Btrfs: fix a use-after-free bug in btrfs_dev_replace_finishing

free_device rcu callback, scheduled from btrfs_rm_dev_replace_srcdev,
can be processed before btrfs_scratch_superblock is called, which would
result in a use-after-free on btrfs_device contents.  Fix this by
zeroing the superblock before the rcu callback is registered.

Cc: Stefan Behrens <sbehrens@giantdisaster.de>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>
Signed-off-by: Josef Bacik <jbacik@fusionio.com>

fs/btrfs/dev-replace.c

@@ -535,10 +535,7 @@ static int btrfs_dev_replace_finishing(struct btrfs_fs_info *fs_info,
 	list_add(&tgt_device->dev_alloc_list, &fs_info->fs_devices->alloc_list);
 
 	btrfs_rm_dev_replace_srcdev(fs_info, src_device);
-	if (src_device->bdev) {
-		/* zero out the old super */
-		btrfs_scratch_superblock(src_device);
-	}
+
 	/*
 	 * this is again a consistent state where no dev_replace procedure
 	 * is running, the target device is part of the filesystem, the

fs/btrfs/volumes.c

@@ -1716,6 +1716,7 @@ void btrfs_rm_dev_replace_srcdev(struct btrfs_fs_info *fs_info,
 				 struct btrfs_device *srcdev)
 {
 	WARN_ON(!mutex_is_locked(&fs_info->fs_devices->device_list_mutex));
+
 	list_del_rcu(&srcdev->dev_list);
 	list_del_rcu(&srcdev->dev_alloc_list);
 	fs_info->fs_devices->num_devices--;
@@ -1725,9 +1726,13 @@ void btrfs_rm_dev_replace_srcdev(struct btrfs_fs_info *fs_info,
 	}
 	if (srcdev->can_discard)
 		fs_info->fs_devices->num_can_discard--;
-	if (srcdev->bdev)
+	if (srcdev->bdev) {
 		fs_info->fs_devices->open_devices--;
 
+		/* zero out the old super */
+		btrfs_scratch_superblock(srcdev);
+	}
+
 	call_rcu(&srcdev->rcu, free_device);
 }