[PATCH 2/2] block: fix use after free for bd_holder_dir

[Yu Kuai <yukuai1@xxxxxxxxxxxxxxx>]

From: Yu Kuai <yukuai3@xxxxxxxxxx>

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@xxxxxxxxxx>
---
 block/holder.c | 23 ++++++++++++++++-------
 1 file changed, 16 insertions(+), 7 deletions(-)

diff --git a/block/holder.c b/block/holder.c
index 5fc68238ce3a..1c6c5b132a92 100644
--- a/block/holder.c
+++ b/block/holder.c
@@ -78,19 +78,32 @@ int bd_link_disk_holder(struct block_device *bdev, struct gendisk *disk)
 	if (bdev->bd_disk == disk)
 		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;
+	}
 
-	WARN_ON_ONCE(!bdev->bd_holder);
+	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) {
 		holder->refcnt++;
+		kobject_put(bdev->bd_holder_dir);
 		goto out_unlock;
 	}
 
 	holder = kzalloc(sizeof(*holder), GFP_KERNEL);
 	if (!holder) {
 		ret = -ENOMEM;
+		kobject_put(bdev->bd_holder_dir);
 		goto out_unlock;
 	}
 
@@ -101,16 +114,12 @@ int bd_link_disk_holder(struct block_device *bdev, struct gendisk *disk)
 		ret = __link_disk_holder(bdev, disk);
 		if (ret) {
 			kfree(holder);
+			kobject_put(bdev->bd_holder_dir);
 			goto out_unlock;
 		}
 	}
 
 	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);
 
 out_unlock:
 	mutex_unlock(&disk->open_mutex);
-- 
2.31.1