.. _module-pw_fuzzer: --------- pw_fuzzer --------- ``pw_fuzzer`` provides developers with tools to write `libFuzzer`_ based fuzzers. Fuzzing or fuzz testing is style of testing that stochastically generates inputs to targeted interfaces in order to automatically find defects and/or vulnerabilities. In other words, fuzzing is simply an automated way of testing APIs with generated data. A fuzzer is a program that is used to fuzz a interface. It typically has three steps that it executes repeatedly: #. Generate a new, context-free input. This is the *fuzzing engine*. For ``pw_fuzzer``, this is `libFuzzer`_. #. Use the input to exercise the targeted interface, or code being tested. This is the *fuzz target function*. For ``pw_fuzzer``, these are the GN ``sources`` and/or ``deps`` that define `LLVMFuzzerTestOneInput`_. #. Monitor the code being tested for any abnormal conditions. This is the *instrumentation*. For ``pw_fuzzer``, these are sanitizer runtimes from LLVM's `compiler_rt`_. .. note:: ``pw_fuzzer`` is currently only supported on Linux using clang. .. image:: doc_resources/pw_fuzzer_coverage_guided.png :alt: Coverage Guided Fuzzing with libFuzzer :align: left Writing fuzzers =============== To write a fuzzer, a developer needs to write a fuzz target function follwing the `fuzz target function`__ guidelines given by libFuzzer: .. code:: extern "C" int LLVMFuzzerTestOneInput(const uint8_t *data, size_t size) { DoSomethingInterestingWithMyAPI(data, size); return 0; // Non-zero return values are reserved for future use. } .. __: LLVMFuzzerTestOneInput_ When writing you fuzz target function, you may want to consider: - It is acceptable to return early if the input doesn't mean some constraints, e.g. it is too short. - If your fuzzer accepts data with a well-defined format, you can bootstrap coverage by crafting examples and adding them to a `corpus`_. - There are tools to `split a fuzzing input`_ into multiple fields if needed; the `FuzzedDataProvider`_ is particularly easy to use. - If your code acts on "transformed" inputs, such as encoded or compressed inputs, you may want to try `structure aware fuzzing`. - You can do `startup initialization`_ if you need to. - If your code is non-deterministic or uses checksums, you may want to disable those **only** when fuzzing by using LLVM's `FUZZING_BUILD_MODE_UNSAFE_FOR_PRODUCTION`_ .. _build: Building fuzzers ================ To build a fuzzer, do the following: 1. Add the GN target using ``pw_fuzzer`` GN template, and add it to your the test group of the module: .. code:: # In $dir_my_module/BUILD.gn import("$dir_pw_fuzzer/fuzzer.gni") pw_fuzzer("my_fuzzer") { sources = [ "my_fuzzer.cc" ] deps = [ ":my_lib" ] } pw_test_group("tests") { tests = [ ":existing_tests", ... ":my_fuzzer", # <- Added! ] } 2. Select the clang toolchain and a sanitizer of your choice. See LLVM for `valid options`_. .. code:: sh $ gn gen out --args='pw_toolchain_SANITIZER="address"' 3. Build normally, e.g. using ``pw watch``. .. _run: Running fuzzers locally ======================= Based on the example above, the fuzzer output will be at ``out/host/obj/my_module/my_fuzzer``. It can be invoked using the normal `libFuzzer options`_ and `sanitizer runtime flags`_. For even more details, see the libFuzzer section on `running a fuzzer`_. For example, the following invocation disables "one definition rule" detection, saves failing inputs to ``artifacts/``, treats any input that takes longer than 10 seconds as a failure, and stores the working corpus in ``corpus/``. .. code:: $ mkdir -p corpus $ ASAN_OPTIONS=detect_odr_violation=0 \ out/host_clang_debug/obj/pw_fuzzer/bin/toy_fuzzer \ -artifact_prefix=artifacts/ \ -timeout=10 \ corpus INFO: Seed: 305325345 INFO: Loaded 1 modules (46 inline 8-bit counters): 46 [0x38dfc0, 0x38dfee), INFO: Loaded 1 PC tables (46 PCs): 46 [0x23aaf0,0x23add0), INFO: 0 files found in corpus INFO: -max_len is not provided; libFuzzer will not generate inputs larger than 4096 bytes INFO: A corpus is not provided, starting from an empty corpus #2 INITED cov: 2 ft: 3 corp: 1/1b exec/s: 0 rss: 27Mb #4 NEW cov: 3 ft: 4 corp: 2/3b lim: 4 exec/s: 0 rss: 27Mb L: 2/2 MS: 2 ShuffleBytes-InsertByte- #11 NEW cov: 7 ft: 8 corp: 3/7b lim: 4 exec/s: 0 rss: 27Mb L: 4/4 MS: 2 EraseBytes-CrossOver- #27 REDUCE cov: 7 ft: 8 corp: 3/6b lim: 4 exec/s: 0 rss: 27Mb L: 3/3 MS: 1 EraseBytes- #29 REDUCE cov: 7 ft: 8 corp: 3/5b lim: 4 exec/s: 0 rss: 27Mb L: 2/2 MS: 2 ChangeBit-EraseBytes- #445 REDUCE cov: 9 ft: 10 corp: 4/13b lim: 8 exec/s: 0 rss: 27Mb L: 8/8 MS: 1 InsertRepeatedBytes- #12104 NEW cov: 11 ft: 12 corp: 5/24b lim: 122 exec/s: 0 rss: 28Mb L: 11/11 MS: 4 CMP-InsertByte-ShuffleBytes-ChangeByte- DE: "\xff\xff"- #12321 NEW cov: 12 ft: 13 corp: 6/31b lim: 122 exec/s: 0 rss: 28Mb L: 7/11 MS: 2 CopyPart-EraseBytes- #12459 REDUCE cov: 12 ft: 13 corp: 6/28b lim: 122 exec/s: 0 rss: 28Mb L: 8/8 MS: 3 CMP-InsertByte-EraseBytes- DE: "\x00\x00"- #12826 REDUCE cov: 12 ft: 13 corp: 6/26b lim: 122 exec/s: 0 rss: 28Mb L: 5/8 MS: 2 ShuffleBytes-EraseBytes- #14824 REDUCE cov: 12 ft: 13 corp: 6/25b lim: 135 exec/s: 0 rss: 28Mb L: 4/8 MS: 3 PersAutoDict-ShuffleBytes-EraseBytes- DE: "\x00\x00"- #15106 REDUCE cov: 12 ft: 13 corp: 6/24b lim: 135 exec/s: 0 rss: 28Mb L: 3/8 MS: 2 ChangeByte-EraseBytes- ... #197809 REDUCE cov: 35 ft: 36 corp: 22/129b lim: 1800 exec/s: 0 rss: 79Mb L: 9/9 MS: 1 InsertByte- #216250 REDUCE cov: 35 ft: 36 corp: 22/128b lim: 1980 exec/s: 0 rss: 87Mb L: 8/8 MS: 1 EraseBytes- #242761 REDUCE cov: 35 ft: 36 corp: 22/127b lim: 2237 exec/s: 0 rss: 101Mb L: 7/8 MS: 1 EraseBytes- ==126148== ERROR: libFuzzer: deadly signal #0 0x35b981 in __sanitizer_print_stack_trace ../recipe_cleanup/clangFu99hg/llvm_build_dir/tools/clang/stage2-bins/runtimes/runtimes-x86_64-unknown-linux-gnu-bins/compiler-rt/lib/asan/asan_stack.cpp:86:3 #1 0x2bcdb5 in fuzzer::PrintStackTrace() (/home/aarongreen/src/pigweed/out/host/obj/pw_fuzzer/toy_fuzzer+0x2bcdb5) #2 0x2a2ac9 in fuzzer::Fuzzer::CrashCallback() (/home/aarongreen/src/pigweed/out/host/obj/pw_fuzzer/toy_fuzzer+0x2a2ac9) #3 0x7f866684151f (/lib/x86_64-linux-gnu/libpthread.so.0+0x1351f) #4 0x3831df in (anonymous namespace)::toy_example(char const*, char const*) /home/aarongreen/src/pigweed/out/host/../../pw_fuzzer/examples/toy_fuzzer.cc:49:15 #5 0x3831df in LLVMFuzzerTestOneInput /home/aarongreen/src/pigweed/out/host/../../pw_fuzzer/examples/toy_fuzzer.cc:80:3 #6 0x2a4025 in fuzzer::Fuzzer::ExecuteCallback(unsigned char const*, unsigned long) (/home/aarongreen/src/pigweed/out/host/obj/pw_fuzzer/toy_fuzzer+0x2a4025) #7 0x2a3774 in fuzzer::Fuzzer::RunOne(unsigned char const*, unsigned long, bool, fuzzer::InputInfo*, bool*) (/home/aarongreen/src/pigweed/out/host/obj/pw_fuzzer/toy_fuzzer+0x2a3774) #8 0x2a5769 in fuzzer::Fuzzer::MutateAndTestOne() (/home/aarongreen/src/pigweed/out/host/obj/pw_fuzzer/toy_fuzzer+0x2a5769) #9 0x2a6185 in fuzzer::Fuzzer::Loop(std::__Fuzzer::vector >&) (/home/aarongreen/src/pigweed/out/host/obj/pw_fuzzer/toy_fuzzer+0x2a6185) #10 0x294c8a in fuzzer::FuzzerDriver(int*, char***, int (*)(unsigned char const*, unsigned long)) (/home/aarongreen/src/pigweed/out/host/obj/pw_fuzzer/toy_fuzzer+0x294c8a) #11 0x2bd422 in main ../recipe_cleanup/clangFu99hg/llvm_build_dir/tools/clang/stage2-bins/runtimes/runtimes-x86_64-unknown-linux-gnu-bins/compiler-rt/lib/fuzzer/FuzzerMain.cpp:19:10 #12 0x7f8666684bba in __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x26bba) #13 0x26ae19 in _start (/home/aarongreen/src/pigweed/out/host/obj/pw_fuzzer/toy_fuzzer+0x26ae19) NOTE: libFuzzer has rudimentary signal handlers. Combine libFuzzer with AddressSanitizer or similar for better crash reports. SUMMARY: libFuzzer: deadly signal MS: 1 CrossOver-; base unit: 9f479f7a6e3a21363397a25da3168218ba182a16 0x68,0x65,0x6c,0x6c,0x6f,0x0,0x77,0x6f,0x72,0x6c,0x64,0x0,0x0,0x0, hello\x00world\x00\x00\x00 artifact_prefix='artifacts'; Test unit written to artifacts/crash-6e4fdc7ffd04131ea15dd243a0890b1b606f4831 Base64: aGVsbG8Ad29ybGQAAAA= Running fuzzers on OSS-Fuzz =========================== Pigweed is integrated with `OSS-Fuzz`_, a continuous fuzzing infrastructure for open source software. Fuzzers listed in in ``pw_test_groups`` will automatically start being run within a day or so of appearing in the git repository. Bugs produced by OSS-Fuzz can be found in its `Monorail instance`_. These bugs include: * A detailed report, including a symbolized backtrace. * A revision range indicating when the bug has been detected. * A minimized testcase, which is a fuzzer input that can be used to reproduce the bug. To reproduce a bug: #. Build_ the fuzzers as described above. #. Download the minimized testcase. #. Run_ the fuzzer with the testcase as an argument. For example, if the testcase is saved as "~/Downloads/testcase" and the fuzzer is the same as in the examples above, you could run: .. code:: $ ./out/host/obj/pw_fuzzer/toy_fuzzer ~/Downloads/testcase If you need to recreate the OSS-Fuzz environment locally, you can use its documentation on `reproducing`_ issues. In particular, you can recreate the OSS-Fuzz environment using: .. code:: $ python infra/helper.py pull_images $ python infra/helper.py build_image pigweed $ python infra/helper.py build_fuzzers --sanitizer
pigweed With that environment, you can run the reproduce bugs using: .. code:: python infra/helper.py reproduce pigweed _ ~/Downloads/testcase You can even verify fixes in your local source checkout: .. code:: $ python infra/helper.py build_fuzzers --sanitizer
pigweed $PW_ROOT $ python infra/helper.py reproduce pigweed _ ~/Downloads/testcase .. _compiler_rt: https://compiler-rt.llvm.org/ .. _corpus: https://llvm.org/docs/LibFuzzer.html#corpus .. _FUZZING_BUILD_MODE_UNSAFE_FOR_PRODUCTION: https://llvm.org/docs/LibFuzzer.html#fuzzer-friendly-build-mode .. _FuzzedDataProvider: https://github.com/llvm/llvm-project/blob/master/compiler-rt/include/fuzzer/FuzzedDataProvider.h .. _libFuzzer: https://llvm.org/docs/LibFuzzer.html .. _libFuzzer options: https://llvm.org/docs/LibFuzzer.html#options .. _LLVMFuzzerTestOneInput: https://llvm.org/docs/LibFuzzer.html#fuzz-target .. _monorail instance: https://bugs.chromium.org/p/oss-fuzz .. _oss-fuzz: https://github.com/google/oss-fuzz .. _reproducing: https://google.github.io/oss-fuzz/advanced-topics/reproducing/ .. _running a fuzzer: https://llvm.org/docs/LibFuzzer.html#running .. _sanitizer runtime flags: https://github.com/google/sanitizers/wiki/SanitizerCommonFlags .. _split a fuzzing input: https://github.com/google/fuzzing/blob/master/docs/split-inputs.md .. _startup initialization: https://llvm.org/docs/LibFuzzer.html#startup-initialization .. _structure aware fuzzing: https://github.com/google/fuzzing/blob/master/docs/structure-aware_fuzzing.md .. _valid options: https://gcc.gnu.org/onlinedocs/gcc/Instrumentation-Options.html