Re: [PATCH 4/4] block, bfq: update pos_root for idle bfq_queue in bfq_bfqq_move()

[yukuai (C)]

在 2021/12/21 19:50, Jan Kara 写道:
> On Tue 21-12-21 11:21:35, Yu Kuai wrote:
> > During code review, we found that if bfqq is not busy in
> > bfq_bfqq_move(), bfq_pos_tree_add_move() won't be called for the bfqq,
> > thus bfqq->pos_root still points to the old bfqg. However, the ref
> > that bfqq hold for the old bfqg will be released, so it's possible
> > that the old bfqg can be freed. This is problematic because the freed
> > bfqg can still be accessed by bfqq->pos_root.
> >
> > Fix the problem by calling bfq_pos_tree_add_move() for idle bfqq
> > as well.
> >
> > Fixes: e21b7a0b9887 ("block, bfq: add full hierarchical scheduling and cgroups support")
> > Signed-off-by: Yu Kuai <yukuai3@xxxxxxxxxx>
>
> I'm just wondering, how can it happen that !bfq_bfqq_busy() queue is in
> pos_tree? Because bfq_remove_request() takes care to remove bfqq from the
> pos_tree...

Hi,

It's right this is not a problem in common case. The problem seems to
relate to queue merging and task migration. Because I once reporduced
it with the same reporducer for the problem that offlined bfqg can be
inserted into service tree. The uaf is exactly in
bfq_remove_request->rb_rease(). However I didn't save the stack...

I guess this is because bfq_del_bfqq_busy() is called from
bfq_release_process_ref(), and queue merging prevert sunch bfqq to be
freed, thus such bfqq is not in service tree, and it's pos_root can
point to the old bfqg after bfq_bic_update_cgroup->bfq_bfqq_move.

I haven't confirmed this, however, this patch itself only cleared
bfqq->pos_root for idle bfqq, there should be no harm.

Thanks,
Kuai
> 								Honza
>
> > ---
> >   block/bfq-cgroup.c | 14 +++++++++-----
> >   1 file changed, 9 insertions(+), 5 deletions(-)
> >
> > diff --git a/block/bfq-cgroup.c b/block/bfq-cgroup.c
> > index 8e8cf6b3d946..822dd28ecf53 100644
> > --- a/block/bfq-cgroup.c
> > +++ b/block/bfq-cgroup.c
> > @@ -677,7 +677,6 @@ void bfq_bfqq_move(struct bfq_data *bfqd, struct bfq_queue *bfqq,
> >   		bfq_deactivate_bfqq(bfqd, bfqq, false, false);
> >   	else if (entity->on_st_or_in_serv)
> >   		bfq_put_idle_entity(bfq_entity_service_tree(entity), entity);
> > -	bfqg_and_blkg_put(old_parent);
> >   
> >   	if (entity->parent &&
> >   	    entity->parent->last_bfqq_created == bfqq)
> > @@ -690,11 +689,16 @@ void bfq_bfqq_move(struct bfq_data *bfqd, struct bfq_queue *bfqq,
> >   	/* pin down bfqg and its associated blkg  */
> >   	bfqg_and_blkg_get(bfqg);
> >   
> > -	if (bfq_bfqq_busy(bfqq)) {
> > -		if (unlikely(!bfqd->nonrot_with_queueing))
> > -			bfq_pos_tree_add_move(bfqd, bfqq);
> > +	/*
> > +	 * Don't leave the pos_root to old bfqg, since the ref to old bfqg will
> > +	 * be released and the bfqg might be freed.
> > +	 */
> > +	if (unlikely(!bfqd->nonrot_with_queueing))
> > +		bfq_pos_tree_add_move(bfqd, bfqq);
> > +	bfqg_and_blkg_put(old_parent);
> > +
> > +	if (bfq_bfqq_busy(bfqq))
> >   		bfq_activate_bfqq(bfqd, bfqq);
> > -	}
> >   
> >   	if (!bfqd->in_service_queue && !bfqd->rq_in_driver)
> >   		bfq_schedule_dispatch(bfqd);
> > --
> > 2.31.1