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


Jiyong Yu and Aishani Dutta, University of Illinois Urbana-Champaign; Trent Jaeger, Pennsylvania State University; David Kohlbrenner, University of Washington; Christopher W. Fletcher, University of Illinois Urbana-Champaign


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 timerless cache attacks targeting Apple M1 CPUs named Synchronization Storage Channels (S 2C). 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.

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@inproceedings {291182,
author = {Jiyong Yu and Aishani Dutta and Trent Jaeger and David Kohlbrenner and Christopher W. Fletcher},
title = {Synchronization Storage Channels ({{{{{S2C)}}}}}: Timer-less Cache {Side-Channel} Attacks on the Apple M1 via Hardware Synchronization Instructions},
booktitle = {32nd USENIX Security Symposium (USENIX Security 23)},
year = {2023},
isbn = {978-1-939133-37-3},
address = {Anaheim, CA},
pages = {1973--1990},
url = {},
publisher = {USENIX Association},
month = aug

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