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

[Jan Kara]

On Sat 25-12-21 15:44:54, yukuai (C) wrote:
> 在 2021/12/25 9:19, yukuai (C) 写道:
> > 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@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.
> > > 
> > 
> > Hi,
> > 
> > I reporduced this problem on v4.19, here is the stack:
> > 
> > [34094.992162]
> > ==================================================================
> > [34094.993121] BUG: KASAN: use-after-free in rb_erase+0x4e0/0x8c0
> > [34094.993121] Write of size 8 at addr ffff888126528258 by task
> > kworker/3:1H/554
> > [34094.993121]
> > [34094.993121] CPU: 3 PID: 554 Comm: kworker/3:1H Not tainted 4.19.90+ #2
> > [34094.993121] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996),
> > BIOS ?-20190727_073836-4
> > [34094.993121] Workqueue: kblockd blk_mq_run_work_fn
> > [34094.993121] Call Trace:
> > [34094.993121]  dump_stack+0x76/0xa0
> > [34094.993121]  print_address_description+0x6c/0x237
> > [34094.993121]  ? rb_erase+0x4e0/0x8c0
> > [34094.993121]  kasan_report.cold+0x88/0x2a0
> > [34094.993121]  rb_erase+0x4e0/0x8c0
> > [34094.993121]  bfq_remove_request+0x239/0x4c0
> > [34094.993121]  bfq_dispatch_request+0x658/0x17b0
> > [34094.993121]  blk_mq_do_dispatch_sched+0x183/0x220
> > [34094.993121]  ? blk_mq_sched_free_hctx_data+0xe0/0xe0
> > [34094.993121]  ? __switch_to+0x3b2/0x6c0
> > [34094.993121]  blk_mq_sched_dispatch_requests+0x2ac/0x310
> > [34094.993121]  ? finish_task_switch+0xa4/0x370
> > [34094.993121]  ? dequeue_task_fair+0x216/0x360
> > [34094.993121]  ? blk_mq_sched_restart+0x40/0x40
> > [34094.993121]  ? __schedule+0x588/0xc10
> > [34094.993121]  __blk_mq_run_hw_queue+0x82/0x140
> > [34094.993121]  process_one_work+0x39d/0x770
> > [34094.993121]  worker_thread+0x78/0x5c0
> > [34094.993121]  ? process_one_work+0x770/0x770
> > [34094.993121]  kthread+0x1af/0x1d0
> > [34094.993121]  ? kthread_create_worker_on_cpu+0xd0/0xd0
> > [34094.993121]  ret_from_fork+0x1f/0x30
> > [34094.993121]
> > [34094.993121] Allocated by task 19184:
> > [34094.993121]  kasan_kmalloc+0xc2/0xe0
> > [34094.993121]  kmem_cache_alloc_node_trace+0xf9/0x220
> > [34094.993121]  bfq_pd_alloc+0x4c/0x510
> > [34094.993121]  blkg_alloc+0x237/0x310
> > [34094.993121]  blkg_create+0x499/0x5f0
> > [34094.993121]  blkg_lookup_create+0x140/0x1b0
> > [34094.993121]  generic_make_request_checks+0x5ce/0xad0
> > [34094.993121]  generic_make_request+0xd9/0x6b0
> > [34094.993121]  submit_bio+0xa6/0x240
> > [34094.993121]  mpage_readpages+0x29e/0x3b0
> > [34094.993121]  read_pages+0xdf/0x3a0
> > [34094.993121]  __do_page_cache_readahead+0x278/0x290
> > [34094.993121]  ondemand_readahead+0x275/0x460
> > [34094.993121]  generic_file_read_iter+0xc4a/0x1790
> > [34094.993121]  blkdev_read_iter+0x8c/0xc0
> > [34094.993121]  aio_read+0x174/0x260
> > [34094.993121]  io_submit_one+0x7d3/0x14b0
> > [34094.993121]  __x64_sys_io_submit+0xfe/0x230
> > [34094.993121]  do_syscall_64+0x6f/0x280
> > [34094.993121]  entry_SYSCALL_64_after_hwframe+0x44/0xa9
> > [34094.993121]
> > [34094.993121] Freed by task 9:
> > [34094.993121]  __kasan_slab_free+0x12f/0x180
> > [34094.993121]  kfree+0x92/0x1b0
> > [34094.993121]  blkg_free.part.0+0x4a/0xe0
> > [34094.993121]  rcu_process_callbacks+0x420/0x6c0
> > [34094.993121]  __do_softirq+0x109/0x36c
