Abstract
Enclaved execution environments, such as Intel SGX, enable secure, hardware-enforced isolated execution of critical application components without having to trust the underlying operating system or hypervisor. A recent line of research, however, explores innovative controlled-channel attacks mounted by untrusted system software to partially compromise the confidentiality of enclave programs. Apart from exploiting relatively well-known side-channels like the CPU cache and branch predictor, these attacks have so far focused on tracking side-effects from enclaved address translations via the paging unit.
This paper shows, however, that for 32-bit SGX enclaves the unacclaimed x86 segmentation unit can be abused as a novel controlled-channel to reveal enclaved memory accesses at a page-level granularity, and in restricted circumstances even at a very precise byte-level granularity. While the x86 paging unit has been extensively studied from both an attack as well as a defense perspective, we are the first to show that address translation side-channels are not limited to paging. Our findings furthermore confirm that largely abandoned legacy x86 processor features, included for backwards compatibility, suggest new and unexpected side-channels.
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Notes
- 1.
Note that we assume here that the next instruction is located immediately after the current one in memory. We explain in the next section how segmentation-based attacks can infer secret target addresses in case of jump instructions.
- 2.
The cmove instruction packs a condition and move into a single instruction. The move is only performed when the equal flag in the processor’s status register is set.
- 3.
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Acknowledgements
This work was partially supported by the Research Fund KU Leuven. Jo Van Bulck and Raoul Strackx are supported by a grant of the Research Foundation – Flanders (FWO).
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A Vulnerable Microcode Versions
A Vulnerable Microcode Versions
Only very recently, Intel provided microcode revisions to foil our segmentation-based attacks. We tested the following microcode revisions for our Skylake machine:
Version | Release date | CPUSVN | Vulnerable |
|---|---|---|---|
0x1E | unknown | 020202ffffff00000000000000000000 | Yes |
0x2E | unknown | 020202ffffff00000000000000000000 | Yes |
0x9E | unknown | 020202ffffff00000000000000000000 | Yes |
0x4A | unknown | 020202ffffff00000000000000000000 | Yes |
0x8A | unknown | 020202ffffff00000000000000000000 | Yes |
0xBA | April 9th, 2017 | 020202ffffff00000000000000000000 | No |
0xC2 | November 16th, 2017 | 020702ffffff00000000000000000000 | No |
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Gyselinck, J., Van Bulck, J., Piessens, F., Strackx, R. (2018). Off-Limits: Abusing Legacy x86 Memory Segmentation to Spy on Enclaved Execution. In: Payer, M., Rashid, A., Such, J. (eds) Engineering Secure Software and Systems. ESSoS 2018. Lecture Notes in Computer Science(), vol 10953. Springer, Cham. https://doi.org/10.1007/978-3-319-94496-8_4
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