An Exploratory Analysis of Microcode as a Building Block for System Defenses

TL;DR

This paper explores the potential of microcode as a flexible platform for implementing system security defenses, reverse engineering its inner workings and demonstrating practical security features on commercial CPUs.

Contribution

It provides new reverse engineering insights into proprietary microcode and demonstrates how to implement security defenses and a trusted environment using microcode on off-the-shelf CPUs.

Findings

Microcode can implement timing attack mitigations.

Microcode enables hardware-assisted address sanitization.

Microcode can support a microcode-assisted instrumentation framework.

Abstract

Microcode is an abstraction layer used by modern x86 processors that interprets user-visible CISC instructions to hardware-internal RISC instructions. The capability to update x86 microcode enables a vendor to modify CPU behavior in-field, and thus patch erroneous microarchitectural processes or even implement new features. Most prominently, the recent Spectre and Meltdown vulnerabilities were mitigated by Intel via microcode updates. Unfortunately, microcode is proprietary and closed source, and there is little publicly available information on its inner workings. In this paper, we present new reverse engineering results that extend and complement the public knowledge of proprietary microcode. Based on these novel insights, we show how modern system defenses and tools can be realized in microcode on a commercial, off-the-shelf AMD x86 CPU. We demonstrate how well-established system security defenses such as timing attack mitigations, hardware-assisted address sanitization, and instruction set randomization can be realized in microcode. We also present a proof-of-concept implementation of a microcode-assisted instrumentation framework. Finally, we show how a secure microcode update mechanism and enclave functionality can be implemented in microcode to realize a small trusted execution environment. All microcode programs and the whole infrastructure needed to reproduce and extend our results are publicly available.