Control-Flow Integrity (CFI) — QEMU documentation

# Control-Flow Integrity (CFI)

This document describes the current control-flow integrity (CFI) mechanism in QEMU. How it can be enabled, its benefits and deficiencies, and how it affects new and existing code in QEMU

## Basics

CFI is a hardening technique that focusing on guaranteeing that indirect function calls have not been altered by an attacker. The type used in QEMU is a forward-edge control-flow integrity that ensures function calls performed through function pointers, always call a “compatible” function. A compatible function is a function with the same signature of the function pointer declared in the source code.

This type of CFI is entirely compiler-based and relies on the compiler knowing the signature of every function and every function pointer used in the code. As of now, the only compiler that provides support for CFI is Clang.

CFI is best used on production binaries, to protect against unknown attack vectors.

In case of a CFI violation (i.e. call to a non-compatible function) QEMU will terminate abruptly, to stop the possible attack.

## Building with CFI

NOTE: CFI requires the use of link-time optimization. Therefore, when CFI is selected, LTO will be automatically enabled.

To build with CFI, the minimum requirement is Clang 6+. If you are planning to also enable fuzzing, then Clang 11+ is needed (more on this later).

Given the use of LTO, a version of AR that supports LLVM IR is required. The easies way of doing this is by selecting the AR provided by LLVM:

    AR=llvm-ar-9 CC=clang-9 CXX=clang++-9 /path/to/configure --enable-cfi

CFI is enabled on every binary produced.

If desired, an additional flag to increase the verbosity of the output in case of a CFI violation is offered (`--enable-debug-cfi`).

## Using QEMU built with CFI

A binary with CFI will work exactly like a standard binary. In case of a CFI violation, the binary will terminate with an illegal instruction signal.

## Incompatible code with CFI

As mentioned above, CFI is entirely compiler-based and therefore relies on compile-time knowledge of the code. This means that, while generally supported for most code, some specific use pattern can break CFI compatibility, and create false-positives. The two main patterns that can cause issues are:

- Just-in-time compiled code: since such code is created at runtime, the jump to the buffer containing JIT code will fail.

- Libraries loaded dynamically, e.g. with dlopen/dlsym, since the library was not known at compile time.

Current areas of QEMU that are not entirely compatible with CFI are:

1. TCG, since the idea of TCG is to pre-compile groups of instructions at runtime to speed-up interpretation, quite similarly to a JIT compiler

2. TCI, where the interpreter has to interpret the generic call operation

3. Plugins, since a plugin is implemented as an external library

4. Modules, since they are implemented as an external library

5. Directly calling signal handlers from the QEMU source code, since the signal handler may have been provided by an external library or even plugged at runtime.

## Disabling CFI for a specific function

If you are working on function that is performing a call using an incompatible way, as described before, you can selectively disable CFI checks for such function by using the decorator `QEMU_DISABLE_CFI` at function definition, and add an explanation on why the function is not compatible with CFI. An example of the use of `QEMU_DISABLE_CFI` is provided here:

    /*
     * Disable CFI checks.
     * TCG creates binary blobs at runtime, with the transformed code.
     * A TB is a blob of binary code, created at runtime and called with an
     * indirect function call. Since such function did not exist at compile time,
     * the CFI runtime has no way to verify its signature and would fail.
     * TCG is not considered a security-sensitive part of QEMU so this does not
     * affect the impact of CFI in environment with high security requirements
     */
    QEMU_DISABLE_CFI
    static inline tcg_target_ulong cpu_tb_exec(CPUState *cpu, TranslationBlock *itb)

NOTE: CFI needs to be disabled at the caller function, (i.e. a compatible cfi function that calls a non-compatible one), since the check is performed when the function call is performed.

## CFI and fuzzing

There is generally no advantage of using CFI and fuzzing together, because they target different environments (production for CFI, debug for fuzzing).

CFI could be used in conjunction with fuzzing to identify a broader set of bugs that may not end immediately in a segmentation fault or triggering an assertion. However, other sanitizers such as address and ub sanitizers can identify such bugs in a more precise way than CFI.

There is, however, an interesting use case in using CFI in conjunction with fuzzing, that is to make sure that CFI is not triggering any false positive in remote-but-possible parts of the code.

CFI can be enabled with fuzzing, but with some caveats: 1. Fuzzing relies on the linker performing function wrapping at link-time. The standard BFD linker does not support function wrapping when LTO is also enabled. The workaround is to use LLVM’s lld linker. 2. Fuzzing also relies on a custom linker script, which is only supported by lld with version 11+.

In other words, to compile with fuzzing and CFI, clang 11+ is required, and lld needs to be used as a linker:

    AR=llvm-ar-11 CC=clang-11 CXX=clang++-11 /path/to/configure --enable-cfi \
                              -enable-fuzzing --extra-ldflags="-fuse-ld=lld"

and then, compile the fuzzers as usual.