SynthCT: Towards Portable Constant-Time Code

Sushant Dinesh (University of Illinois at Urbana Champaign), Grant Garrett-Grossman (University of Illinois at Urbana Champaign), Christopher W. Fletcher (University of Illinois at Urbana Champaign)

Recent attacks have demonstrated that modern microarchitectures are fraught with microarchitectural side channels. Constant-time (CT) programming is a software development methodology where programs are carefully written to avoid these channels. In a nutshell, the idea is to only pass secret data to safe instructions, i.e., those whose execution creates operand-
independent hardware resource usage.

Yet, current CT programming practices have significant security and performance issues. CT code is written and compiled once, but may execute on multiple different microarchitectures. Yet, what instructions are safe vs. unsafe is fundamentally a microarchitecture-specific issue. A new microarchitectural optimization (or vulnerability) may change the set of safe instructions and break CT guarantees.

In this work, we develop SynthCT to address the above issues. Given a specification of safe/unsafe instructions, SynthCT automatically synthesizes translations for all unsafe instructions in the ISA using only instructions from the safe set. The synthesized translations can be used as a part of a late-stage compiler pass to generate hardened binaries for a specific microarchitecture. This closes the security hole as the specification, and hence the safe translations, can target each microarchitecture individually. This also allows CT code to reclaim some performance, e.g., use more complex/higher-performing instructions, when they are deemed safe for a specific microarchitecture.

Using the techniques we develop in SynthCT, we are able to synthesize translations for a majority of the x86 64 ISA. Specifically, SynthCT is able to generate safe translations for 75% of the ISA using only the remaining 25% of the ISA. Interestingly, the majority of the instructions that SynthCT was unable to generate translations for are instructions that experts believe are safe instructions on today’s x86 64 microarchitectures.