Messages in this thread | | | Subject | Re: [PATCH 4/4] block, bfq: update pos_root for idle bfq_queue in bfq_bfqq_move() | From | "yukuai (C)" <> | Date | Thu, 23 Dec 2021 09:03:55 +0800 |
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在 2021/12/22 22:17, Jan Kara 写道: > 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@huawei.com> >>> >>> 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.
Hi,
I'll try to reporduce the UAF, and take a look at it.
Thanks, Kuai > > Honza >
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