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FUZZING.md

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Fuzz testing

Modern fuzz testers are very effective and we wish to use them to ensure that no silly bugs creep into BoringSSL.

We use Clang's libFuzzer for fuzz testing and there are a number of fuzz testing functions in fuzz/. They are not built by default because they require that the rest of BoringSSL be built with some changes that make fuzzing much more effective, but are completely unsafe for real use.

In order to build the fuzz tests you will need at least Clang 6.0. Pass -DFUZZ=1 on the CMake command line to enable building BoringSSL with coverage and AddressSanitizer, and to build the fuzz test binaries. You'll probably need to set the CC and CXX environment variables too, like this:

CC=clang CXX=clang++ cmake -GNinja -DFUZZ=1 -B build
ninja -C build

From the build/ directory, you can then run the fuzzers. For example:

./fuzz/cert -max_len=10000 -jobs=32 -workers=32 ../fuzz/cert_corpus/

The arguments to jobs and workers should be the number of cores that you wish to dedicate to fuzzing. By default, libFuzzer uses the largest test in the corpus (or 64 if empty) as the maximum test case length. The max_len argument overrides this.

The recommended values of max_len for each test are:

Test max_len value
bn_div 384
bn_mod_exp 4096
cert 10000
client 20000
pkcs8 2048
privkey 2048
server 4096
session 8192
spki 1024
read_pem 512
ssl_ctx_api 256

These were determined by rounding up the length of the largest case in the corpus.

There are directories in fuzz/ for each of the fuzzing tests which contain seed files for fuzzing. Some of the seed files were generated manually but many of them are “interesting” results generated by the fuzzing itself. (Where “interesting” means that it triggered a previously unknown path in the code.)

Minimising the corpora

When a large number of new seeds are available, it's a good idea to minimise the corpus so that different seeds that trigger the same code paths can be deduplicated.

In order to minimise all the corpora, build for fuzzing and run ./fuzz/minimise_corpora.sh. Note that minimisation is, oddly, often not idempotent for unknown reasons.

Fuzzer mode

When -DFUZZ=1 is passed into CMake, BoringSSL builds with BORINGSSL_UNSAFE_FUZZER_MODE and BORINGSSL_UNSAFE_DETERMINISTIC_MODE defined. This modifies the library to be more friendly to fuzzers. If BORINGSSL_UNSAFE_DETERMINISTIC_MODE is set, BoringSSL will:

  • Replace RAND_bytes with a deterministic PRNG. Call RAND_reset_for_fuzzing() at the start of fuzzers which use RAND_bytes to reset the PRNG state.

  • Use a hard-coded time instead of the actual time.

Additionally, if BORINGSSL_UNSAFE_FUZZER_MODE is set, BoringSSL will:

  • Modify the TLS stack to perform all signature checks (CertificateVerify and ServerKeyExchange) and the Finished check, but always act as if the check succeeded.

  • Treat every cipher as the NULL cipher.

  • Tickets are unencrypted and the MAC check is performed but ignored.

  • renegotiation_info checks are ignored.

This is to prevent the fuzzer from getting stuck at a cryptographic invariant in the protocol.

TLS transcripts

The client and server corpora are seeded from the test suite. The test suite has a -fuzzer flag which mirrors the fuzzer mode changes above and a -deterministic flag which removes all non-determinism on the Go side. Not all tests pass, so ssl/test/runner/fuzzer_mode.json contains the necessary suppressions. The run_tests target will pass appropriate command-line flags.

There are separate corpora, client_corpus_no_fuzzer_mode and server_corpus_no_fuzzer_mode. These are transcripts for fuzzers with only BORINGSSL_UNSAFE_DETERMINISTIC_MODE defined. To build in this mode, pass -DNO_FUZZER_MODE=1 into CMake. This configuration is run in the same way but without -fuzzer and -shim-config flags.

If both sets of tests pass, refresh the fuzzer corpora with refresh_ssl_corpora.sh:

cd fuzz
./refresh_ssl_corpora.sh /path/to/fuzzer/mode/build /path/to/non/fuzzer/mode/build