|Control Flow Verification Tool Design Document
|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.
|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
|- 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.
|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
|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
|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 analysis for indirect
|control flow operations across DSO boundaries. Currently, these CFI features are
|only experimental with an unstable ABI, making them unsuitable for analysis.
|The tool currently only supports the x86, x86_64, and AArch64 architectures.