Automatically eliminating speculative leaks from cryptographic code with blade

Marco Vassena, Craig Disselkoen, Klaus von Gleissenthall, Sunjay Cauligi, Rami Gökhan Klcl, Ranjit Jhala, Dean Tullsen, Deian Stefan

Research output: Contribution to JournalArticleAcademicpeer-review

Abstract

We introduce Blade, a new approach to automatically and efficiently eliminate speculative leaks from cryptographic code. Blade is built on the insight that to stop leaks via speculative execution, it suffices to cut the dataflow from expressions that speculatively introduce secrets (sources) to those that leak them through the cache (sinks), rather than prohibit speculation altogether. We formalize this insight in a static type system that (1) types each expression as either transient, i.e., possibly containing speculative secrets or as being stable, and (2) prohibits speculative leaks by requiring that all sink expressions are stable. Blade relies on a new abstract primitive, protect, to halt speculation at fine granularity. We formalize and implement protect using existing architectural mechanisms, and show how Blade's type system can automatically synthesize a minimal number of protects to provably eliminate speculative leaks. We implement Blade in the Cranelift WebAssembly compiler and evaluate our approach by repairing several verified, yet vulnerable WebAssembly implementations of cryptographic primitives. We find that Blade can fix existing programs that leak via speculation automatically, without user intervention, and efficiently even when using fences to implement protect.

Original languageEnglish
Article number49
Pages (from-to)1-30
Number of pages30
JournalProceedings of the ACM on Programming Languages
Volume5
Issue numberPOPL
DOIs
Publication statusPublished - Jan 2021

Keywords

  • Constant-time
  • Spectre
  • Speculative execution
  • Type system

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