MicroScope, and microarchitectural replay attacks in general, take advantage of the characteristics of speculative execution to trap the execution of the victim application in an infinite loop, enabling the attacker to amplify a side-channel attack by executing it indefinitely. Due to the nature of the replay, it can be used to effectively attack security critical trusted execution environments (secure enclaves), even under conditions where a side-channel attack would not be possible. At the same time, unlike speculative side-channel attacks, MicroScope can be used to amplify the correct path of execution, rendering many existing speculative side-channel defences ineffective. In this work, we generalize microarchitectural replay attacks beyond MicroScope and present an efficient defence against them. We make the observation that such attacks rely on repeated squashes of so-called "replay handles" and that the instructions causing the side-channel must reside in the same reorder buffer window as the handles. We propose Delay-on-Squash, a technique for tracking squashed instructions and preventing them from being replayed by speculative replay handles. Our evaluation shows that it is possible to achieve full security against microarchitectural replay attacks with very modest hardware requirements, while still maintaining 97% of the insecure baseline performance.
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