genhd: Fix BUG in blkdev_open()

When two blkdev_open() calls for a partition race with device removal
and recreation, we can hit BUG_ON(!bd_may_claim(bdev, whole, holder)) in
blkdev_open(). The race can happen as follows:

CPU0 CPU1 CPU2
del_gendisk()
bdev_unhash_inode(part1);

blkdev_open(part1, O_EXCL) blkdev_open(part1, O_EXCL)
bdev = bd_acquire() bdev = bd_acquire()
blkdev_get(bdev)
bd_start_claiming(bdev)
- finds old inode 'whole'
bd_prepare_to_claim() -> 0
bdev_unhash_inode(whole);
<device removed>
<new device under same
number created>
blkdev_get(bdev);
bd_start_claiming(bdev)
- finds new inode 'whole'
bd_prepare_to_claim()
- this also succeeds as we have
different 'whole' here...
- bad things happen now as we
have two exclusive openers of
the same bdev

The problem here is that block device opens can see various intermediate
states while gendisk is shutting down and then being recreated.

We fix the problem by introducing new lookup_sem in gendisk that
synchronizes gendisk deletion with get_gendisk() and furthermore by
making sure that get_gendisk() does not return gendisk that is being (or
has been) deleted. This makes sure that once we ever manage to look up
newly created bdev inode, we are also guaranteed that following
get_gendisk() will either return failure (and we fail open) or it
returns gendisk for the new device and following bdget_disk() will
return new bdev inode (i.e., blkdev_open() follows the path as if it is
completely run after new device is created).

Reported-and-analyzed-by: Hou Tao <[e-mail]>
Tested-by: Hou Tao <[e-mail]>
Signed-off-by: Jan Kara <[e-mail]>
Signed-off-by: Jens Axboe <[e-mail]>

block/genhd.c

···
 	blk_integrity_del(disk);
 	disk_del_events(disk);
 
+	/*
+	 * Block lookups of the disk until all bdevs are unhashed and the
+	 * disk is marked as dead (GENHD_FL_UP cleared).
+	 */
+	down_write(&disk->lookup_sem);
 	/* invalidate stuff */
 	disk_part_iter_init(&piter, disk,
 			     DISK_PITER_INCL_EMPTY | DISK_PITER_REVERSE);
···
 	bdev_unhash_inode(disk_devt(disk));
 	set_capacity(disk, 0);
 	disk->flags &= ~GENHD_FL_UP;
+	up_write(&disk->lookup_sem);
 
 	if (!(disk->flags & GENHD_FL_HIDDEN))
 		sysfs_remove_link(&disk_to_dev(disk)->kobj, "bdi");
···
 		spin_unlock_bh(&ext_devt_lock);
 	}
 
-	if (disk && unlikely(disk->flags & GENHD_FL_HIDDEN)) {
+	if (!disk)
+		return NULL;
+
+	/*
+	 * Synchronize with del_gendisk() to not return disk that is being
+	 * destroyed.
+	 */
+	down_read(&disk->lookup_sem);
+	if (unlikely((disk->flags & GENHD_FL_HIDDEN) ||
+		     !(disk->flags & GENHD_FL_UP))) {
+		up_read(&disk->lookup_sem);
 		put_disk_and_module(disk);
 		disk = NULL;
+	} else {
+		up_read(&disk->lookup_sem);
 	}
 	return disk;
 }
···
 			kfree(disk);
 			return NULL;
 		}
+		init_rwsem(&disk->lookup_sem);
 		disk->node_id = node_id;
 		if (disk_expand_part_tbl(disk, 0)) {
 			free_part_stats(&disk->part0);

include/linux/genhd.h

···
 	void *private_data;
 
 	int flags;
+	struct rw_semaphore lookup_sem;
 	struct kobject *slave_dir;
 
 	struct timer_rand_state *random;