block: fix use after free for bd_holder_dir

Currently, the caller of bd_link_disk_holer() get 'bdev' by
blkdev_get_by_dev(), which will look up 'bdev' by inode number 'dev'.
Howerver, it's possible that del_gendisk() can be called currently, and
'bd_holder_dir' can be freed before bd_link_disk_holer() access it, thus
use after free is triggered.

t1:				t2:
bdev = blkdev_get_by_dev
				del_gendisk
				 kobject_put(bd_holder_dir)
				  kobject_free()
bd_link_disk_holder

Fix the problem by checking disk is still live and grabbing a reference
to 'bd_holder_dir' first in bd_link_disk_holder().
Signed-off-by: Yu Kuai <yukuai3@huawei.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20221115141054.1051801-9-yukuai1@huaweicloud.comSigned-off-by: Jens Axboe <axboe@kernel.dk>

block/holder.c

@@ -65,12 +65,24 @@ int bd_link_disk_holder(struct block_device *bdev, struct gendisk *disk)
 	if (WARN_ON_ONCE(!disk->slave_dir))
 		return -EINVAL;
 
-	mutex_lock(&disk->open_mutex);
+	/*
+	 * del_gendisk drops the initial reference to bd_holder_dir, so we
+	 * need to keep our own here to allow for cleanup past that point.
+	 */
+	mutex_lock(&bdev->bd_disk->open_mutex);
+	if (!disk_live(bdev->bd_disk)) {
+		mutex_unlock(&bdev->bd_disk->open_mutex);
+		return -ENODEV;
+	}
+	kobject_get(bdev->bd_holder_dir);
+	mutex_unlock(&bdev->bd_disk->open_mutex);
 
+	mutex_lock(&disk->open_mutex);
 	WARN_ON_ONCE(!bdev->bd_holder);
 
 	holder = bd_find_holder_disk(bdev, disk);
 	if (holder) {
+		kobject_put(bdev->bd_holder_dir);
 		holder->refcnt++;
 		goto out_unlock;
 	}
@@ -92,11 +104,6 @@ int bd_link_disk_holder(struct block_device *bdev, struct gendisk *disk)
 		goto out_del_symlink;
 	list_add(&holder->list, &disk->slave_bdevs);
 
-	/*
-	 * del_gendisk drops the initial reference to bd_holder_dir, so we need
-	 * to keep our own here to allow for cleanup past that point.
-	 */
-	kobject_get(bdev->bd_holder_dir);
 	mutex_unlock(&disk->open_mutex);
 	return 0;
 
@@ -106,6 +113,8 @@ int bd_link_disk_holder(struct block_device *bdev, struct gendisk *disk)
 	kfree(holder);
 out_unlock:
 	mutex_unlock(&disk->open_mutex);
+	if (ret)
+		kobject_put(bdev->bd_holder_dir);
 	return ret;
 }
 EXPORT_SYMBOL_GPL(bd_link_disk_holder);