MDL-28350 prevent bogus role assignment via externallib

enrol/externallib.php

@@ -175,6 +175,12 @@ public static function role_assign($assignments) {
             self::validate_context($context);
             require_capability('moodle/role:assign', $context);
 
+            // throw an exception if user is not able to assign the role in this context
+            $roles = get_assignable_roles($context, ROLENAME_SHORT);
+            if (!key_exists($assignment['roleid'], $roles)) {
+                throw new invalid_parameter_exception('Can not assign roleid='.$assignment['roleid'].' in contextid='.$assignment['contextid']);
+            }
+
             role_assign($assignment['roleid'], $assignment['userid'], $assignment['contextid']);
         }
 
@@ -231,6 +237,12 @@ public static function role_unassign($unassignments) {
             self::validate_context($context);
             require_capability('moodle/role:assign', $context);
 
+            // throw an exception if user is not able to unassign the role in this context
+            $roles = get_assignable_roles($context, ROLENAME_SHORT);
+            if (!key_exists($unassignment['roleid'], $roles)) {
+                throw new invalid_parameter_exception('Can not unassign roleid='.$unassignment['roleid'].' in contextid='.$unassignment['contextid']);
+            }
+
             role_unassign($unassignment['roleid'], $unassignment['userid'], $unassignment['contextid']);
         }