1==============================================
2Control Flow Verification Tool Design Document
3==============================================
4
5.. contents::
6   :local:
7
8Objective
9=========
10
11This document provides an overview of an external tool to verify the protection
12mechanisms implemented by Clang's *Control Flow Integrity* (CFI) schemes
13(``-fsanitize=cfi``). This tool, provided a binary or DSO, should infer whether
14indirect control flow operations are protected by CFI, and should output these
15results in a human-readable form.
16
17This tool should also be added as part of Clang's continuous integration testing
18framework, where modifications to the compiler ensure that CFI protection
19schemes are still present in the final binary.
20
21Location
22========
23
24This tool will be present as a part of the LLVM toolchain, and will reside in
25the "/llvm/tools/llvm-cfi-verify" directory, relative to the LLVM trunk. It will
26be tested in two methods:
27
28- Unit tests to validate code sections, present in
29  "/llvm/unittests/tools/llvm-cfi-verify".
30- Integration tests, present in "/llvm/tools/clang/test/LLVMCFIVerify". These
31  integration tests are part of clang as part of a continuous integration
32  framework, ensuring updates to the compiler that reduce CFI coverage on
33  indirect control flow instructions are identified.
34
35Background
36==========
37
38This tool will continuously validate that CFI directives are properly
39implemented around all indirect control flows by analysing the output machine
40code. The analysis of machine code is important as it ensures that any bugs
41present in linker or compiler do not subvert CFI protections in the final
42shipped binary.
43
44Unprotected indirect control flow instructions will be flagged for manual
45review. These unexpected control flows may simply have not been accounted for in
46the compiler implementation of CFI (e.g. indirect jumps to facilitate switch
47statements may not be fully protected).
48
49It may be possible in the future to extend this tool to flag unnecessary CFI
50directives (e.g. CFI directives around a static call to a non-polymorphic base
51type). This type of directive has no security implications, but may present
52performance impacts.
53
54Design Ideas
55============
56
57This tool will disassemble binaries and DSO's from their machine code format and
58analyse the disassembled machine code. The tool will inspect virtual calls and
59indirect function calls. This tool will also inspect indirect jumps, as inlined
60functions and jump tables should also be subject to CFI protections. Non-virtual
61calls (``-fsanitize=cfi-nvcall``) and cast checks (``-fsanitize=cfi-*cast*``)
62are not implemented due to a lack of information provided by the bytecode.
63
64The tool would operate by searching for indirect control flow instructions in
65the disassembly. A control flow graph would be generated from a small buffer of
66the instructions surrounding the 'target' control flow instruction. If the
67target instruction is branched-to, the fallthrough of the branch should be the
68CFI trap (on x86, this is a ``ud2`` instruction). If the target instruction is
69the fallthrough (i.e. immediately succeeds) of a conditional jump, the
70conditional jump target should be the CFI trap. If an indirect control flow
71instruction does not conform to one of these formats, the target will be noted
72as being CFI-unprotected.
73
74Note that in the second case outlined above (where the target instruction is the
75fallthrough of a conditional jump), if the target represents a vcall that takes
76arguments, these arguments may be pushed to the stack after the branch but
77before the target instruction. In these cases, a secondary 'spill graph' in
78constructed, to ensure the register argument used by the indirect jump/call is
79not spilled from the stack at any point in the interim period. If there are no
80spills that affect the target register, the target is marked as CFI-protected.
81
82Other Design Notes
83~~~~~~~~~~~~~~~~~~
84
85Only machine code sections that are marked as executable will be subject to this
86analysis. Non-executable sections do not require analysis as any execution
87present in these sections has already violated the control flow integrity.
88
89Suitable extensions may be made at a later date to include analysis for indirect
90control flow operations across DSO boundaries. Currently, these CFI features are
91only experimental with an unstable ABI, making them unsuitable for analysis.
92
93The tool currently only supports the x86, x86_64, and AArch64 architectures.
94