Measuring Long Wire Leakage with Ring Oscillators in Cloud FPGAs

Recent investigations into FPGA routing resources have shown that long wires in FPGAs leak information about their state in a way which can be measured using ring oscillators. Although in many cases this leakage does not pose a security threat, the possibility of multi-tenant use of FPGA resources invites potential side-and covert-channel attacks exploiting long wire leakage. However, prior work has ignored the realities of cloud environments, which may pose restrictions on the generated bitstreams, such as disallowing combinatorial loops. In this paper, we first demonstrate that the long wire leakage phenomenon persists even in the high-end Virtex UltraScale+ FPGA family. We then evaluate two ring oscillator designs that overcome combinatorial loop restrictions employed by cloud FPGA providers. We experimentally measure the long wire leakage of Virtex UltraScale+ FPGAs in the lab as well as in the Amazon and Huawei FPGA clouds. We show that the two new ring oscillator designs provide almost-identical estimates for the strength of the leakage as traditional ring oscillators, allowing us to measure femtosecond-scale changes in the delays of the long wires. We finally present a set of defense mechanisms that can prevent the new ring oscillator designs from being instantiated in the cloud and the long wire leakage from being exploited.