Control Flow Integrity — Clang 3.7 documentation

# Clang 3.7 documentation

## Control Flow Integrity

- Introduction
  - Forward-Edge CFI for Virtual Calls
    - Performance
  - Bad Cast Checking
  - Non-Virtual Member Function Call Checking
    - Strictness
  - Design
  - Publications

## Introduction

Clang includes an implementation of a number of control flow integrity (CFI) schemes, which are designed to abort the program upon detecting certain forms of undefined behavior that can potentially allow attackers to subvert the program’s control flow. These schemes have been optimized for performance, allowing developers to enable them in release builds.

To enable Clang’s available CFI schemes, use the flag `-fsanitize=cfi`. As currently implemented, CFI relies on link-time optimization (LTO); so it is required to specify `-flto`, and the linker used must support LTO, for example via the gold plugin. To allow the checks to be implemented efficiently, the program must be structured such that certain object files are compiled with CFI enabled, and are statically linked into the program. This may preclude the use of shared libraries in some cases.

Clang currently implements forward-edge CFI for member function calls and bad cast checking. More schemes are under development.

### Forward-Edge CFI for Virtual Calls

This scheme checks that virtual calls take place using a vptr of the correct dynamic type; that is, the dynamic type of the called object must be a derived class of the static type of the object used to make the call. This CFI scheme can be enabled on its own using `-fsanitize=cfi-vcall`.

For this scheme to work, all translation units containing the definition of a virtual member function (whether inline or not) must be compiled with `-fsanitize=cfi-vcall` enabled and be statically linked into the program. Classes in the C++ standard library (under namespace `std`) are exempted from checking, and therefore programs may be linked against a pre-built standard library, but this may change in the future.

#### Performance

A performance overhead of less than 1% has been measured by running the Dromaeo benchmark suite against an instrumented version of the Chromium web browser. Another good performance benchmark for this mechanism is the virtual-call-heavy SPEC 2006 xalancbmk.

Note that this scheme has not yet been optimized for binary size; an increase of up to 15% has been observed for Chromium.

### Bad Cast Checking

This scheme checks that pointer casts are made to an object of the correct dynamic type; that is, the dynamic type of the object must be a derived class of the pointee type of the cast. The checks are currently only introduced where the class being casted to is a polymorphic class.

Bad casts are not in themselves control flow integrity violations, but they can also create security vulnerabilities, and the implementation uses many of the same mechanisms.

There are two types of bad cast that may be forbidden: bad casts from a base class to a derived class (which can be checked with `-fsanitize=cfi-derived-cast`), and bad casts from a pointer of type `void*` or another unrelated type (which can be checked with `-fsanitize=cfi-unrelated-cast`).

The difference between these two types of casts is that the first is defined by the C++ standard to produce an undefined value, while the second is not in itself undefined behavior (it is well defined to cast the pointer back to its original type).

If a program as a matter of policy forbids the second type of cast, that restriction can normally be enforced. However it may in some cases be necessary for a function to perform a forbidden cast to conform with an external API (e.g. the `allocate` member function of a standard library allocator). Such functions may be blacklisted using a Sanitizer special case list.

For this scheme to work, all translation units containing the definition of a virtual member function (whether inline or not) must be compiled with `-fsanitize=cfi-derived-cast` or `-fsanitize=cfi-unrelated-cast` enabled and be statically linked into the program. Classes in the C++ standard library (under namespace `std`) are exempted from checking, and therefore programs may be linked against a pre-built standard library, but this may change in the future.

### Non-Virtual Member Function Call Checking

This scheme checks that non-virtual calls take place using an object of the correct dynamic type; that is, the dynamic type of the called object must be a derived class of the static type of the object used to make the call. The checks are currently only introduced where the object is of a polymorphic class type. This CFI scheme can be enabled on its own using `-fsanitize=cfi-nvcall`.

For this scheme to work, all translation units containing the definition of a virtual member function (whether inline or not) must be compiled with `-fsanitize=cfi-nvcall` enabled and be statically linked into the program. Classes in the C++ standard library (under namespace `std`) are exempted from checking, and therefore programs may be linked against a pre-built standard library, but this may change in the future.

#### Strictness

If a class has a single non-virtual base and does not introduce or override virtual member functions or fields other than an implicitly defined virtual destructor, it will have the same layout and virtual function semantics as its base. By default, casts to such classes are checked as if they were made to the least derived such class.

Casting an instance of a base class to such a derived class is technically undefined behavior, but it is a relatively common hack for introducing member functions on class instances with specific properties that works under most compilers and should not have security implications, so we allow it by default. It can be disabled with `-fsanitize=cfi-cast-strict`.

### Design

Please refer to the design document.

### Publications

Control-Flow Integrity: Principles, Implementations, and Applications. Martin Abadi, Mihai Budiu, Úlfar Erlingsson, Jay Ligatti.

Enforcing Forward-Edge Control-Flow Integrity in GCC & LLVM. Caroline Tice, Tom Roeder, Peter Collingbourne, Stephen Checkoway, Úlfar Erlingsson, Luis Lozano, Geoff Pike.