Synchronization Storage Channels (S²C): Timer-less Cache Side-Channel Attacks on the Apple M1 via Hardware Synchronization Instructions

Shared caches have been a prime target for mounting crossprocess/core side-channel attacks. Fundamentally, these attacks require a mechanism to accurately observe changes in cache state. Most cache attacks rely on timing measurements to indirectly infer cache state changes, and attack success hinges on the reliability/availability of accurate timing sources. Far fewer techniques have been proposed to directly observe cache state changes without reliance on timers. Further, none of said 'timer-less' techniques are accessible to userspace attackers targeting modern CPUs. This paper proposes a novel technique for mounting timer-less cache attacks targeting Apple M1 CPUs named Synchronization Storage Channels (S2C). The key observation is that the implementation of synchronization instructions, specifically Load-Linked/Store-Conditional (LL/SC), makes architectural state changes when L1 cache evictions occur. This by itself is a useful starting point for attacks, however faces multiple technical challenges when being used to perpetrate cross-core cache attacks. Specifically, LL/SC only observes L1 evictions (not shared L2 cache evictions). Further, each attacker thread can only simultaneously monitor one address at a time through LL/SC (as opposed to many). We propose a suite of techniques and reverse engineering to overcome these limitations, and demonstrate how a single-threaded userspace attacker can use LL/SC to simultaneously monitor multiple (up to 11) victim L2 sets and succeed at standard cache-attack applications, such as breaking cryptographic implementations and constructing covert channels.