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 dc9edc44de6c ("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 dc9edc44de6c ("block: Fix a blk_exit_rl() regression")
Reported-by: syzbot <redacted>
Signed-off-by: yu kuai <redacted>
---
Changes in V2:
 add device name to the new dir name
 fix compile err when 'CONFIG_BLK_DEBUG_FS' is not enabled
 add treatment of 'q->blk_trace->dir'

 block/blk-sysfs.c | 42 ++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 42 insertions(+)

diff --git a/block/blk-sysfs.c b/block/blk-sysfs.c
index fca9b158f4a0..1da8d4a4915a 100644
--- a/block/blk-sysfs.c
+++ b/block/blk-sysfs.c

@@ -11,6 +11,7 @@
 #include <linux/blktrace_api.h>
 #include <linux/blk-mq.h>
 #include <linux/blk-cgroup.h>
+#include <linux/debugfs.h>
 
 #include "blk.h"
 #include "blk-mq.h"

@@ -1011,6 +1012,46 @@ int blk_register_queue(struct gendisk *disk)
 }
 EXPORT_SYMBOL_GPL(blk_register_queue);
 
+#ifdef CONFIG_DEBUG_FS
+/*
+ * blk_prepare_release_queue - rename q->debugfs_dir and q->blk_trace->dir
+ * @q: request_queue of which the dir to be renamed belong to.
+ *
+ * Because the final release of request_queue is in a workqueue, the
+ * cleanup might not been finished yet while the same device start to
+ * create. It's not correct if q->debugfs_dir still exist while trying
+ * to create a new one.
+ */
+static void blk_prepare_release_queue(struct request_queue *q)
+{
+	struct dentry *new = NULL;
+	struct dentry **old = NULL;
+	char name[DNAME_INLINE_LEN];
+	int i = 0;
+
+#ifdef CONFIG_BLK_DEBUG_FS
+	if (!IS_ERR_OR_NULL(q->debugfs_dir))
+		old = &q->debugfs_dir;
+#endif
+#ifdef CONFIG_BLK_DEV_IO_TRACE
+	/* q->debugfs_dir and q->blk_trace->dir can't both exist */
+	if (q->blk_trace && !IS_ERR_OR_NULL(q->blk_trace->dir))
+		old = &q->blk_trace->dir;
+#endif