ovl: copy_up_xattr(): use strnlen

Be defensive about what underlying fs provides us in the returned xattr
list buffer.  strlen() may overrun the buffer, so use strnlen() and WARN if
the contents are not properly null terminated.
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>
Cc: <stable@vger.kernel.org>

fs/overlayfs/copy_up.c

@@ -57,6 +57,7 @@ int ovl_copy_xattr(struct dentry *old, struct dentry *new)
 	ssize_t list_size, size, value_size = 0;
 	char *buf, *name, *value = NULL;
 	int uninitialized_var(error);
+	size_t slen;
 
 	if (!old->d_inode->i_op->getxattr ||
 	    !new->d_inode->i_op->getxattr)
@@ -79,7 +80,16 @@ int ovl_copy_xattr(struct dentry *old, struct dentry *new)
 		goto out;
 	}
 
-	for (name = buf; name < (buf + list_size); name += strlen(name) + 1) {
+	for (name = buf; list_size; name += slen) {
+		slen = strnlen(name, list_size) + 1;
+
+		/* underlying fs providing us with an broken xattr list? */
+		if (WARN_ON(slen > list_size)) {
+			error = -EIO;
+			break;
+		}
+		list_size -= slen;
+
 		if (ovl_is_private_xattr(name))
 			continue;
 retry: