CheckedCBox: Type Directed Program Partitioning with Checked C for Incremental Spatial Memory Safety

Abstract: Spatial memory safety violation is still a major issue for C programs. Checked-C is a safe dialect of C and extends it with Checked pointer types and annotations that guarantee spatial memory safety in a backward-compatible manner, allowing the mix of checked pointers and regular (unchecked) pointer types. However, unchecked code vulnerabilities can violate the checked code's spatial safety guarantees. We present CheckedCBox, which adds a flexible, type-directed program partitioning mechanism to Checked-C, by enhancing the Checked-C type system with tainted types that enable flexible partitioning of the program into checked and unchecked regions, in a manner such that unchecked region code does not affect the spatial safety in the checked region. We formalize our type system and prove the non-crashing and non-exposure properties of a well-typed CheckedCBox program. We implemented CheckedCBox in a configurable manner, which enables us to use existing sandbox mechanisms (eg WebAssembly) to execute programs. Consequently, in doing so, CheckedCBox has prevented four known vulnerabilities by efficiently partitioning the program.