In virtual machine (VM) systems, mandatory access control (MAC) enforcement is possible now. This technique is both stronger and more flexible than traditional VM isolation, even if network communication is controlled. Unfortunately all of the VM systems with the MAC enforcement does not consider that the MAC controls may be distorted by covert channels, which constitute an important risk in VM systems. Traditional MAC models have difficulties being enforced to reduce the risk of covert flows in VM systems due to the many constraints and the lack of flexibility. In this paper, we identify access control requirements for managing covert channels in VM systems through a critical analysis of the ways by which classical models constrain the covert information flows and we propose a model called the Prioritized Chinese Wall model (PCW) to reduce the risk of covert flows in VM systems while preserving the flexibility. Furthermore, we enforce the policy in sHype/Xen VM system.
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