Load-Step: A Precise TrustZone Execution Control Framework for Exploring New Side-channel Attacks Like Flush+Evict

Trusted execution environments (TEEs) are imported into processors to protect sensitive programs against a potentially malicious operating system (OS), though, they are announced not effective in defending microarchitecture ($\mu$ arch) side-channel attacks. Furthermore, TEE attackers often utilize their high privilege to strengthen attacks by interrupting the execution of victim programs. Maximum temporal resolution is achieved on the x86 platform, which interrupts and measures by every instruction. However, the capability of $\mu$ arch side-channel attacks and the precision a kernel-privileged attacker can achieve in the TrustZone system are still unexplored. In this paper, we propose Load-Step, a precise framework that periodically interrupts the victim program in the TrustZone system and then conducts $\mu$ arch side-channel attacks. Our self-designed benchmark shows that Load-Step can invoke interrupts with load-instruction precision. Based on Load-Step, we present Flush+Evict, a new side-channel attack detecting the Arm Cache Coherent Interconnect (ArmCCI). It outperforms Prime+Probe with much higher precision and 282 % of the profiling speed. When attacking the RSA decryption in the latest MbedTLS library, Load-Step can recover the full key by only a single trace in 7.5 seconds. Our work thus breaches the exponent blinding, which aims to defend RSA decryption against side-channel attacks in the MbedTLS library.