Secure hybrid replacement policy: Mitigating conflict-based cache side channel attacks

Conflict-based attacks on the last-level cache (LLC) are the most threatening cache side channel attack scenarios, where attackers exploit cache set conflicts as a channel to obtain sensitive cache accesses of victims. In this paper, we observe that conflict-based attacks cause exploited cache sets to experience an abnormally large number of accesses. Protecting these sets with suspicious access patterns effectively interferes with the attacker's observation and thus mitigates such attacks. Based on this insight, we propose Secure Hybrid Replacement Policy (SHRP). SHRP captures suspicious sets by counting the number of accesses and alters their replacement policy to introduce noise into the attacker's probe. Specifically, When a request occurs an LLC hit, SHRP has a certain probability of sending an invalidation request to the suspicious set. If a request triggers an LLC replacement, SHRP has a certain probability of turning the request uncacheable. Because of these probabilistic obscuring actions, attackers are impossible to accurately infer the victim's behaviors on the protected cache set. The evaluation demonstrates that SHRP efficiently mitigates conflict-based attacks and induces negligible performance impacts and acceptable storage overhead.