Control Flow Integrity

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. You can also enable a subset of available schemes. As currently implemented, all schemes rely 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.

The compiler will only produce CFI checks for a class if it can infer hidden LTO visibility for that class. LTO visibility is a property of a class that is inferred from flags and attributes. For more details, see the documentation for LTO visibility.

The -fsanitize=cfi-{vcall,nvcall,derived-cast,unrelated-cast} flags require that a -fvisibility= flag also be specified. This is because the default visibility setting is -fvisibility=default, which would disable CFI checks for classes without visibility attributes. Most users will want to specify -fvisibility=hidden, which enables CFI checks for such classes.

Experimental support for cross-DSO control flow integrity exists that does not require classes to have hidden LTO visibility. This cross-DSO support has unstable ABI at this time.

Available schemes

Available schemes are:

  • -fsanitize=cfi-cast-strict: Enables strict cast checks.
  • -fsanitize=cfi-derived-cast: Base-to-derived cast to the wrong dynamic type.
  • -fsanitize=cfi-unrelated-cast: Cast from void* or another unrelated type to the wrong dynamic type.
  • -fsanitize=cfi-nvcall: Non-virtual call via an object whose vptr is of the wrong dynamic type.
  • -fsanitize=cfi-vcall: Virtual call via an object whose vptr is of the wrong dynamic type.
  • -fsanitize=cfi-icall: Indirect call of a function with wrong dynamic type.
  • -fsanitize=cfi-mfcall: Indirect call via a member function pointer with wrong dynamic type.

You can use -fsanitize=cfi to enable all the schemes and use -fno-sanitize flag to narrow down the set of schemes as desired. For example, you can build your program with -fsanitize=cfi -fno-sanitize=cfi-nvcall,cfi-icall to use all schemes except for non-virtual member function call and indirect call checking.

Remember that you have to provide -flto or -flto=thin if at least one CFI scheme is enabled.

Trapping and Diagnostics

By default, CFI will abort the program immediately upon detecting a control flow integrity violation. You can use the -fno-sanitize-trap= flag to cause CFI to print a diagnostic similar to the one below before the program aborts.

bad-cast.cpp:109:7: runtime error: control flow integrity check for type 'B' failed during base-to-derived cast (vtable address 0x000000425a50)
0x000000425a50: note: vtable is of type 'A'
 00 00 00 00  f0 f1 41 00 00 00 00 00  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  20 5a 42 00
              ^

If diagnostics are enabled, you can also configure CFI to continue program execution instead of aborting by using the -fsanitize-recover= flag.

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), other than members of ignored types or types with public LTO visibility, must be compiled with -flto or -flto=thin enabled and be statically linked into the program.

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) unless the object is uninitialized and the cast is a static_cast (see C++14 [basic.life]p5).

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 ignored.

For this scheme to work, all translation units containing the definition of a virtual member function (whether inline or not), other than members of ignored types or types with public LTO visibility, must be compiled with -flto or -flto=thin enabled and be statically linked into the program.

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), other than members of ignored types or types with public LTO visibility, must be compiled with -flto or -flto=thin enabled and be statically linked into the program.

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.

Indirect Function Call Checking

This scheme checks that function calls take place using a function of the correct dynamic type; that is, the dynamic type of the function must match the static type used at the call. This CFI scheme can be enabled on its own using -fsanitize=cfi-icall.

For this scheme to work, each indirect function call in the program, other than calls in ignored functions, must call a function which was either compiled with -fsanitize=cfi-icall enabled, or whose address was taken by a function in a translation unit compiled with -fsanitize=cfi-icall.

If a function in a translation unit compiled with -fsanitize=cfi-icall takes the address of a function not compiled with -fsanitize=cfi-icall, that address may differ from the address taken by a function in a translation unit not compiled with -fsanitize=cfi-icall. This is technically a violation of the C and C++ standards, but it should not affect most programs.

Each translation unit compiled with -fsanitize=cfi-icall must be statically linked into the program or shared library, and calls across shared library boundaries are handled as if the callee was not compiled with -fsanitize=cfi-icall.

This scheme is currently supported on a limited set of targets: x86, x86_64, arm, arch64 and wasm.

-fsanitize-cfi-icall-generalize-pointers

Mismatched pointer types are a common cause of cfi-icall check failures. Translation units compiled with the -fsanitize-cfi-icall-generalize-pointers flag relax pointer type checking for call sites in that translation unit, applied across all functions compiled with -fsanitize=cfi-icall.

Specifically, pointers in return and argument types are treated as equivalent as long as the qualifiers for the type they point to match. For example, char*, char**, and int* are considered equivalent types. However, char* and const char* are considered separate types.

-fsanitize-cfi-icall-generalize-pointers is not compatible with -fsanitize-cfi-cross-dso.

