datAFLow

DatAFLow is a fuzzer built on top of AFL++. However, instead of a control-flow-based feedback mechanism (e.g., based on control-flow edge coverage), datAFLow uses a data-flow-based feedback mechanism; specifically, data flows based on def-use associations.

To enable performant fuzzing, datAFLow uses a custom low-fat pointer memory allocator for efficiently tracking data flows at runtime. This is achieved via two mechanisms: a runtime replacement for malloc and friends, libfuzzalloc, and a set of LLVM passes to transform your target to use libfuzzalloc.

More details are available in our registered report, published at the 1st International Fuzzing Workshop (FUZZING) 2022. You can read our report here.

Building

The datAFLow fuzzer requires a custom version of clang. Once this is built, the fuzzalloc toolchain can be built. FUZZALLOC_SRC variable refers to this directory.

Patching clang

fuzzalloc requires a patch to the clang compiler to disable turning constant arrays into packed constant structs.

To build the custom clang:

# Get the LLVM source code and update the clang source code
mkdir llvm
cd llvm
$FUZZALLOC_SRC/llvm-scripts/get_llvm_src.sh
$FUZZALLOC_SRC/llvm-scripts/update_clang_src.sh

# Build and install LLVM/clang/etc.
mkdir build
mkdir install
cd build
# If debugging you can also add -DCMAKE_BUILD_TYPE=Debug -DCOMPILER_RT_DEBUG=On
# Note that if you're going to use gclang, things seem to work better if you use
# the gold linker (https://llvm.org/docs/GoldPlugin.html)
cmake ../llvm -DLLVM_ENABLE_PROJECTS="clang;compiler-rt"        \
    -DLLVM_BUILD_EXAMPLES=Off -DLLVM_INCLUDE_EXAMPLES=Off       \
    -DLLVM_TARGETS_TO_BUILD="X86" -DCMAKE_INSTALL_PREFIX=$(realpath ../install)
cmake --build .
cmake --build . --target install

# Add the install directory to your path so that you use the correct clang
export PATH=$(realpath ../install):$PATH

With AddressSanitizer (ASan)

Fuzzing is typically performed in conjunction with a sanitizer so that "silent" bugs can be uncovered. Sanitizers such as ASan typically hook and replace dynamic memory allocation routines such as malloc/free so that they can detect buffer over/under flows, use-after-frees, etc. Unfortunately, this means that we lose the ability to track dataflow (as we rely on the memory allocator to do this). Therefore, we must use a custom version of ASan in order to (a) detect bugs and (b) track dataflow.

To build the custom ASan, run the following after running get_llvm_src.sh and update_clang_src.sh above:

cd llvm
$FUZZALLOC_SRC/llvm-scripts/update_compiler_rt_src.sh
$FUZZALLOC_SRC/llvm-scripts/update_llvm_src.sh

# Build and install LLVM/clang/etc.
cd build
# If debugging you can also add -DCMAKE_BUILD_TYPE=Debug -DCOMPILER_RT_DEBUG=On
cmake ../llvm -DLLVM_ENABLE_PROJECTS="clang;compiler-rt"                \
    -DFUZZALLOC_ASAN=On -DLIBFUZZALLOC_PATH=/path/to/libfuzzalloc.so    \
    -DLLVM_BUILD_EXAMPLES=Off -DLLVM_INCLUDE_EXAMPLES=Off               \
    -DLLVM_TARGETS_TO_BUILD="X86" -DCMAKE_INSTALL_PREFIX=$(realpath ../install)
cmake --build .
cmake --build . --target install

# Make sure the install path is available in $PATH

Note that after building LLVM with the custom ASan, you will have to rebuild fuzzalloc with the new clang/clang++ (found under install/bin).

Building fuzzalloc

mkdir build
cd build
cmake -DCMAKE_C_COMPILER=clang -DCMAKE_CXX_COMPILER=clang++ -DAFL_PATH=/path/to./afl++/source $FUZZALLOC_SRC
make -j

Usage

libfuzzalloc is a drop-in replacement for malloc and friends. When using gcc, it's safest to pass in the flags

-fno-builtin-malloc -fno-builtin-calloc -fno-builtin-realloc -fno-builtin-free

All you have to do is link your target with -lfuzzalloc.

Instrumenting a Target

The dataflow-cc (and dataflow-cc++) tools can be used as dropin replacements for clang (and clang++).

Note that this typically requires running dataflow-preprocess before running dataflow-cc to collect the allocation sites to tag.

dataflow-preprocess

If the target uses custom memory allocation routines (i.e., wrapping malloc, calloc, etc.), then a special case list containing a list of these routines should be provided to dataflow-preprocess. Doing so ensures dynamically-allocated variable def sites are appropriately tagged. The list is provided via the FUZZALLOC_MEM_FUNCS environment variable; i.e., FUZZALLOC_MEM_FUNCS=/path/to/special/case/list. The special case list must be formatted as:

[fuzzalloc]
fun:malloc_wrapper
fun:calloc_wrapper
fun:realloc_wrapper

The locations of variable tag sites are stored in a file specified by the FUZZALLOC_TAG_LOG environment variable.

dataflow-cc

dataflow-cc is a drop-in replacement for clang. To use the tag list generated by dataflow-preprocess, set it in the FUZZALLOC_TAG_LOG environment variable (e.g., FUZZALLOC_TAG_LOG=/path/to/tags).

Other useful environment variables include:

• FUZZALLOC_FUZZER: Sets the fuzzer instrumentation to use. Valid fuzzers include: debug-log (logging to stderr. This requires fuzzalloc be built in debug mode; i.e., with -DCMAKE_BUILD_TYPE=Debug), AFL, and libfuzzer.

• FUZZALLOC_SENSITIVITY: Sets the use site sensitivity. Valid sensitivities are: mem-read, mem-write, mem-access, mem-read-offset, mem-write-offset, and mem-access-offset.

With libFuzzer (Experimental)

The following flags are added to libFuzzer:

• use_dataflow: Enable dataflow-based coverage
• print_dataflows: Print out covered def/use chains
• job_prefix: fuzz-JOB.log prefix