TimeGaps Channels: Exploiting CPU Halted Time for Fun and Profit

What do computers do when they do not compute? To answer this question, we investigate TimeGaps, periods during program execution, in which the timestamp counter progresses while the CPU is halted. We develop techniques for identifying TimeGaps and find that on Intel processors, TimeGaps amount to over 1% of the elapsed time. We further find that TimeGaps occurrence correlate with frequency transitions at either the CPU or at the Integrated Graphics Processing Unit (iGPU). We then turn our attention to the security impact of TimeGaps under two settings: default Dynamic Voltage and Frequency Scaling (DVFS) configuration, and fixed-frequency countermeasures. Under default DVFS settings, TimeGaps exhibit leakage capabilities comparable to state-of-the-art CPU-frequency-based side channels, i.e., Hertzbleed. Leveraging this, we infer website visits with an accuracy of 98.0% on Chrome and 85.2% on Tor, and extract cryptographic keys from Cloudflare's CIRCL library. Under fixed CPU frequency, where Hertzbleed is no longer effective, TimeGaps induced by iGPU frequency transitions continue to leak iGPU instruction and operand-level information. Moreover, TimeGaps re-enable three frequency-based sidechannel attacks previously believed to be mitigated by fixing CPU frequency, including pixel stealing with a high accuracy of 98.2%, robust website fingerprinting (92.2% on Chrome, 87.4% on Tor), and keystroke detection with a precision of over 84.6%.