mtd: break circular locks in register_mtd_blktrans

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 holding mtd_table_mutex only
for the required critical section in register_mtd_blktrans. This
avoids the mtd_table_mutex -> major_names_lock dependency.

Reported-and-tested-by: syzbot+6a8a0d93c91e8fbf2e80@syzkaller.appspotmail.com
Co-developed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Desmond Cheong Zhi Xi <desmondcheongzx@gmail.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20210617160904.570111-1-desmondcheongzx@gmail.com

drivers/mtd/mtd_blkdevs.c

@@ -526,14 +526,10 @@ int register_mtd_blktrans(struct mtd_blktrans_ops *tr)
 	if (!blktrans_notifier.list.next)
 		register_mtd_user(&blktrans_notifier);
 
-
-	mutex_lock(&mtd_table_mutex);
-
 	ret = register_blkdev(tr->major, tr->name);
 	if (ret < 0) {
 		printk(KERN_WARNING "Unable to register %s block device on major %d: %d\n",
 		       tr->name, tr->major, ret);
-		mutex_unlock(&mtd_table_mutex);
 		return ret;
 	}
 
@@ -543,12 +539,12 @@ int register_mtd_blktrans(struct mtd_blktrans_ops *tr)
 	tr->blkshift = ffs(tr->blksize) - 1;
 
 	INIT_LIST_HEAD(&tr->devs);
-	list_add(&tr->list, &blktrans_majors);
 
+	mutex_lock(&mtd_table_mutex);
+	list_add(&tr->list, &blktrans_majors);
 	mtd_for_each_device(mtd)
 		if (mtd->type != MTD_ABSENT)
 			tr->add_mtd(tr, mtd);
-
 	mutex_unlock(&mtd_table_mutex);
 	return 0;
 }