Uncore Encore: Covert Channels Exploiting Uncore Frequency Scaling

Modern processors dynamically adjust clock frequencies and voltages to reduce energy consumption. Recent Intel processors separate the uncore frequency from the core frequency, using Uncore Frequency Scaling (UFS) to adapt the uncore frequency to various workloads. While UFS improves power efficiency, it also introduces security vulnerabilities. In this paper, we study the feasibility of covert channels exploiting UFS. First, we conduct a series of experiments to understand the details of UFS, such as the factors that can cause uncore frequency variations. Then, based on the results, we build the first UFS-based covert channel, UF-variation, which works both across-cores and across-processors. Finally, we analyze the robustness of UF-variation under known defense mechanisms against uncore covert channels, and show that UF-variation remains functional even with those defenses in place.