120 lines
3.7 KiB
Markdown
120 lines
3.7 KiB
Markdown
# Fuzzing
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Each fuzzing target can be built with multiple engines.
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Zstd provides a fuzz corpus for each target that can be downloaded with
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the command:
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```
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make corpora
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```
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It will download each corpus into `./corpora/TARGET`.
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## fuzz.py
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`fuzz.py` is a helper script for building and running fuzzers.
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Run `./fuzz.py -h` for the commands and run `./fuzz.py COMMAND -h` for
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command specific help.
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### Generating Data
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`fuzz.py` provides a utility to generate seed data for each fuzzer.
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```
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make -C ../tests decodecorpus
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./fuzz.py gen TARGET
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```
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By default it outputs 100 samples, each at most 8KB into `corpora/TARGET-seed`,
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but that can be configured with the `--number`, `--max-size-log` and `--seed`
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flags.
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### Build
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It respects the usual build environment variables `CC`, `CFLAGS`, etc.
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The environment variables can be overridden with the corresponding flags
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`--cc`, `--cflags`, etc.
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The specific fuzzing engine is selected with `LIB_FUZZING_ENGINE` or
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`--lib-fuzzing-engine`, the default is `libregression.a`.
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Alternatively, you can use Clang's built in fuzzing engine with
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`--enable-fuzzer`.
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It has flags that can easily set up sanitizers `--enable-{a,ub,m}san`, and
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coverage instrumentation `--enable-coverage`.
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It sets sane defaults which can be overridden with flags `--debug`,
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`--enable-ubsan-pointer-overflow`, etc.
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Run `./fuzz.py build -h` for help.
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### Running Fuzzers
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`./fuzz.py` can run `libfuzzer`, `afl`, and `regression` tests.
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See the help of the relevant command for options.
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Flags not parsed by `fuzz.py` are passed to the fuzzing engine.
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The command used to run the fuzzer is printed for debugging.
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Here's a helpful command to fuzz each target across all cores,
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stopping only if a bug is found:
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```
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for target in $(./fuzz.py list); do
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./fuzz.py libfuzzer $target -jobs=10 -workers=10 -max_total_time=1000 || break;
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done
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```
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Alternatively, you can fuzz all targets in parallel, using one core per target:
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```
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python3 ./fuzz.py list | xargs -P$(python3 ./fuzz.py list | wc -l) -I__ sh -c "python3 ./fuzz.py libfuzzer __ 2>&1 | tee __.log"
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```
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Either way, to double-check that no crashes were found, run `ls corpora/*crash`.
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If any crashes were found, you can use the hashes to reproduce them.
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## LibFuzzer
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```
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# Build the fuzz targets
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./fuzz.py build all --enable-fuzzer --enable-asan --enable-ubsan --cc clang --cxx clang++
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# OR equivalently
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CC=clang CXX=clang++ ./fuzz.py build all --enable-fuzzer --enable-asan --enable-ubsan
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# Run the fuzzer
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./fuzz.py libfuzzer TARGET <libfuzzer args like -jobs=4>
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```
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where `TARGET` could be `simple_decompress`, `stream_round_trip`, etc.
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### MSAN
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Fuzzing with `libFuzzer` and `MSAN` is as easy as:
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```
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CC=clang CXX=clang++ ./fuzz.py build all --enable-fuzzer --enable-msan
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./fuzz.py libfuzzer TARGET <libfuzzer args>
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```
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`fuzz.py` respects the environment variables / flags `MSAN_EXTRA_CPPFLAGS`,
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`MSAN_EXTRA_CFLAGS`, `MSAN_EXTRA_CXXFLAGS`, `MSAN_EXTRA_LDFLAGS` to easily pass
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the extra parameters only for MSAN.
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## AFL
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The default `LIB_FUZZING_ENGINE` is `libregression.a`, which produces a binary
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that AFL can use.
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```
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# Build the fuzz targets
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CC=afl-clang CXX=afl-clang++ ./fuzz.py build all --enable-asan --enable-ubsan
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# Run the fuzzer without a memory limit because of ASAN
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./fuzz.py afl TARGET -m none
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```
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## Regression Testing
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The regression test supports the `all` target to run all the fuzzers in one
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command.
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```
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CC=clang CXX=clang++ ./fuzz.py build all --enable-asan --enable-ubsan
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./fuzz.py regression all
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CC=clang CXX=clang++ ./fuzz.py build all --enable-msan
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./fuzz.py regression all
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```
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## Fuzzing a custom sequence producer plugin
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Sequence producer plugin authors can use the zstd fuzzers to stress-test their code.
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See the documentation in `fuzz_third_party_seq_prod.h` for details.
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