AFLGo is an extension of American Fuzzy Lop (AFL).
Given a set of target locations (e.g., folder/file.c:582
), AFLGo generates inputs specifically with the objective to exercise these target locations.
Unlike AFL, AFLGo spends most of its time budget on reaching specific target locations without wasting resources stressing unrelated program components. This is particularly interesting in the context of
- patch testing by setting changed statements as targets. When a critical component is changed, we would like to check whether this introduced any vulnerabilities. AFLGo, a fuzzer that can focus on those changes, has a higher chance of exposing the regression.
- static analysis report verification by setting statements as targets that a static analysis reports as potentially dangerous or vulnerability-inducing. When assessing the security of a program, static analysis tools might identify dangerous locations, such as critical system calls. AFLGo can generate inputs that actually show that this is indeed no false positive.
- information flow detection by setting sensitive sources and sinks as targets. To expose data leakage vulnerabilities, a security researcher would like to generate executions that exercise sensitive sources containing private information and sensitive sinks where data becomes visible to the outside world. A directed fuzzer can be used to generate such executions efficiently.
- crash reproduction by setting method calls in the stack-trace as targets. When in-field crashes are reported, only the stack-trace and some environmental parameters are sent to the in-house development team. To preserve the user's privacy, the specific crashing input is often not available. AFLGo could help the in-house team to swiftly reproduce these crashes.
AFLGo is based on AFL from Michał Zaleski <lcamtuf@coredump.cx>. Checkout the project awesome-directed-fuzzing for related work on directed greybox/whitebox fuzzing.
Let's get started with building AFLGo (on Ubuntu 20.04) and fuzz the target libxml2:
git clone https://github.com/aflgo/aflgo.git
cd aflgo
export AFLGO=$PWD
# Build AFLGo
sudo ./build.sh
# When you fuzz for the very first time...
sudo sh -c 'echo core > /proc/sys/kernel/core_pattern'
# Fuzz the target libxml2
cd examples
./libxml2-ef709ce2.sh
See the detailed steps discussed below.
The easiest way to use AFLGo is as patch testing tool in OSS-Fuzz. Here is our integration:
- AFLGO_INST_RATIO -- The proportion of basic blocks instrumented with distance values (default: 100).
- AFLGO_SELECTIVE -- Add AFL-trampoline only to basic blocks with distance values? (default: off).
- AFLGO_PROFILER_FILE -- When CFG-tracing is enabled, the data will be stored here. (See
instrument/README.md
)
You can run AFLGo building script to do everything for you instead of manually go through step 1 to step 3. Be careful in these steps we would download, build and install LLVM 11.0.0 from source, which may have unexpected impacts on compiler toolchain in current system.
For step 4 to step 8, we are going to take libxml2 as an example. You can also equivalently run libxml2 fuzzing script instead.
Before we start, make sure that source code tree of AFLGo is ready and we are in its root. Then set the environment variable AFLGO
to it, which will be used in later steps. For example,
git clone https://github.com/aflgo/aflgo.git
cd aflgo
export AFLGO=$PWD
-
Install LLVM 11.0.0 with Gold-plugin. Then make sure that the following commands successfully executed:
# Install LLVMgold into bfd-plugins mkdir /usr/lib/bfd-plugins cp /usr/local/lib/libLTO.so /usr/lib/bfd-plugins cp /usr/local/lib/LLVMgold.so /usr/lib/bfd-plugins
-
Install other prerequisite
sudo apt-get update sudo apt-get install python3 sudo apt-get install python3-dev sudo apt-get install python3-pip sudo apt-get install pkg-config sudo apt-get install autoconf sudo apt-get install automake sudo apt-get install libtool-bin sudo apt-get install gawk sudo apt-get install libboost-all-dev # boost is not required if you use gen_distance_orig.sh in step 7 python3 -m pip install networkx # May vary by different python versions, see the case statement in build.sh python3 -m pip install pydot python3 -m pip install pydotplus
-
Compile AFLGo fuzzer, LLVM-instrumentation pass and the distance calculator
export CXX=`which clang++` export CC=`which clang` export LLVM_CONFIG=`which llvm-config` pushd afl-2.57b; make clean all; popd; pushd instrument; make clean all; popd; pushd distance/distance_calculator; cmake ./; cmake --build ./; popd;
-
Download subject libxml2.
# Clone subject repository git clone https://gitlab.gnome.org/GNOME/libxml2 export SUBJECT=$PWD/libxml2
-
Set targets (e.g., changed statements in commit ef709ce2). Writes
BBtargets.txt
.# Setup directory containing all temporary files mkdir temp export TMP_DIR=$PWD/temp # Download commit-analysis tool wget https://raw.githubusercontent.com/jay/showlinenum/develop/showlinenum.awk chmod +x showlinenum.awk mv showlinenum.awk $TMP_DIR # Generate BBtargets from commit ef709ce2 pushd $SUBJECT git checkout ef709ce2 git diff -U0 HEAD^ HEAD > $TMP_DIR/commit.diff popd cat $TMP_DIR/commit.diff | $TMP_DIR/showlinenum.awk show_header=0 path=1 | grep -e "\.[ch]:[0-9]*:+" -e "\.cpp:[0-9]*:+" -e "\.cc:[0-9]*:+" | cut -d+ -f1 | rev | cut -c2- | rev > $TMP_DIR/BBtargets.txt # Print extracted targets. echo "Targets:" cat $TMP_DIR/BBtargets.txt
Note: If there are no targets, there is nothing to instrument!
