btrfs: hold ref on root in btrfs_ioctl_default_subvol

We look up an arbitrary fs root here, we need to hold a ref on the root
for the duration.
Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/ioctl.c

@@ -3986,7 +3986,7 @@ static long btrfs_ioctl_default_subvol(struct file *file, void __user *argp)
 	struct btrfs_root *new_root;
 	struct btrfs_dir_item *di;
 	struct btrfs_trans_handle *trans;
-	struct btrfs_path *path;
+	struct btrfs_path *path = NULL;
 	struct btrfs_key location;
 	struct btrfs_disk_key disk_key;
 	u64 objectid = 0;
@@ -4017,44 +4017,50 @@ static long btrfs_ioctl_default_subvol(struct file *file, void __user *argp)
 		ret = PTR_ERR(new_root);
 		goto out;
 	}
-	if (!is_fstree(new_root->root_key.objectid)) {
+	if (!btrfs_grab_fs_root(new_root)) {
 		ret = -ENOENT;
 		goto out;
 	}
+	if (!is_fstree(new_root->root_key.objectid)) {
+		ret = -ENOENT;
+		goto out_free;
+	}
 
 	path = btrfs_alloc_path();
 	if (!path) {
 		ret = -ENOMEM;
-		goto out;
+		goto out_free;
 	}
 	path->leave_spinning = 1;
 
 	trans = btrfs_start_transaction(root, 1);
 	if (IS_ERR(trans)) {
-		btrfs_free_path(path);
 		ret = PTR_ERR(trans);
-		goto out;
+		goto out_free;
 	}
 
 	dir_id = btrfs_super_root_dir(fs_info->super_copy);
 	di = btrfs_lookup_dir_item(trans, fs_info->tree_root, path,
 				   dir_id, "default", 7, 1);
 	if (IS_ERR_OR_NULL(di)) {
-		btrfs_free_path(path);
+		btrfs_release_path(path);
 		btrfs_end_transaction(trans);
 		btrfs_err(fs_info,
 			  "Umm, you don't have the default diritem, this isn't going to work");
 		ret = -ENOENT;
-		goto out;
+		goto out_free;
 	}
 
 	btrfs_cpu_key_to_disk(&disk_key, &new_root->root_key);
 	btrfs_set_dir_item_key(path->nodes[0], di, &disk_key);
 	btrfs_mark_buffer_dirty(path->nodes[0]);
-	btrfs_free_path(path);
+	btrfs_release_path(path);
 
 	btrfs_set_fs_incompat(fs_info, DEFAULT_SUBVOL);
 	btrfs_end_transaction(trans);
+out_free:
+	btrfs_put_fs_root(new_root);
+	btrfs_free_path(path);
 out:
 	mnt_drop_write_file(file);
 	return ret;