Control Flow Verification Tool Design Document

Objective

This document provides an overview of an external tool to verify the protection mechanisms implemented by Clang’s Control Flow Integrity (CFI) schemes (-fsanitize=cfi). This tool, provided a binary or DSO, should infer whether indirect control flow operations are protected by CFI, and should output these results in a human-readable form.

This tool should also be added as part of Clang’s continuous integration testing framework, where modifications to the compiler ensure that CFI protection schemes are still present in the final binary.

Location

This tool will be present as a part of the LLVM toolchain, and will reside in the “/llvm/tools/llvm-cfi-verify” directory, relative to the LLVM trunk. It will be tested in two methods:

  • Unit tests to validate code sections, present in “/llvm/unittests/tools/llvm-cfi-verify”.
  • Integration tests, present in “/llvm/tools/clang/test/LLVMCFIVerify”. These integration tests are part of clang as part of a continuous integration framework, ensuring updates to the compiler that reduce CFI coverage on indirect control flow instructions are identified.

Background

This tool will continuously validate that CFI directives are properly implemented around all indirect control flows by analysing the output machine code. The analysis of machine code is important as it ensures that any bugs present in linker or compiler do not subvert CFI protections in the final shipped binary.

Unprotected indirect control flow instructions will be flagged for manual review. These unexpected control flows may simply have not been accounted for in the compiler implementation of CFI (e.g. indirect jumps to facilitate switch statements may not be fully protected).

It may be possible in the future to extend this tool to flag unnecessary CFI directives (e.g. CFI directives around a static call to a non-polymorphic base type). This type of directive has no security implications, but may present performance impacts.

Design Ideas

This tool will disassemble binaries and DSO’s from their machine code format and analyse the disassembled machine code. The tool will inspect virtual calls and indirect function calls. This tool will also inspect indirect jumps, as inlined functions and jump tables should also be subject to CFI protections. Non-virtual calls (-fsanitize=cfi-nvcall) and cast checks (-fsanitize=cfi-*cast*) are not implemented due to a lack of information provided by the bytecode.

The tool would operate by searching for indirect control flow instructions in the disassembly. A control flow graph would be generated from a small buffer of the instructions surrounding the ‘target’ control flow instruction. If the target instruction is branched-to, the fallthrough of the branch should be the CFI trap (on x86, this is a ud2 instruction). If the target instruction is the fallthrough (i.e. immediately succeeds) of a conditional jump, the conditional jump target should be the CFI trap. If an indirect control flow instruction does not conform to one of these formats, the target will be noted as being CFI-unprotected.

Note that in the second case outlined above (where the target instruction is the fallthrough of a conditional jump), if the target represents a vcall that takes arguments, these arguments may be pushed to the stack after the branch but before the target instruction. In these cases, a secondary ‘spill graph’ in constructed, to ensure the register argument used by the indirect jump/call is not spilled from the stack at any point in the interim period. If there are no spills that affect the target register, the target is marked as CFI-protected.

Other Design Notes

Only machine code sections that are marked as executable will be subject to this analysis. Non-executable sections do not require analysis as any execution present in these sections has already violated the control flow integrity.

Suitable extensions may be made at a later date to include anaylsis for indirect control flow operations across DSO boundaries. Currently, these CFI features are only experimental with an unstable ABI, making them unsuitable for analysis.