New Attacks and Defense for Encrypted-Address Cache

Conflict-based cache attacks can allow an adversary to infer the access pattern of a co-running application by orchestrating evictions via cache conflicts. Such attacks can be mitigated by randomizing the location of the lines in the cache. Our recent proposal, CEASER, makes cache randomization practical by accessing the cache using an encrypted address and periodically changing the encryption key. CEASER was analyzed with the state-of-the-art algorithm on forming eviction sets, and the analysis showed that CEASER with a Remap-Rate of 1% is sufficient to tolerate years of attack. In this paper, we present two new attacks that significantly push the state-of-the-art in forming eviction sets. Our first attack reduces the time required to form the eviction set from O (L
²
) to O(L), where L is the number of lines in the attack. This attack is 35x faster than the best-known attack and requires that the Remap-Rate of CEASER be increased to 35%. Our second attack exploits the replacement policy (we analyze LRU, RRIP, and Random) to form eviction set quickly and requires that the Remap-Rate of CEASER be increased to more than 100%, incurring impractical overheads. To improve the robustness of CEASER against these attacks in a practical manner, we propose Skewed-CEASER (CEASER-S), which divides the cache ways into multiple partitions and maps the cache line to be resident in a different set in each partition. This design significantly improves the robustness of CEASER, as the attacker must form an eviction set that can dislodge the line from multiple possible locations. We show that CEASER-S can tolerate years of attacks while retaining a Remap-Rate of 1%. CEASER-S incurs negligible slowdown (within 1%) and a storage overhead of less than 100 bytes for the newly added structures.