> > [34094.993121]
> > [34094.993121] The buggy address belongs to the object at ffff888126528000
> > [34094.993121]  which belongs to the cache kmalloc-2048 of size 2048
> > [34094.993121] The buggy address is located 600 bytes inside of
> > [34094.993121]  2048-byte region [ffff888126528000, ffff888126528800)
> > [34094.993121] The buggy address belongs to the page:
> > [34094.993121] page:ffffea0004994a00 count:1 mapcount:0
> > mapping:ffff88810000e800 index:0xffff0
> > [34094.993121] flags: 0x17ffffc0008100(slab|head)
> > [34094.993121] raw: 0017ffffc0008100 dead000000000100 dead000000000200
> > ffff88810000e800
> > [34094.993121] raw: ffff88812652c400 00000000800f0009 00000001ffffffff
> > 0000000000000000
> > [34094.993121] page dumped because: kasan: bad access detected
> > [34094.993121]
> > [34094.993121] Memory state around the buggy address:
> > [34094.993121]  ffff888126528100: fb fb fb fb fb fb fb fb fb fb fb fb fb
> > fb fb fb
> > [34094.993121]  ffff888126528180: fb fb fb fb fb fb fb fb fb fb fb fb fb
> > fb fb fb
> > [34094.993121] >ffff888126528200: fb fb fb fb fb fb fb fb fb fb fb fb fb
> > fb fb fb
> > [34094.993121]                                                     ^
> > [34094.993121]  ffff888126528280: fb fb fb fb fb fb fb fb fb fb fb fb fb
> > fb fb fb
> > [34094.993121]  ffff888126528300: fb fb fb fb fb fb fb fb fb fb fb fb fb
> > fb fb fb
> > [34094.993121]
> > ==================================================================
> > 
> > I'll try to figure out the root cause, in the meantime, feel free to
> > kick around if you have any througts.
> > 
> > Thansk,
> > Kuai
> > >                                 Honza
> > > 
> 
> Hi,
> 
> I finally figure out the root cause... This is introduced by a temporary
> fix of the problem that offlined bfqg is reinserted into service tree.
> 
> The temporary fix is as follow:
> 
> diff --git a/block/bfq-cgroup.c b/block/bfq-cgroup.c
> index 24a5c5329bcd..ee1933cd9a43 100644
> --- a/block/bfq-cgroup.c
> +++ b/block/bfq-cgroup.c
> @@ -935,6 +935,7 @@ static void bfq_pd_offline(struct blkg_policy_data *pd)
> 
>  put_async_queues:
>         bfq_put_async_queues(bfqd, bfqg);
> +       pd->plid = BLKCG_MAX_POLS;
> 
>         spin_unlock_irqrestore(&bfqd->lock, flags);
>         /*
> diff --git a/block/bfq-wf2q.c b/block/bfq-wf2q.c
> index b74cc0da118e..fa2062244805 100644
> --- a/block/bfq-wf2q.c
> +++ b/block/bfq-wf2q.c
> @@ -1692,6 +1692,15 @@ void bfq_del_bfqq_busy(struct bfq_data *bfqd, struct
> bfq_queue *bfqq,
>   */
>  void bfq_add_bfqq_busy(struct bfq_data *bfqd, struct bfq_queue *bfqq)
>  {
> +#ifdef CONFIG_BFQ_GROUP_IOSCHED
> +       /* If parent group is offlined, move the bfqq to root group */
> +       if (bfqq->entity.parent) {
> +               struct bfq_group *bfqg = bfq_bfqq_to_bfqg(bfqq);
> +
> +               if (bfqg->pd.plid >= BLKCG_MAX_POLS)
> +                       bfq_bfqq_move(bfqd, bfqq, bfqd->root_group);
> +       }
> +#endif
>         bfq_log_bfqq(bfqd, bfqq, "add to busy");
> 
> I add bfq_bfqq_move() here before bfq_mark_bfqq_busy(), which cause
> the problem...

OK, thanks for following up on this. So do I understand you correctly that
the problem with empty bfqq being in pos_tree does not exist in current
upstream kernel?

								Honza
-- 
Jan Kara <jack@xxxxxxxx>
SUSE Labs, CR