From mboxrd@z Thu Jan 1 00:00:00 1970 From: Jan Kara Subject: Re: [PATCH 4/4] block, bfq: update pos_root for idle bfq_queue in bfq_bfqq_move() Date: Wed, 22 Dec 2021 15:17:22 +0100 Message-ID: <20211222141722.GC685@quack2.suse.cz> References: <20211221032135.878550-1-yukuai3@huawei.com> <20211221032135.878550-5-yukuai3@huawei.com> <20211221115001.GD24748@quack2.suse.cz> <6ca1e924-47fa-b94e-598c-69a9549eb68e@huawei.com> Mime-Version: 1.0 Content-Transfer-Encoding: 8bit Return-path: DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=suse.cz; s=susede2_rsa; t=1640182648; h=from:from:reply-to:date:date:message-id:message-id:to:to:cc:cc: mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=idiSby8trVHShWK13Y0N9hJM/pCoT2M/uIPXzb4ZYSA=; b=XDsNO3+6DoHh9p+11+8PK6Gn9zpjZeUbct6b6ph3OmJRYuyc9lSQprir/4vmItc06j8BtB XOEVxllFEWsgILJ8W7Nr+s/AAWK/QhZCWFoz7vYeG3K0SuXxsuHodfbKv1NpYkQ55dlK48 CrCYotIwo+k+loQFNJJz2LZNfYBOBWo= DKIM-Signature: v=1; a=ed25519-sha256; c=relaxed/relaxed; d=suse.cz; s=susede2_ed25519; t=1640182648; h=from:from:reply-to:date:date:message-id:message-id:to:to:cc:cc: mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=idiSby8trVHShWK13Y0N9hJM/pCoT2M/uIPXzb4ZYSA=; b=gqT1jGNfT+HgU3YyxXQ1XixhKQerBZcrY72CDNrrn+PaBGBW7lhNdsvIge7o/1p7kZMKkL 1Q1dAFAsOMJg56Aw== Content-Disposition: inline In-Reply-To: <6ca1e924-47fa-b94e-598c-69a9549eb68e-hv44wF8Li93QT0dZR+AlfA@public.gmane.org> List-ID: Content-Type: text/plain; charset="utf-8" To: "yukuai (C)" Cc: Jan Kara , tj-DgEjT+Ai2ygdnm+yROfE0A@public.gmane.org, axboe-tSWWG44O7X1aa/9Udqfwiw@public.gmane.org, paolo.valente-QSEj5FYQhm4dnm+yROfE0A@public.gmane.org, fchecconi-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org, avanzini.arianna-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org, cgroups-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, linux-block-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, linux-kernel-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, yi.zhang-hv44wF8Li93QT0dZR+AlfA@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 > > > > 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 SUSE Labs, CR