Flush+Flush: A Stealthier Last-Level Cache Attack

Research on cache attacks has shown that CPU caches leak significant information. Recent attacks either use the Flush+Reload technique on read-only shared memory or the Prime+Probe technique without shared memory, to derive encryption keys or eavesdrop on user input. Efficient countermeasures against these powerful attacks that do not cause a loss of performance are a challenge. In this paper, we use hardware performance counters as a means to detect access-based cache attacks. Indeed, existing attacks cause numerous cache references and cache misses and can subsequently be detected. We propose a new criteria that uses these events for ad-hoc detection. These findings motivate the development of a novel attack technique: the Flush+Flush attack. The Flush+Flush attack only relies on the execution time of the flush instruction, that depends on whether the data is cached or not. Like Flush+Reload, it monitors when a process loads read-only shared memory into the CPU cache. However, Flush+Flush does not have a reload step, thus causing no cache misses compared to typical Flush+Reload and Prime+Probe attacks. We show that the significantly lower impact on the hardware performance counters therefore evades detection mechanisms. The Flush+Flush attack has a performance close to state-of-the-art side channels in existing cache attack scenarios, while reducing cache misses significantly below the border of detectability. Our Flush+Flush covert channel achieves a transmission rate of 496KB/s which is 6.7 times faster than any previously published cache covert channel. To the best of our knowledge, this is the first work discussing the stealthiness of cache attacks both from the attacker and the defender perspective.