Update documentation and testing integration for fuzzing
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Nick Mathewson
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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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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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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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Right now we support three different kinds of fuzzer.
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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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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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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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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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== Writing Tor fuzzers
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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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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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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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== 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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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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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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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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== 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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@ -246,3 +246,5 @@ noinst_LIBRARIES += $(OSS_FUZZ_FUZZERS)
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oss-fuzz-fuzzers: oss-fuzz-prereqs $(OSS_FUZZ_FUZZERS)
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fuzzers: $(FUZZERS) $(LIBFUZZER_FUZZERS)
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fuzz: $(FUZZERS)
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$(top_srcdir)/src/test/fuzz_static_testcases.sh
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@ -5,11 +5,23 @@
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set -e
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if [ -z "${TOR_FUZZ_CORPORA}" ] || [ ! -d "${TOR_FUZZ_CORPORA}" ] ; then
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echo "You need to set TOR_FUZZ_CORPORA to point to a checkout of "
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echo "the 'fuzzing-corpora' repository."
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exit 77
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fi
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for fuzzer in "${builddir:-.}"/src/test/fuzz/fuzz-* ; do
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f=`basename $fuzzer`
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case="${f#fuzz-}"
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echo "Running tests for ${case}"
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for entry in ${abs_top_srcdir:-.}/src/test/fuzz/data/${case}/*; do
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"${fuzzer}" "--err" < "$entry"
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done
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if [ -d "${TOR_FUZZ_CORPORA}/${case}" ]; then
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echo "Running tests for ${case}"
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for entry in "${TOR_FUZZ_CORPORA}/${case}/"*; do
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"${fuzzer}" "--err" < "$entry"
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done
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else
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echo "No tests found for ${case}"
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fi
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done
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