STM/PE & XHIM

Eugene Myers
U.S. National Security Agency

We describe our work to demonstrate an enhanced SMI transfer monitor (STM) to provide protected execution services on the x86 platform. An STM is a hypervisor that executes in x86 system management mode (SMM) and functions as a peer to the hypervisor or operating system. The STM constrains the SMI handler, by hosting the handler in a virtual machine (VM). Otherwise, the SMI handler holds unconstrained access to the platform, which could undermine the assurance provided by DRTM or TXT.

Our STM enhancements create a protected execution capability by extending the STM to support additional VMs (PE/VM). These enhancements utilize the existing capabilities of the x86 processor and, thus, require no additional hardware. We modified an existing hypervisor integrity measurement engine to function in a PE/VM. The related discussion explains how the module can be loaded from a guest virtual machine and how page tables are used to restrict the access that the measurement engine is allowed to memory.


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