| # 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, given 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 the 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 not have 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. 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. |