Inferring Fine-grained Control Flow Inside SGX Enclaves with Branch Shadowing


Sangho Lee, Ming-Wei Shih, Prasun Gera, Taesoo Kim, and Hyesoon Kim, Georgia Institute of Technology; Marcus Peinado, Microsoft Research


Intel has introduced a hardware-based trusted execution environment, Intel Software Guard Extensions (SGX), that provides a secure, isolated execution environment, or enclave, for a user program without trusting any underlying software (e.g., an operating system) or firmware. Researchers have demonstrated that SGX is vulnerable to a page-fault-based attack. However, the attack only reveals page-level memory accesses within an enclave.

In this paper, we explore a new, yet critical, sidechannel attack, branch shadowing, that reveals fine-grained control flows (branch granularity) in an enclave. The root cause of this attack is that SGX does not clear branch history when switching from enclave to non-enclave mode, leaving fine-grained traces for the outside world to observe, which gives rise to a branch-prediction side channel. However, exploiting this channel in practice is challenging because 1) measuring branch execution time is too noisy for distinguishing fine-grained control-flow changes and 2) pausing an enclave right after it has executed the code block we target requires sophisticated control. To overcome these challenges, we develop two novel exploitation techniques: 1) a last branch record (LBR)-based history-inferring technique and 2) an advanced programmable interrupt controller (APIC)-based technique to control the execution of an enclave in a fine-grained manner. An evaluation against RSA shows that our attack infers each private key bit with 99.8% accuracy. Finally, we thoroughly study the feasibility of hardware-based solutions (i.e., branch history flushing) and propose a software-based approach that mitigates the attack.

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@inproceedings {203698,
author = {Sangho Lee and Ming-Wei Shih and Prasun Gera and Taesoo Kim and Hyesoon Kim and Marcus Peinado},
title = {Inferring Fine-grained Control Flow Inside {SGX} Enclaves with Branch Shadowing},
booktitle = {26th {USENIX} Security Symposium ({USENIX} Security 17)},
year = {2017},
isbn = {978-1-931971-40-9},
address = {Vancouver, BC},
pages = {557--574},
url = {},
publisher = {{USENIX} Association},