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Recent years have seen a growing concern over System Management Mode (SMM) and its broad access to platform resources. The SMI Transfer Monitor (STM) is Intel's most powerful executing CPU context. The STM is a firmware-based hypervisor that applies the principle of least privilege to powerful System Management Interrupt (SMI) handlers that control runtime firmware. These handlers have traditionally had full access to memory as well as the register state of applications and kernel code even when their functionality did not require it. The STM has been been enabled for UEFI and, most recently, coreboot firmware, adding protection against runtime SMM-based attacks as well as establishing a firmware-based Trusted Execution Environment (TEE) capability. We provide a detailed overview of the STM architecture, evaluate its protections, and quantify its performance. Our results show the STM can protect against published SMM vulnerabilities with tolerable performance overheads.

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