Taming Parallelism in a Multi-Variant Execution Environment.

Exploit mitigations, by themselves, do not stop determined and well-resourced adversaries from compromising vulnerable software through memory corruption. Multi-variant execution environments (MVEEs) add additional assurance by executing multiple, diversified copies (variants) of the same program in lockstep while monitoring their behavior for signs of attacks (divergence). While executing multiple copies of the same program requires additional computational resources, modern MVEEs run many workloads at near-native speed and can detect adversaries before they leak secrets or achieve persistence on the host system. Multi-threaded programs are challenging to execute in lockstep by an MVEE. If the threads in a set of variants are not scheduled in the exact same order, the variants will diverge from each other in terms of the system calls they make. While benign, such divergence undermines the MVEEs ability detect divergence caused by malicious program inputs. To address this problem, we developed an MVEE-specific synchronization scheme that lets us execute a set of multithreaded variants in lockstep without causing benign divergence. Our fully-fledged MVEE runs the PARSEC 2.1 and SPLASH-2x parallel benchmarks (with four worker threads per variant) with a slowdown of less than 15% relative to unprotected execution. Addressing this longstanding compatibility issue makes MVEEs a viable defense for a far greater range of realistic workloads.