Levioso: Efficient Compiler-Informed Secure Speculation

Spectre-type attacks have exposed a major class of vulnerabilities arising from speculative execution of instructions, the main performance enabler of modern CPUs. These attacks speculatively leak secrets that have been either speculatively loaded (seen in sandboxed programs) or non-speculatively loaded (seen in constant-time programs). Various hardware-only defenses have been proposed to mitigate both speculative and non-speculative secrets via all potential transmission channels. However, limited program knowledge is exposed to the hardware and these solutions conservatively restrict the execution of all instructions that can potentially leak.

In this work, we show that not all instructions depend on older unresolved branches and they can safely execute without leaking speculative information. We present Levioso, a novel hardware/software co-design, that provides comprehensive secure speculation guarantees while reducing performance overhead compared to existing defenses. Levioso informs the hardware about true branch dependencies and applies restrictions only when necessary. Our evaluations demonstrate that Levioso is able to significantly reduce the performance overhead compared to two prior defenses from 51% and 43% to just 23%.