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This paper describes context-sensitive fencing (CSF), a microcode-level defense against multiple variants of Spectre. CSF leverages the ability to dynamically alter the decoding of the instruction stream, 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, but with minimal impact on the efficacy of key performance features such as speculative execution. This research also examines several alternative fence implementations, and introduces three new types of fences which allow most dynamic reorderings of loads and stores, but in a way that prevents speculative accesses from changing visible cache state. These optimizations reduce the performance overhead of the defense mechanism, compared to state-of-the-art software-based fencing mechanisms by a factor of six. CCS Concepts • Security and privacy → Side-channel analysis and countermeasures; Systems security; Information flow control; • Computer systems organization → Architectures.

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