MIRAGE: Mitigating Conflict-Based Cache Attacks with a Practical Fully-Associative Design
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Shared caches in processors are vulnerable to conflict-based side-channel attacks, whereby an attacker can monitor the access pattern of a victim by evicting victim cache lines using cache-set conflicts. Recent mitigations propose a randomized mapping of addresses to cache lines, to obfuscate locations of set-conflicts. However, newer attacks are able to discover conflicting addresses despite such mitigations, because these designs select candidates for eviction only from a small set of conflicting lines. This paper presents Mirage, a practical design for a fully associative cache, wherein eviction candidates are selected randomly from among all the lines resident in the cache, to be immune to set-conflicts. A key challenge in enabling such a design for large shared caches (containing tens of thousands of resident cache lines) is managing the complexity of cache-lookup, as a naive design can require searching through all the resident lines. Mirage achieves full-associativity while retaining practical set-associative lookups by decoupling placement and replacement, using pointer-based indirection from tag-store to data-store to allow a newly installed address to globally evict data of any random resident line. To eliminate set-conflicts, Mirage provisions extra invalid tags and a skewed-associative tag-store design where lines can be installed without set-conflict, along with a load-aware skew-selection policy that guarantees the availability of sets with invalid tags. Our analysis shows Mirage provides the global eviction property of a fully-associative cache throughout system-lifetime (violations of full-associativity, i.e set-conflicts, occur less than once in 10^4 to 10^17 years), thus offering a principled defense against any eviction-set discovery and any potential conflict based attacks. Mirage incurs limited slowdown (2 storage compared to a non-secure cache.
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