1 files changed, 123 insertions, 0 deletions
diff --git a/doc/HACKING/Fuzzing.md b/doc/HACKING/Fuzzing.md
new file mode 100644
index 0000000000..2039d6a4c0
--- /dev/null
+++ b/doc/HACKING/Fuzzing.md
@@ -0,0 +1,123 @@
+= Fuzzing Tor
+
+== The simple version (no fuzzing, only tests)
+
+Check out fuzzing-corpora, and set TOR_FUZZ_CORPORA to point to the place
+where you checked it out.
+
+To run the fuzzing test cases in a deterministic fashion, use:
+      make test-fuzz-corpora
+
+This won't actually fuzz Tor!  It will just run all the fuzz binaries
+on our existing set of testcases for the fuzzer.
+
+
+== Different kinds of fuzzing
+
+Right now we support three different kinds of fuzzer.
+
+First, there's American Fuzzy Lop (AFL), a fuzzer that works by forking
+a target binary and passing it lots of different inputs on stdin.  It's the
+trickiest one to set up, so I'll be describing it more below.
+
+Second, there's libFuzzer, a llvm-based fuzzer that you link in as a library,
+and it runs a target function over and over.  To use this one, you'll need to
+have a reasonably recent clang and libfuzzer installed.  At that point, you
+just build with --enable-expensive-hardening and --enable-libfuzzer.  That
+will produce a set of binaries in src/test/fuzz/lf-fuzz-* .  These programs
+take as input a series of directories full of fuzzing examples.  For more
+information on libfuzzer, see http://llvm.org/docs/LibFuzzer.html
+
+Third, there's Google's OSS-Fuzz infrastructure, which expects to get all of
+its.  For more on this, see https://github.com/google/oss-fuzz and the
+projects/tor subdirectory.  You'll need to mess around with Docker a bit to
+test this one out; it's meant to run on Google's infrastructure.
+
+In all cases, you'll need some starting examples to give the fuzzer when it
+starts out.  There's a set in the "fuzzing-corpora" git repository.  Try
+setting TOR_FUZZ_CORPORA to point to a checkout of that repository
+
+== Writing Tor fuzzers
+
+A tor fuzzing harness should have:
+* a fuzz_init() function to set up any necessary global state.
+* a fuzz_main() function to receive input and pass it to a parser.
+* a fuzz_cleanup() function to clear global state.
+
+Most fuzzing frameworks will produce many invalid inputs - a tor fuzzing
+harness should rejecting invalid inputs without crashing or behaving badly.
+
+But the fuzzing harness should crash if tor fails an assertion, triggers a
+bug, or accesses memory it shouldn't. This helps fuzzing frameworks detect
+"interesting" cases.
+
+
+== Guided Fuzzing with AFL
+
+There is no HTTPS, hash, or signature for American Fuzzy Lop's source code, so
+its integrity can't be verified. That said, you really shouldn't fuzz on a
+machine you care about, anyway.
+
+To Build:
+  Get AFL from http://lcamtuf.coredump.cx/afl/ and unpack it
+  cd afl
+  make
+  cd ../tor
+  PATH=$PATH:../afl/ CC="../afl/afl-gcc" ./configure --enable-expensive-hardening
+  AFL_HARDEN=1 make clean fuzzers
+
+To Find The ASAN Memory Limit: (64-bit only)
+
+On 64-bit platforms, afl needs to know how much memory ASAN uses,
+because ASAN tends to allocate a ridiculous amount of virtual memory,
+and then not actually use it.
+
+Read afl/docs/notes_for_asan.txt for more details.
+
+  Download recidivm from http://jwilk.net/software/recidivm
+  Download the signature
+  Check the signature
+  tar xvzf recidivm*.tar.gz
+  cd recidivm*
+  make
+  /path/to/recidivm -v src/test/fuzz/fuzz-http
+  Use the final "ok" figure as the input to -m when calling afl-fuzz
+  (Normally, recidivm would output a figure automatically, but in some cases,
+  the fuzzing harness will hang when the memory limit is too small.)
+
+You could also just say "none" instead of the memory limit below, if you
+don't care about memory limits.
+
+
+To Run:
+  mkdir -p src/test/fuzz/fuzz_http_findings
+  ../afl/afl-fuzz -i ${TOR_FUZZ_CORPORA}/http -o src/test/fuzz/fuzz_http_findings -m <asan-memory-limit> -- src/test/fuzz/fuzz-http
+
+
+AFL has a multi-core mode, check the documentation for details.
+You might find the included fuzz-multi.sh script useful for this.
+
+macOS (OS X) requires slightly more preparation, including:
+* using afl-clang (or afl-clang-fast from the llvm directory)
+* disabling external crash reporting (AFL will guide you through this step)
+
+== Triaging Issues
+
+Crashes are usually interesting, particularly if using AFL_HARDEN=1 and --enable-expensive-hardening. Sometimes crashes are due to bugs in the harness code.
+
+Hangs might be interesting, but they might also be spurious machine slowdowns.
+Check if a hang is reproducible before reporting it. Sometimes, processing
+valid inputs may take a second or so, particularly with the fuzzer and
+sanitizers enabled.
+
+To see what fuzz-http is doing with a test case, call it like this:
+  src/test/fuzz/fuzz-http --debug < /path/to/test.case
+
+(Logging is disabled while fuzzing to increase fuzzing speed.)
+
+== Reporting Issues
+
+Please report any issues discovered using the process in Tor's security issue
+policy:
+
+https://trac.torproject.org/projects/tor/wiki/org/meetings/2016SummerDevMeeting/Notes/SecurityIssuePolicy