MIFP: Selective Fat-Pointer Bounds Compression for Accurate Bounds Checking

Shengjie Xu, Eric Liu, Wei Huang, David Lie

Abstract: Bounds compression for fat pointers can reduce the memory and performance overhead of maintaining pointer bounds and is necessary for efficient hardware implementation. However, compression can introduce inaccuracy to the bounds, making certain out-of-bounds accesses undetectable. Although the security threat can be mitigated by padding the objects, no known mitigations can detect these out-of-bounds accesses deterministically. We present MIFP, a method that automatically mixes both compressed and uncompressed bounds to preserve the performance benefits of bounds compression while ensuring accurate bounds checking. Given a program using a single fat pointer representation (e.g., all compressed bounds), MIFP performs whole-program analysis to expand potentially unsafe and inaccurate fat pointers such that they carry accurate uncompressed bounds. To minimize the number of pointers to expand, MIFP adds instrumentation on a per-allocation-site granularity; objects of the same type but different code allocation locations can have their pointer members transformed differently depending on how the pointers are used. We describe our algorithm and supporting data structures, and show that utilizing multiple fat-pointer representations reduces the runtime and memory overheads of uncompressed bounds by 79% and 93% respectively.