capstone/suite/fuzz

Fuzzing

Build the fuzz target

To build the fuzz target, you can simply run make with appropriate flags set :

ASAN_OPTIONS=detect_leaks=0 CXXFLAGS="-DFUZZING_BUILD_MODE_UNSAFE_FOR_PRODUCTION -fsanitize=address -fsanitize=fuzzer-no-link" CFLAGS="-DFUZZING_BUILD_MODE_UNSAFE_FOR_PRODUCTION -fsanitize=address -fsanitize=fuzzer-no-link" LDFLAGS="-fsanitize=address" make

You can replace address with another sanitizer : memory or undefined The fuzz target is then suite/fuzz/fuzz_bindisasm2

You can find this in travis configuration .travis.yml

Another way is to use oss-fuzz, see https://github.com/google/oss-fuzz/blob/master/projects/capstone/build.sh

Troubleshooting

If you get cc: error: unrecognized argument to ‘-fsanitize=’ option: ‘fuzzer’ check if you have a workable version of libfuzz installed. Also try to build with CC=clang make

Interpret OSS-Fuzz report

A reported bug by OSS-fuzz looks usually like this:

...
    #20 0x7f3a42062082 in __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x24082) (BuildId: 0702430aef5fa3dda43986563e9ffcc47efbd75e)
    #21 0x55ad814876dd in _start (build-out/fuzz_disasmnext+0x5246dd)

DEDUP_TOKEN: raise--abort--
AddressSanitizer can not provide additional info.
SUMMARY: AddressSanitizer: ABRT (/lib/x86_64-linux-gnu/libc.so.6+0x4300b) (BuildId: 0702430aef5fa3dda43986563e9ffcc47efbd75e) in raise
==62==ABORTING
MS: 0 ; base unit: 0000000000000000000000000000000000000000
0x7,0xe8,0x3,0x4e,0xc0,0xf8,
\007\350\003N\300\370

It emits the bytes fed to Capstone in the last two lines.

The first byte determines the arch+mode. The following bytes the actual data producing the crash.

You can run ./fuzz_decode_platform to get the arch+mode used:

./fuzz_decode_platform 0x7
cstool arch+mode = aarch64

And reproduce the bug with cstool:

# Make sureevery hex number has two digits!
cstool -d aarch64 0xe8,0x03,0x4e,0xc0,0xf8,

Make sure the every hex number has two digits (0x3 -> 0x03)! cstool won't parse it correctly otherwise.

Fuzz drivers

There are custom drivers :

• driverbin.c : prints cstool command before running one input
• drivermc.c : converts MC test data to raw binary data before running as many inputs as there are lines in a file
• onefile.c : simple one file driver

For libfuzzer, the preferred main function is now to use linker option -fsanitize=fuzzer

Fuzzit integration

Travis will build the fuzz target with the different sanitizers. Then, Travis will launch sanity fuzzit jobs as part of continuous integration (for each of the sanitizers) The fuzzit target ids are stored in a configuration file fuzzitid.txt and used by fuzzit.sh