Flush+Flush: A Fast and Stealthy Cache Attack

TL;DR

The paper introduces Flush+Flush, a novel cache attack that is faster and more stealthy than existing methods by relying solely on flush instruction timing, evading detection and enabling high-speed covert channels.

Contribution

It presents the first cache attack that does not cause cache misses and is undetectable by current detection mechanisms, significantly increasing attack speed and stealth.

Findings

Flush+Flush causes no cache misses

It achieves 496 KB/s in covert channels, 6.7 times faster than previous methods

It evades all current cache attack detection mechanisms

Abstract

Research on cache attacks has shown that CPU caches leak significant information. Proposed detection mechanisms assume that all cache attacks cause more cache hits and cache misses than benign applications and use hardware performance counters for detection. In this article, we show that this assumption does not hold by developing a novel attack technique: the Flush+Flush attack. The Flush+Flush attack only relies on the execution time of the flush instruction, which depends on whether data is cached or not. Flush+Flush does not make any memory accesses, contrary to any other cache attack. Thus, it causes no cache misses at all and the number of cache hits is reduced to a minimum due to the constant cache flushes. Therefore, Flush+Flush attacks are stealthy, i.e., the spy process cannot be detected based on cache hits and misses, or state-of-the-art detection mechanisms. The Flush+Flush attack runs in a higher frequency and thus is faster than any existing cache attack. With 496 KB/s in a cross-core covert channel it is 6.7 times faster than any previously published cache covert channel.