block: rename 'q->debugfs_dir' and 'q->blk_trace->dir' in blk_unregister_queue()

hulk inclusion
category: bugfix
bugzilla: 30213
CVE: CVE-2019-19770

---------------------------

syzbot is reporting use after free bug in debugfs_remove[1].

This is because in request_queue, 'q->debugfs_dir' and
'q->blk_trace->dir' could be the same dir. And in __blk_release_queue(),
blk_mq_debugfs_unregister() will remove everything inside the dir.

With futher investigation of the reporduce repro, the problem can be
reporduced by following procedure:

1. LOOP_CTL_ADD, create a request_queue q1, blk_mq_debugfs_register() will
create the dir.
2. LOOP_CTL_REMOVE, blk_release_queue() will add q1 to release queue.
3. LOOP_CTL_ADD, create another request_queue q2,blk_mq_debugfs_register()
will fail because the dir aready exist.
4. BLKTRACESETUP, create two files(msg and dropped) inside the dir.
5. call __blk_release_queue() for q1, debugfs_remove_recursive() will
delete the files created in step 4.
6. LOOP_CTL_REMOVE, blk_release_queue() will add q2 to release queue.
And when __blk_release_queue() is called for q2, blk_trace_shutdown() will
try to release the two files created in step 4, wich are aready released
in step 5.

thread1                  |kworker                   |thread2
 ----------------------- | ------------------------ | --------------------
loop_control_ioctl       |                          |
 loop_add                |                          |
  blk_mq_debugfs_register|                          |
   debugfs_create_dir    |                          |
loop_control_ioctl       |                          |
 loop_remove             |                          |
  blk_release_queue      |                          |
   schedule_work         |                          |
                         |                          |loop_control_ioctl
                         |                          | loop_add
                         |                          |  ...
                         |                          |blk_trace_ioctl
                         |                          | __blk_trace_setup
                         |                          |   debugfs_create_file
                         |__blk_release_queue       |
                         | blk_mq_debugfs_unregister|
                         |  debugfs_remove_recursive|
                         |                          |loop_control_ioctl
                         |                          | loop_remove
                         |                          |  ...
                         |__blk_release_queue       |
                         | blk_trace_shutdown       |
                         |  debugfs_remove          |

commit dc9edc44 ("block: Fix a blk_exit_rl() regression") pushed the
final release of request_queue to a workqueue, so, when loop_add() is
called again(step 3), __blk_release_queue() might not been called yet,
which causes the problem.

Fix the problem by renaming 'q->debugfs_dir' or 'q->blk_trace->dir'
in blk_unregister_queue() if they exist.

[1] https://syzkaller.appspot.com/bug?extid=903b72a010ad6b7a40f2


References: CVE-2019-19770
Fixes: commit dc9edc44 ("block: Fix a blk_exit_rl() regression")
Reported-by: syzbot <syz...@syzkaller.appspotmail.com>
Signed-off-by: yu kuai <yukuai3@huawei.com>
Reviewed-by: Jason Yan <yanaijie@huawei.com>
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>