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Title: MPTEE: Bringing Flexible and Efficient Memory Protection to Intel SGX

Source: EuroSys ’20

Authors: Wenjia Zhao, Kangjie Lu, Yong Qi and Saiyu Qi

Summary

  • The problem the paper aims to solve

    • The applications of SGX have a fundamental need of flexible memory protection that, while preventing itself from being attacked, can update memory-page permissions dynamically.

    • Existing systems can not provide the flexible and efficient memory-protection mechanism due to the lack of hardware support and the untrustedness of operating systems.

  • How can the paper address the problem? What is the main idea of this paper?

    • Observation

      In enclave, the memory sections that have same permissions are adjacent.

    • Cross-Region Bound Check(CRBC)

      (1) Use three bound registers (R/W/X) to offer six overlapping permission regions (X/R/RX/RW/RWX/No).

      (2) By changing the boundary value of three registers, the ranges of memory permission regions can be flexibly changed at runtime.

    • Adaptive Permission Enforcement

      While one permission region has no change requirement at runtime, they configure the SGX permission bits in config.xml, relying on hardware check to reduce the performance overhead of CRBC.

    • Memory Isolation & Control-data Integrity

      (1) Save all the code and data related to the permission enforcement in the isolated no-permisson region offered by CRBC.

      (2) Use a trampoline table to check indirect calls and jumps.

  • Validations

    They extensively evaluate MPTEE from performance, utility, and effectiveness.

    The results of experimentation and analysis show that MPTEE can achieve a flexible and secure memory protection while just imposing a small overhead (avg 6.6%. With Adaptive Permission Enforcement, the overhead caused by CRBC can be improved more efficiently).

Strengthens

  • The observation that in enclave, the memory sections that have the same permissions are adjacent.
  • The MPTEE actually develop a flexible and secure memory-protection mechanism with less hardware support and just impose a small overhead.

Weaknesses

  • In 2018, Intel MPX has been eliminated by Microsoft.
  • This paper argues that as the hardware component, SGX cannot be easily upgraded. Old version SGX will still be the dominating version for a while. And they think that for enforcement integrity, MPTEE’s design is compatible with new SGX.

Comments

  • Intel MPX itself has many deficiencies (such as maintenance burdens), but this paper does not mention the solutions MPTEE has taken to address these deficiencies.