VoltJockey: Breaching TrustZone by Software-Controlled Voltage Manipulation over Multi-core Frequencies

ARM TrustZone builds a trusted execution environment based on the concept of hardware separation. It has been quite successful in defending against various software attacks and forcing attackers to explore vulnerabilities in interface designs and side channels. The recently reported CLKscrew attack breaks TrustZone through software by overclocking CPU to generate hardware faults. However, overclocking makes the processor run at a very high frequency, which is relatively easy to detect and prevent, for example by hardware frequency locking. In this paper, we propose an innovative software-controlled hardware fault-based attack, VoltJockey, on multi-core processors that adopt dynamic voltage and frequency scaling (DVFS) techniques for energy efficiency. Unlike CLKscrew, we manipulate the voltages rather than the frequencies via DVFS unit to generate hardware faults on the victim cores, which makes VoltJockey stealthier and harder to prevent than CLKscrew. We deliberately control the fault generation to facilitate differential fault analysis to break TrustZone. The entire attack process is based on software without any involvement of hardware. We implement VoltJockey on an ARM-based Krait processor from a commodity Android phone and demonstrate how to reveal the AES key from TrustZone and how to breach the RSA-based TrustZone authentication. These results suggest that VoltJockey has a comparable efficiency to side channels in obtaining TrustZone-guarded credentials, as well as the potential of bypassing the RSA-based verification to load untrusted applications into TrustZone. We also discuss both hardware-based and software-based countermeasures and their limitations.