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On Performing Accurate Time Measurements of SGX Enclave Instructions

This repository contains my bachelor thesis that I did with the System Security Group at ETH Zurich.

Short Summary

SGX enclaves allow to securely execute a program on hardware owned by an untrusted entity. Since precise timestamps are not available inside enclaves and not even the OS can manipulate their computations, it is difficult to measure the timings of instructions executing inside enclaves.

This thesis serves as a reference on how to obtain accurate timings in general and especially in connection with SGX enclaves. We built on top of SGX-Step, a practical attack framework for precise enclave execution control. Nemesis goes in the same direction as our work and also modifies SGX-Step to do time measurements. However, our project further improves the observation accuracy and runs more in-depth test cases. We describe the following improvements over SGX-Step (with references to the corresponding sections in the thesis):

  1. We noted that different enclaves have varying enclave entry and exit times, which makes it hard to compare instruction timings of different enclaves. We added the measurement of multiple test cases inside the same enclave as well as automated plotting. (4.1)
  2. Better instruction serialization barriers before and after the measurement to reduce noise. (3.2.1, 3.3.3)
  3. Reduced cache pollution due to (almost) constant time measurement code and delayed instruction filtering and logging. (4.3, 4.4)
  4. Multiple consistency checks to detect imprecise APIC timers, misconfigured tests and instructions that can execute in two parts. (4.5, 4.6)
  5. Support for more complex test cases. (3.3.1, 4.8, 4.9, 5.1)

Furthermore, we discuss the following enhancements and new features:

  1. New plot types to gain more insights about measurements. (3.4.2, 3.4.3, 3.4.4, 5.1)
  2. Alternative measurement options that all share the same test case specifications:
    1. Measure timings outside the enclave to see differences to instructions inside the enclave. (3.3.2)
    2. Counter method to cross validate results of SGX-Step (although they have lower precision). (3.3.4)
  3. Filtering noise and especialy outliers at page boundaries to get a meaningful variance and compact plots. (3.4, 4.2, 4.10)

By improving the instruction granular time measurements of SGX enclaves, we show that even more detailed information about the enclave’s execution state can be leaked than previously assumed. We describe the discovery of the phenomenon that led to the Frontal Attack paper.

The entire thesis is provided in this repository.

Frontal Attack

Abstract

We introduce a new timing side-channel attack on Intel CPU processors. Our Frontal attack exploits the way that CPU frontend fetches and processes instructions while being interrupted. In particular, we observe that in modern Intel CPUs, some instruction's execution times will depend on which operations precede and succeed them, and on their virtual addresses. Unlike previous attacks that could only profile branches if they contained different code or were based on conditional jumps, the attack allows the adversary to distinguish between instruction-wise identical branches. As the attack requires OS capabilities to set the interrupts, we use it to exploit SGX enclaves. Our attack demonstrates that a realistic SGX attacker can always observe the full enclave instruction trace, and secret-depending branching should not be used even alongside defenses to current controlled-channel attacks. We show that the adversary can use the Frontal attack to extract a secret from an SGX enclave if that secret was used as a branching condition for two instruction-wise identical branches. The attack can be exploited against several crypto libraries and affects all Intel CPUs.

-- <cite>Frontal Attack</cite>

The source code of a prove of concept implementation of the Frontal attack is published here and includes part of the code of this thesis.

Acknowledgement

I would like to thank Ivan Puddu and Moritz Schneider for their extraordinary support and countless discussions and inputs. Moreover, I thank Prof. Dr. Srdjan Capkun for making this project possible.

Contact

In case of questions or comments, you can contact me using the following email address: miro.haller@alumni.ethz.ch