core.c
> >>> @@ -9506,13 +9506,25 @@ void sched_offline_group(struct task_group *tg)
> >>>  {
> >>>       unsigned long flags;
> >>>
> >>> -     /* End participation in shares distribution: */
> >>> -     unregister_fair_sched_group(tg);
> >>> -
> >>> +     /*
> >>> +      * Unlink first, to avoid walk_tg_tree_from() from finding us (via
> >>> +      * sched_cfs_period_timer()).
> >>> +      */
> >>>       spin_lock_irqsave(&task_group_lock, flags);
> >>>       list_del_rcu(&tg->list);
> >>>       list_del_rcu(&tg->siblings);
> >>>       spin_unlock_irqrestore(&task_group_lock, flags);
> >>> +
> >>> +     /*
> >>> +      * Wait for all pending users of this task group to leave their RCU
> >>> +      * critical section to ensure no new user will see our dying task
> >>> +      * group any more. Specifically ensure that tg_unthrottle_up() won't
> >>> +      * add decayed cfs_rq's to it.
> >>> +      */
> >>> +     synchronize_rcu();
> >>
> >> I was going to suggest that we could just clear all of avg.load_sum/etc, but
> >> that breaks the speculative on_list read. Currently the final avg update
> >> just races, but that's not good enough if we wanted to rely on it to
> >> prevent UAF. synchronize_rcu() doesn't look so bad if the alternative is
> >> taking every rqlock anyways.
> >>
> >> I do wonder if we can move the relevant part of
> >> unregister_fair_sched_group into sched_free_group_rcu. After all
> >> for_each_leaf_cfs_rq_safe is not _rcu and update_blocked_averages does
> >> in fact hold the rqlock (though print_cfs_stats thinks it is _rcu and
> >> should be updated).
> >
> > I was wondering the same thing.
> > we would have to move unregister_fair_sched_group() completely in
> > sched_free_group_rcu() and probably in cpu_cgroup_css_free() too.
>
> Well, the point is, print_cfs_stats() pretty much relies on the list to
> be stable, i.e. safe to traverse. It doesn't take locks while walking it
> (beside the RCU lock), so if we would modify it concurrently, bad things
> would happen.
>
> Now, all manipulations of &cfs_rq->leaf_cfs_rq_list happen via the
> list_*_rcu() variants, so that assumption in print_cfs_stats() is
> actually sound -- the list chain can be walked without fearing to
> dereference bad pointers. But if we would move the unchaining of
> cfs_rq's down to sched_free_group_rcu() (or free_fair_sched_group(), as
> that gets called more or less directly from sched_free_group_rcu()), we
> would have the unchaining and kfree() of cfs_rq's in the same RCU GP.
> Or, ignoring RCU for a moment, print_cfs_stats() may see a cfs_rq on one
> CPU we're about to kfree() on the other. So, at least, the
> kfree(tg->cfs_rq[i]) would need to become a kfree_rcu(). But likely all
> the others as well, as print_cfs_stats() looks at cfs_rq->tg and
> cfs_rq->tg->se[cpu], too.
>
> > remove_entity_load_avg(tg->se[cpu]); has to be called only once we are
> > sure that se->my_q will not be updated which means no more in the
> > leaf_list
>
> And that would be only be after tq was unlinked in sched_offline_group()
> so walk_tg_tree_from() can't find it any more (to prevent re-linking the
> cfs_rq) and after unregister_fair_sched_group() has run. But between the
> two an RCU GP is needed. We can, sure, use the existing one by moving
> unregister_fair_sched_group() to sched_free_group_rcu() /
> cpu_cgroup_css_free(). But then we also need another RCU GP after that,
> prior to releasing the involved objects, to ensure print_cfs_stats()
> won't be the new UAF subject.

Ok so we must have 2 GPs:

list_del_rcu(&tg->siblings);
GP to wait for the end of ongoing walk_tg_tree_from : synchronize_rcu
in your patch
list_del_leaf_cfs_rq(tg->cfs_rq[cpu]); if on_list
remove_entity_load_avg(tg->se[cpu]);
GP to wait for the end of ongoing for_each_leaf_cfs_rq_safe (print_cfs_stats)
kfree everything


>
> Thanks,
> Mathias