-fsanitize-cfi-canonical-jump-tables

The default behavior of Clang’s indirect function call checker will replace the address of each CFI-checked function in the output file’s symbol table with the address of a jump table entry which will pass CFI checks. We refer to this as making the jump table canonical. This property allows code that was not compiled with -fsanitize=cfi-icall to take a CFI-valid address of a function, but it comes with a couple of caveats that are especially relevant for users of cross-DSO CFI:

  • There is a performance and code size overhead associated with each exported function, because each such function must have an associated jump table entry, which must be emitted even in the common case where the function is never address-taken anywhere in the program, and must be used even for direct calls between DSOs, in addition to the PLT overhead.
  • There is no good way to take a CFI-valid address of a function written in assembly or a language not supported by Clang. The reason is that the code generator would need to insert a jump table in order to form a CFI-valid address for assembly functions, but there is no way in general for the code generator to determine the language of the function. This may be possible with LTO in the intra-DSO case, but in the cross-DSO case the only information available is the function declaration. One possible solution is to add a C wrapper for each assembly function, but these wrappers can present a significant maintenance burden for heavy users of assembly in addition to adding runtime overhead.

For these reasons, we provide the option of making the jump table non-canonical with the flag -fno-sanitize-cfi-canonical-jump-tables. When the jump table is made non-canonical, symbol table entries point directly to the function body. Any instances of a function’s address being taken in C will be replaced with a jump table address.

This scheme does have its own caveats, however. It does end up breaking function address equality more aggressively than the default behavior, especially in cross-DSO mode which normally preserves function address equality entirely.

Furthermore, it is occasionally necessary for code not compiled with -fsanitize=cfi-icall to take a function address that is valid for CFI. For example, this is necessary when a function’s address is taken by assembly code and then called by CFI-checking C code. The __attribute__((cfi_canonical_jump_table)) attribute may be used to make the jump table entry of a specific function canonical so that the external code will end up taking an address for the function that will pass CFI checks.

-fsanitize=cfi-icall and -fsanitize=function

This tool is similar to -fsanitize=function in that both tools check the types of function calls. However, the two tools occupy different points on the design space; -fsanitize=function is a developer tool designed to find bugs in local development builds, whereas -fsanitize=cfi-icall is a security hardening mechanism designed to be deployed in release builds.

-fsanitize=function has a higher space and time overhead due to a more complex type check at indirect call sites, as well as a need for run-time type information (RTTI), which may make it unsuitable for deployment. Because of the need for RTTI, -fsanitize=function can only be used with C++ programs, whereas -fsanitize=cfi-icall can protect both C and C++ programs.

On the other hand, -fsanitize=function conforms more closely with the C++ standard and user expectations around interaction with shared libraries; the identity of function pointers is maintained, and calls across shared library boundaries are no different from calls within a single program or shared library.

Member Function Pointer Call Checking

This scheme checks that indirect calls via a member function pointer take place using an object of the correct dynamic type. Specifically, we check that the dynamic type of the member function referenced by the member function pointer matches the “function pointer” part of the member function pointer, and that the member function’s class type is related to the base type of the member function. This CFI scheme can be enabled on its own using -fsanitize=cfi-mfcall.

The compiler will only emit a full CFI check if the member function pointer’s base type is complete. This is because the complete definition of the base type contains information that is necessary to correctly compile the CFI check. To ensure that the compiler always emits a full CFI check, it is recommended to also pass the flag -fcomplete-member-pointers, which enables a non-conforming language extension that requires member pointer base types to be complete if they may be used for a call.

For this scheme to work, all translation units containing the definition of a virtual member function (whether inline or not), other than members of ignored types or types with public LTO visibility, must be compiled with -flto or -flto=thin enabled and be statically linked into the program.

This scheme is currently not compatible with cross-DSO CFI or the Microsoft ABI.

Ignorelist

A Sanitizer special case list can be used to relax CFI checks for certain source files, functions and types using the src, fun and type entity types. Specific CFI modes can be be specified using [section] headers.

# Suppress all CFI checking for code in a file.
src:bad_file.cpp
src:bad_header.h
# Ignore all functions with names containing MyFooBar.
fun:*MyFooBar*
# Ignore all types in the standard library.
type:std::*
# Disable only unrelated cast checks for this function
[cfi-unrelated-cast]
fun:*UnrelatedCast*
# Disable CFI call checks for this function without affecting cast checks
[cfi-vcall|cfi-nvcall|cfi-icall]
fun:*BadCall*

Shared library support

Use -f[no-]sanitize-cfi-cross-dso to enable the cross-DSO control flow integrity mode, which allows all CFI schemes listed above to apply across DSO boundaries. As in the regular CFI, each DSO must be built with -flto or -flto=thin.

Normally, CFI checks will only be performed for classes that have hidden LTO visibility. With this flag enabled, the compiler will emit cross-DSO CFI checks for all classes, except for those which appear in the CFI ignorelist or which use a no_sanitize attribute.

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.