| .. _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 and MacOS 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 meet 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 with GN |
| ======================== |
| |
| 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 your choice of sanitizers ("address" is also the current default). |
| See LLVM for `valid options`_. |
| |
| .. code:: sh |
| |
| $ gn gen out --args='pw_toolchain_SANITIZERS=["address"]' |
| |
| 3. Build normally, e.g. using ``pw watch``. |
| |
| .. _run: |
| |
| Building and running fuzzers with Bazel |
| ======================================= |
| To build a fuzzer, do the following: |
| |
| 1. Add the Bazel target using ``pw_cc_fuzz_test`` macro. |
| |
| .. code:: py |
| |
| load("@pigweed//pw_fuzzer:fuzzer.bzl", "pw_cc_fuzz_test") |
| |
| pw_cc_fuzz_test( |
| name = "my_fuzz_test", |
| srcs = ["my_fuzzer.cc"], |
| deps = [ |
| "@pigweed//pw_fuzzer", |
| ":my_lib", |
| ], |
| ) |
| |
| 2. Build and run the fuzzer. |
| |
| .. code:: sh |
| |
| bazel test //my_module:my_fuzz_test |
| |
| 3. Swap fuzzer backend to use ASAN fuzzing engine. |
| |
| .. code:: |
| |
| # .bazelrc |
| # Define the --config=asan-libfuzzer configuration. |
| build:asan-libfuzzer \ |
| --@rules_fuzzing//fuzzing:cc_engine=@rules_fuzzing//fuzzing/engines:libfuzzer |
| build:asan-libfuzzer \ |
| --@rules_fuzzing//fuzzing:cc_engine_instrumentation=libfuzzer |
| build:asan-libfuzzer --@rules_fuzzing//fuzzing:cc_engine_sanitizer=asan |
| |
| 4. Re-run fuzz tests. |
| |
| .. code:: |
| |
| bazel test //my_module:my_fuzz_test --config asan-libfuzzer |
| |
| 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_fuzz/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<fuzzer::SizedFile, fuzzer::fuzzer_allocator<fuzzer::SizedFile> >&) (/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 <address/undefined> pigweed |
| |
| With that environment, you can run the reproduce bugs using: |
| |
| .. code:: |
| |
| python infra/helper.py reproduce pigweed <pw_module>_<fuzzer_name> ~/Downloads/testcase |
| |
| You can even verify fixes in your local source checkout: |
| |
| .. code:: |
| |
| $ python infra/helper.py build_fuzzers --sanitizer <address/undefined> pigweed $PW_ROOT |
| $ python infra/helper.py reproduce pigweed <pw_module>_<fuzzer_name> ~/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/HEAD/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/HEAD/docs/split-inputs.md |
| .. _startup initialization: https://llvm.org/docs/LibFuzzer.html#startup-initialization |
| .. _structure aware fuzzing: https://github.com/google/fuzzing/blob/HEAD/docs/structure-aware-fuzzing.md |
| .. _valid options: https://gcc.gnu.org/onlinedocs/gcc/Instrumentation-Options.html |