btrfs: fix transaction abort on set received ioctl due to item overflow

If the set received ioctl fails due to an item overflow when attempting to
add the BTRFS_UUID_KEY_RECEIVED_SUBVOL we have to abort the transaction
since we did some metadata updates before.

This means that if a user calls this ioctl with the same received UUID
field for a lot of subvolumes, we will hit the overflow, trigger the
transaction abort and turn the filesystem into RO mode. A malicious user
could exploit this, and this ioctl does not even requires that a user
has admin privileges (CAP_SYS_ADMIN), only that he/she owns the subvolume.

Fix this by doing an early check for item overflow before starting a
transaction. This is also race safe because we are holding the subvol_sem
semaphore in exclusive (write) mode.

A test case for fstests will follow soon.

Fixes: dd5f9615fc ("Btrfs: maintain subvolume items in the UUID tree")
CC: stable@vger.kernel.org # 3.12+
Reviewed-by: Anand Jain <asj@kernel.org>
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/ioctl.c

@@ -3929,6 +3929,25 @@ static long _btrfs_ioctl_set_received_subvol(struct file *file,
 		goto out;
 	}
 
+	received_uuid_changed = memcmp(root_item->received_uuid, sa->uuid,
+				       BTRFS_UUID_SIZE);
+
+	/*
+	 * Before we attempt to add the new received uuid, check if we have room
+	 * for it in case there's already an item. If the size of the existing
+	 * item plus this root's ID (u64) exceeds the maximum item size, we can
+	 * return here without the need to abort a transaction. If we don't do
+	 * this check, the btrfs_uuid_tree_add() call below would fail with
+	 * -EOVERFLOW and result in a transaction abort. Malicious users could
+	 * exploit this to turn the fs into RO mode.
+	 */
+	if (received_uuid_changed && !btrfs_is_empty_uuid(sa->uuid)) {
+		ret = btrfs_uuid_tree_check_overflow(fs_info, sa->uuid,
+						     BTRFS_UUID_KEY_RECEIVED_SUBVOL);
+		if (ret < 0)
+			goto out;
+	}
+
 	/*
 	 * 1 - root item
 	 * 2 - uuid items (received uuid + subvol uuid)
@@ -3944,8 +3963,6 @@ static long _btrfs_ioctl_set_received_subvol(struct file *file,
 	sa->rtime.sec = ct.tv_sec;
 	sa->rtime.nsec = ct.tv_nsec;
 
-	received_uuid_changed = memcmp(root_item->received_uuid, sa->uuid,
-				       BTRFS_UUID_SIZE);
 	if (received_uuid_changed &&
 	    !btrfs_is_empty_uuid(root_item->received_uuid)) {
 		ret = btrfs_uuid_tree_remove(trans, root_item->received_uuid,

fs/btrfs/uuid-tree.c

@@ -199,6 +199,44 @@ int btrfs_uuid_tree_remove(struct btrfs_trans_handle *trans, const u8 *uuid, u8
 	return 0;
 }
 
+/*
+ * Check if we can add one root ID to a UUID key.
+ * If the key does not yet exists, we can, otherwise only if extended item does
+ * not exceeds the maximum item size permitted by the leaf size.
+ *
+ * Returns 0 on success, negative value on error.
+ */
+int btrfs_uuid_tree_check_overflow(struct btrfs_fs_info *fs_info,
+				   const u8 *uuid, u8 type)
+{
+	BTRFS_PATH_AUTO_FREE(path);
+	int ret;
+	u32 item_size;
+	struct btrfs_key key;
+
+	if (WARN_ON_ONCE(!fs_info->uuid_root))
+		return -EINVAL;
+
+	path = btrfs_alloc_path();
+	if (!path)
+		return -ENOMEM;
+
+	btrfs_uuid_to_key(uuid, type, &key);
+	ret = btrfs_search_slot(NULL, fs_info->uuid_root, &key, path, 0, 0);
+	if (ret < 0)
+		return ret;
+	if (ret > 0)
+		return 0;
+
+	item_size = btrfs_item_size(path->nodes[0], path->slots[0]);
+
+	if (sizeof(struct btrfs_item) + item_size + sizeof(u64) >
+	    BTRFS_LEAF_DATA_SIZE(fs_info))
+		return -EOVERFLOW;
+
+	return 0;
+}
+
 static int btrfs_uuid_iter_rem(struct btrfs_root *uuid_root, u8 *uuid, u8 type,
 			       u64 subid)
 {

fs/btrfs/uuid-tree.h

@@ -12,6 +12,8 @@ int btrfs_uuid_tree_add(struct btrfs_trans_handle *trans, const u8 *uuid, u8 typ
 			u64 subid);
 int btrfs_uuid_tree_remove(struct btrfs_trans_handle *trans, const u8 *uuid, u8 type,
 			u64 subid);
+int btrfs_uuid_tree_check_overflow(struct btrfs_fs_info *fs_info,
+				   const u8 *uuid, u8 type);
 int btrfs_uuid_tree_iterate(struct btrfs_fs_info *fs_info);
 int btrfs_create_uuid_tree(struct btrfs_fs_info *fs_info);
 int btrfs_uuid_scan_kthread(void *data);