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@@ -1,12 +1,53 @@
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= Fuzzing Tor
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+== The simple version (no fuzzing, only tests)
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+
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+Check out fuzzing-corpora, and set TOR_FUZZ_CORPORA to point to the place
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+where you checked it out.
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+
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To run the fuzzing test cases in a deterministic fashion, use:
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make fuzz
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- [I've turned this off for now. - NM]
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-To build the fuzzing harness binaries, use:
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- make fuzzers
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+== Different kinds of fuzzing
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+
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+Right now we support three different kinds of fuzzer.
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+
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+First, there's American Fuzzy Lop (AFL), a fuzzer that works by forking
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+a target binary and passing it lots of different inputs on stdin. It's the
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+trickiest one to set up, so I'll be describing it more below.
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+
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+Second, there's libFuzzer, a llvm-based fuzzer that you link in as a library,
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+and it runs a target function over and over. To use this one, you'll need to
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+have a reasonably recent clang and libfuzzer installed. At that point, you
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+just build with --enable-expensive-hardening and --enable-libfuzzer. That
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+will produce a set of binaries in src/test/fuzz/lf-fuzz-* . These programs
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+take as input a series of directories full of fuzzing examples. For more
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+information on libfuzzer, see http://llvm.org/docs/LibFuzzer.html
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+
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+Third, there's Google's OSS-Fuzz infrastructure, which expects to get all of
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+its. For more on this, see https://github.com/google/oss-fuzz and the
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+projects/tor subdirectory. You'll need to mess around with Docker a bit to
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+test this one out; it's meant to run on Google's infrastructure.
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+
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+In all cases, you'll need some starting examples to give the fuzzer when it
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+starts out. There's a set in the "fuzzing-corpora" git repository. Try
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+setting TOR_FUZZ_CORPORA to point to a checkout of that repository
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+
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+== Writing Tor fuzzers
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+
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+A tor fuzzing harness should have:
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+* a fuzz_init() function to set up any necessary global state.
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+* a fuzz_main() function to receive input and pass it to a parser.
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+* a fuzz_cleanup() function to clear global state.
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+
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+Most fuzzing frameworks will produce many invalid inputs - a tor fuzzing
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+harness should rejecting invalid inputs without crashing or behaving badly.
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+
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+But the fuzzing harness should crash if tor fails an assertion, triggers a
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+bug, or accesses memory it shouldn't. This helps fuzzing frameworks detect
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+"interesting" cases.
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+
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== Guided Fuzzing with AFL
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@@ -47,7 +88,7 @@ don't care about memory limits.
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To Run:
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mkdir -p src/test/fuzz/fuzz_http_findings
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- ../afl/afl-fuzz -i src/test/fuzz/data/http -x src/test/fuzz/dict/http -o src/test/fuzz/fuzz_http_findings -m <asan-memory-limit> -- src/test/fuzz_dir
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+ ../afl/afl-fuzz -i ${TOR_FUZZ_CORPORA}/http -o src/test/fuzz/fuzz_http_findings -m <asan-memory-limit> -- src/test/fuzz_dir
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AFL has a multi-core mode, check the documentation for details.
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@@ -57,20 +98,6 @@ macOS (OS X) requires slightly more preparation, including:
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* using afl-clang (or afl-clang-fast from the llvm directory)
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* disabling external crash reporting (AFL will guide you through this step)
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-== Writing Tor fuzzers
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-
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-A tor fuzzing harness should have:
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-* a fuzz_init() function to set up any necessary global state.
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-* a fuzz_main() function to receive input and pass it to a parser.
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-* a fuzz_cleanup() function to clear global state.
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-
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-Most fuzzing frameworks will produce many invalid inputs - a tor fuzzing
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-harness should rejecting invalid inputs without crashing or behaving badly.
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-
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-But the fuzzing harness should crash if tor fails an assertion, triggers a
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-bug, or accesses memory it shouldn't. This helps fuzzing frameworks detect
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-"interesting" cases.
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-
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== Triaging Issues
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Crashes are usually interesting, particularly if using AFL_HARDEN=1 and --enable-expensive-hardening. Sometimes crashes are due to bugs in the harness code.
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