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@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. 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@xxxxxxxx> SUSE Labs, CR