CopyCat: Controlled Instruction-Level Attacks on Enclaves for Maximal Key Extraction
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The adversarial model presented by trusted execution environments (TEEs) has prompted researchers to investigate unusual attack vectors. One particularly powerful class of controlled-channel attacks abuses page-table modifications to reliably track enclave memory accesses at a page-level granularity. Crucially, in contrast to noisy microarchitectural timing leakages, this line of deterministic controlled-channel attacks abuses indispensable architectural interfaces and hence cannot be mitigated by tweaking microarchitectural resources. We propose an innovative controlled-channel attack, named CopyCat, that deterministically counts the number of instructions executed within a single enclave code page. We show that combining the instruction counts harvested by CopyCat with traditional, coarse-grained page-level leakage allows to accurately reconstruct enclave control flow at a maximal instruction-level granularity. CopyCat can identify intra-page and intra-cache line branch decisions which may ultimately differ only in a single instruction, highlighting that even extremely subtle control flow deviations can deterministically be leaked from secure enclaves. We demonstrate the improved resolution and practicality of CopyCat on Intel SGX platforms in an extensive study of single-trace and deterministic attacks against side-channel hardened cryptographic libraries. As a result, we disclose multiple vulnerabilities in the the latest version of widely-used cryptographic libraries: WolfSSL, Libgcrypt and OpenSSL, and propose novel algorithmic attacks to perform single-trace key extraction. Our findings mark the importance of stricter verification of cryptographic implementations, especially in the context of TEEs.
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