Timing Side-channel Attacks and Countermeasures in CPU Microarchitectures

Microarchitectural vulnerabilities, such as Meltdown and Spectre, exploit subtle microarchitecture state to steal the user’s secret data and even compromise the operating systems. In recent years, considerable discussion lies in understanding the attack-defense mechanisms and exploitability of such vulnerabilities. Unfortunately, there have been few investigations into a systematic elaboration of threat models, attack scenarios and requirements, and defense targets of the resulting attacks. In this article, we fill this gap and make the following contributions. We first propose two sets of taxonomies for classifying microarchitectural timing side-channel attacks and their countermeasures according to various attack conditions. Based on the taxonomies proposed, we then review published attacks and existing defenses and systematically analyze their internals. In particular, we also provide a comprehensive analysis of the similarities and differences among those attacks, uncovering the corresponding practicality and severity by identifying the attack targets/platforms and the security boundaries that can be bypassed to reveal information. We further examine the scalability of those defenses through specifying expected defense goals and costs. We also discuss corresponding detection methods based on different classifications. Finally, we propose several key challenges of existing countermeasures and the attack trends, and discuss directions for future research.