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Applying the composition principle to verify a hierarchy of security servers

This paper describes how the composition principle of Abadi and Lamport (1991) can be applied to specify and compose systems where access control policies are distributed among a hierarchy of agents. Examples of such systems are layered secure operating systems, where the mandatory access control policy is enforced by the lowest system layer and discretionary and application-specific policies are implemented by outer layers, and microkernel operating systems, where the access control policy may be distributed among a hierarchy of server processes. We specifically consider the case of a microkernel operating system type architecture, in which resource management policies are enforced by server processes outside of the kernel, and where the system access control policy is a composition of the distinct policies implemented by the servers. As an example, we have specified a two-server system, including both safety and progress properties. We formally verified the composition of the two server processes using the HOL theorem proving system.