core: always create /dev/kdbus/ns (and make it private 0700) after setting up the kdbus system bus

src/core/manager.c

@@ -427,6 +427,12 @@ static int manager_setup_kdbus(Manager *m) {
         }
 
         log_debug("Successfully set up kdbus on %s", p);
+
+        /* Create the namespace directory here, so that the contents
+         * of that directory is not visible to non-root users. This is
+         * necessary to ensure that users cannot get access to busses
+         * of virtualized users when no UID namespacing is used. */
+        mkdir_p_label("/dev/kdbus/ns", 0700);
 #endif
 
         return 0;