The Flask architecture, which mainly contains object manager (OM) and security server (SS), is widely used to support flexible security policies in operating system. In nature, OM and SS should be isolated from each other to separate decision from enforcement. However, current implementation of Flask, such as SELinux and SEBSD, puts both OM and SS in the same address space. If one component is subverted, the whole system will be exposed to the attacker. In this paper, we present hardware assisted in-VM isolation to improve the security of the Flask implementation. The key of our approach is the separation of SS from other parts of guest OS by constructing hardware assisted page tables at the hypervisor level. In this way SS can execute in a strongly isolated address space with respect to its associated guest OS, and therefore can provide a trustworthy and centralized repository for policy and decision-making. Our experiment shows that our method introduces moderate performance overhead.