The importance of security-enhancing mechanisms at the kernel level, such as an access control, has been increasingly emphasized as the weaknesses and limitation of mechanisms at the user level have been revealed. Among many access controls available, role based access control (RBAC) is mandatory and supports the separation of duty when compared to discretionary access control (DAC). With these advantages, RBAC has been widely implemented at various levels of computing environments, such as the operating system and database management system levels. However, the overheads for supporting all of the RBAC features and flexibility are significant. We designed a fast, simple, and mandatory access control model with some RBAC and DAC characteristics, then implemented a prototype and measured its overheads.