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

[Jan Kara <jack-AlSwsSmVLrQ@public.gmane.org>]

On Wed 22-12-21 11:12:45, yukuai (C) wrote:
> 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-hv44wF8Li93QT0dZR+AlfA@public.gmane.org>
> > 
> > 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.

Well, I agree this patch does no harm but in my opinion it is just papering
over the real problem which is that we leave bfqq without any request in
the pos_tree which can have also other unexpected consequences. I don't
think your scenario with bfq_release_process_ref() calling
bfq_del_bfqq_busy() really answers my question because we call
bfq_del_bfqq_busy() only if RB_EMPTY_ROOT(&bfqq->sort_list) (i.e., bfqq has
no requests) and when sort_list was becoming empty, bfq_remove_request()
should have removed bfqq from the pos_tree. So I think proper fix lies
elsewhere and I would not merge this patch until we better understand what
is happening in this case.

								Honza
-- 
Jan Kara <jack-IBi9RG/b67k@public.gmane.org>
SUSE Labs, CR