[PATCH] block: break circular locks in blk_request_module

[Desmond Cheong Zhi Xi]

Syzbot reported a circular locking dependency:
https://syzkaller.appspot.com/bug?id=7bd106c28e846d1023d4ca915718b1a0905444cb

This happens because of the following lock dependencies:

1. loop_ctl_mutex -> bdev->bd_mutex (when loop_control_ioctl calls
loop_remove, which then calls del_gendisk; this also happens in
loop_exit which eventually calls loop_remove)

2. bdev->bd_mutex -> mtd_table_mutex (when blkdev_get_by_dev calls
__blkdev_get, which then calls blktrans_open)

3. mtd_table_mutex -> major_names_lock (when register_mtd_blktrans
calls __register_blkdev)

4. major_names_lock -> loop_ctl_mutex (when blk_request_module calls
loop_probe)

Hence there's an overall dependency of:

loop_ctl_mutex   ----------> bdev->bd_mutex
      ^                            |
      |                            |
      |                            v
major_names_lock <---------  mtd_table_mutex

We can break this circular dependency by saving the reference to
probe in blk_request_module, then calling it after releasing
major_names_lock. This is safe because even if struct blk_major_name
is freed, the reference to the probe function is still valid.

Reported-and-tested-by: syzbot+6a8a0d93c91e8fbf2e80@xxxxxxxxxxxxxxxxxxxxxxxxx
Signed-off-by: Desmond Cheong Zhi Xi <desmondcheongzx@xxxxxxxxx>
---
 block/genhd.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/block/genhd.c b/block/genhd.c
index 9f8cb7beaad1..ccaa5cf620f5 100644
--- a/block/genhd.c
+++ b/block/genhd.c
@@ -676,12 +676,14 @@ void blk_request_module(dev_t devt)
 {
 	unsigned int major = MAJOR(devt);
 	struct blk_major_name **n;
+	void (*probe)(dev_t devt);
 
 	mutex_lock(&major_names_lock);
 	for (n = &major_names[major_to_index(major)]; *n; n = &(*n)->next) {
 		if ((*n)->major == major && (*n)->probe) {
-			(*n)->probe(devt);
+			probe = (*n)->probe;
 			mutex_unlock(&major_names_lock);
+			probe(devt);
 			return;
 		}
 	}
-- 
2.25.1