-
Generate CG and intra-procedural CFGs from the subject.
# Set aflgo-instrumenter export CC=$AFLGO/instrument/aflgo-clang export CXX=$AFLGO/instrument/aflgo-clang++ # Set aflgo-instrumentation flags export COPY_CFLAGS=$CFLAGS export COPY_CXXFLAGS=$CXXFLAGS export ADDITIONAL="-targets=$TMP_DIR/BBtargets.txt -outdir=$TMP_DIR -flto -fuse-ld=gold -Wl,-plugin-opt=save-temps" export CFLAGS="$CFLAGS $ADDITIONAL" export CXXFLAGS="$CXXFLAGS $ADDITIONAL" # Build libxml2 (in order to generate CG and CFGs). # Meanwhile go have a coffee ☕️ export LDFLAGS=-lpthread pushd $SUBJECT ./autogen.sh ./configure --disable-shared make clean make xmllint popd
You can test whether CG/CFG extraction was successful with
$SUBJECT/xmllint --valid --recover $SUBJECT/test/dtd3 ls $TMP_DIR/dot-files echo "Function targets" cat $TMP_DIR/Ftargets.txt
Note:
- If the linker (CCLD) complains that you should run
ranlib
, make sure thatlibLTO.so
andLLVMgold.so
(from Install LLVM 11.0.0 with Gold-plugin in step 1) can be found in/usr/lib/bfd-plugins
. - If the compiler crashes, there is some problem with LLVM not supporting our instrumentation (afl-llvm-pass.so.cc:540-577). LLVM has changed the instrumentation-API very often :( You can check LLVM-version, fix problem, and prepare pull request.
- You can speed up the compilation with a parallel build. However, this may impact which BBs are identified as targets. See #41.
- If the linker (CCLD) complains that you should run
-
Generate distance file. Firstly we need to clean up
BBnames.txt
andBBcalls.txt
, otherwisedistance_calculator
may fail. This is necessary for any subjects, not only for libxml2.# Clean up cat $TMP_DIR/BBnames.txt | grep -v "^$"| rev | cut -d: -f2- | rev | sort | uniq > $TMP_DIR/BBnames2.txt && mv $TMP_DIR/BBnames2.txt $TMP_DIR/BBnames.txt cat $TMP_DIR/BBcalls.txt | grep -Ev "^[^,]*$|^([^,]*,){2,}[^,]*$"| sort | uniq > $TMP_DIR/BBcalls2.txt && mv $TMP_DIR/BBcalls2.txt $TMP_DIR/BBcalls.txt
Then start to generate (this may take a while):
# Generate distance ☕️ # $AFLGO/distance/gen_distance_orig.sh is the original, but significantly slower, version $AFLGO/distance/gen_distance_fast.py $SUBJECT $TMP_DIR xmllint
After that you can check the generated distance file with
echo "Distance values:" head -n5 $TMP_DIR/distance.cfg.txt echo "..." tail -n5 $TMP_DIR/distance.cfg.txt
Note: If
distance.cfg.txt
is empty, there was some problem computing the CG-level and BB-level target distance. See$TMP_DIR/step*.log
. -
Instrument the subject
export CFLAGS="$COPY_CFLAGS -distance=$TMP_DIR/distance.cfg.txt" export CXXFLAGS="$COPY_CXXFLAGS -distance=$TMP_DIR/distance.cfg.txt" # Clean and build subject with distance instrumentation ☕️ pushd $SUBJECT make clean ./configure --disable-shared make xmllint popd
If your compilation crashes in this step, have a look at Issue #4.
- We set the exponential annealing-based power schedule (
-z exp
). - We set the time-to-exploitation to 45min (
-c 45m
), assuming the fuzzer is run for about an hour.
(Still take the previous libxml2 as an example)
# Construct seed corpus
mkdir in
cp -r $SUBJECT/test/dtd* in
cp $SUBJECT/test/dtds/* in
$AFLGO/afl-2.57b/afl-fuzz -S ef709ce2 -z exp -c 45m -i in -o out $SUBJECT/xmllint --valid --recover @@
- Tipp: Concurrently fuzz the most recent version as master with classical AFL :)
$AFL/afl-fuzz -M master -i in -o out $MASTER/xmllint --valid --recover @@
- Run more fuzzing scripts of various real programs like Binutils, jasper, lrzip, libming and DARPA CGC. Those scripts haven't contained any dependencies installing steps yet. So it's recommended that see READMEs of those projects first to check their requirements.