Mitigating Speculative Execution Attacks via Context-Sensitive Fencing

Abstract

This paper presents Context-Sensitive Fencing (CSF), a micro-code-level mitigation for multiple variants of Spectre. CSF leverages the ability to dynamically alter the instruction stream via the decoder, to seamlessly inject new micro-ops, including fences, only when dynamic conditions indicate they are needed. This enables the processor to protect against the attack with minimal impact on the efficacy of key performance features such as speculative execution. This research also examines several alternative fence implementations, and introduces new types of fences which allow most dynamic reorderings of loads and stores while still preventing speculative accesses from changing visible microarchitectural state.