Portable Software Fault Isolation

We present a new technique for architecture portable software fault isolation (SFI), together with a prototype implementation in the Coq proof assistant. Unlike traditional SFI, which relies on analysis of assembly-level programs, we analyze and rewrite programs in a compiler intermediate language, the Cminor language of the Comp Cert C compiler. But like traditional SFI, the compiler remains outside of the trusted computing base. By composing our program transformer with the verified back-end of Comp Cert and leveraging Comp Cert´s formally proved preservation of the behavior of safe programs, we can obtain binary modules that satisfy the SFI memory safety policy for any of Comp Cert´s supported architectures (currently: Power PC, ARM, and x86-32). This allows the same SFI analysis to be used across multiple architectures, greatly simplifying the most difficult part of deploying trustworthy SFI systems.