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176 lines
6.1 KiB
Markdown
176 lines
6.1 KiB
Markdown
Fuzzing OpenSSL
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===============
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OpenSSL can use either LibFuzzer or AFL to do fuzzing.
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LibFuzzer
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---------
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How to fuzz OpenSSL with [libfuzzer](http://llvm.org/docs/LibFuzzer.html),
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starting from a vanilla+OpenSSH server Ubuntu install.
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With `clang` from a package manager
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-----------------------------------
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Install `clang`, which [ships with `libfuzzer`](http://llvm.org/docs/LibFuzzer.html#fuzzer-usage)
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since version 6.0:
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sudo apt-get install clang
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Configure `openssl` for fuzzing. For now, you'll still need to pass in the path
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to the `libFuzzer` library file while configuring; this is represented as
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`$PATH_TO_LIBFUZZER` below. A typical value would be
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`/usr/lib/llvm-7/lib/clang/7.0.1/lib/linux/libclang_rt.fuzzer-x86_64.a`.
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CC=clang ./config enable-fuzz-libfuzzer \
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--with-fuzzer-lib=$PATH_TO_LIBFUZZER \
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-DPEDANTIC enable-asan enable-ubsan no-shared \
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-DFUZZING_BUILD_MODE_UNSAFE_FOR_PRODUCTION \
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-fsanitize=fuzzer-no-link \
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enable-ec_nistp_64_gcc_128 -fno-sanitize=alignment \
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enable-weak-ssl-ciphers enable-rc5 enable-md2 \
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enable-ssl3 enable-ssl3-method enable-nextprotoneg \
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--debug
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Compile:
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sudo apt-get install make
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make clean
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LDCMD=clang++ make -j4
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Finally, perform the actual fuzzing:
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fuzz/helper.py $FUZZER
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where $FUZZER is one of the executables in `fuzz/`.
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It will run until you stop it.
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If you get a crash, you should find a corresponding input file in
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`fuzz/corpora/$FUZZER-crash/`.
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With `clang` from source/pre-built binaries
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-------------------------------------------
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You may also wish to use a pre-built binary from the [LLVM Download
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site](http://releases.llvm.org/download.html), or to [build `clang` from
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source](https://clang.llvm.org/get_started.html). After adding `clang` to your
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path and locating the `libfuzzer` library file, the procedure for configuring
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fuzzing is the same, except that you also need to specify
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a `--with-fuzzer-include` option, which should be the parent directory of the
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prebuilt fuzzer library. This is represented as `$PATH_TO_LIBFUZZER_DIR` below.
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CC=clang ./config enable-fuzz-libfuzzer \
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--with-fuzzer-include=$PATH_TO_LIBFUZZER_DIR \
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--with-fuzzer-lib=$PATH_TO_LIBFUZZER \
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-DPEDANTIC enable-asan enable-ubsan no-shared \
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-DFUZZING_BUILD_MODE_UNSAFE_FOR_PRODUCTION \
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-fsanitize=fuzzer-no-link \
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enable-ec_nistp_64_gcc_128 -fno-sanitize=alignment \
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enable-weak-ssl-ciphers enable-rc5 enable-md2 \
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enable-ssl3 enable-ssl3-method enable-nextprotoneg \
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--debug
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AFL
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---
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This is an alternative to using LibFuzzer.
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Configure for fuzzing:
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sudo apt-get install afl-clang
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CC=afl-clang-fast ./config enable-fuzz-afl no-shared no-module \
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-DPEDANTIC enable-tls1_3 enable-weak-ssl-ciphers enable-rc5 \
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enable-md2 enable-ssl3 enable-ssl3-method enable-nextprotoneg \
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enable-ec_nistp_64_gcc_128 -fno-sanitize=alignment \
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--debug
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make clean
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make
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The following options can also be enabled: enable-asan, enable-ubsan, enable-msan
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Run one of the fuzzers:
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afl-fuzz -i fuzz/corpora/$FUZZER -o fuzz/corpora/$FUZZER/out fuzz/$FUZZER
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Where $FUZZER is one of the executables in `fuzz/`.
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Reproducing issues
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------------------
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If a fuzzer generates a reproducible error, you can reproduce the problem using
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the fuzz/*-test binaries and the file generated by the fuzzer. They binaries
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don't need to be built for fuzzing, there is no need to set CC or the call
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config with enable-fuzz-* or -fsanitize-coverage, but some of the other options
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above might be needed. For instance the enable-asan or enable-ubsan option might
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be useful to show you when the problem happens. For the client and server fuzzer
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it might be needed to use -DFUZZING_BUILD_MODE_UNSAFE_FOR_PRODUCTION to
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reproduce the generated random numbers.
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To reproduce the crash you can run:
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fuzz/$FUZZER-test $file
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To do all the tests of a specific fuzzer such as asn1 you can run
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fuzz/asn1-test fuzz/corpora/asn1
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or
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make test TESTS=fuzz_test_asn1
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To run several fuzz tests you can use for instance:
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make test TESTS='test_fuzz_cmp test_fuzz_cms'
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To run all fuzz tests you can use:
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make test TESTS='test_fuzz_*'
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Random numbers
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--------------
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The client and server fuzzer normally generate random numbers as part of the TLS
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connection setup. This results in the coverage of the fuzzing corpus changing
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depending on the random numbers. This also has an effect for coverage of the
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rest of the test suite and you see the coverage change for each commit even when
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no code has been modified.
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Since we want to maximize the coverage of the fuzzing corpus, the client and
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server fuzzer will use predictable numbers instead of the random numbers. This
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is controlled by the FUZZING_BUILD_MODE_UNSAFE_FOR_PRODUCTION define.
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The coverage depends on the way the numbers are generated. We don't disable any
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check of hashes, but the corpus has the correct hash in it for the random
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numbers that were generated. For instance the client fuzzer will always generate
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the same client hello with the same random number in it, and so the server, as
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emulated by the file, can be generated for that client hello.
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Coverage changes
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----------------
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Since the corpus depends on the default behaviour of the client and the server,
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changes in what they send by default will have an impact on the coverage. The
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corpus will need to be updated in that case.
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Updating the corpus
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-------------------
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The client and server corpus is generated with multiple config options:
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- The options as documented above
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- Without enable-ec_nistp_64_gcc_128 and without --debug
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- With no-asm
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- Using 32 bit
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- A default config, plus options needed to generate the fuzzer.
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The libfuzzer merge option is used to add the additional coverage
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from each config to the minimal set.
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Minimizing the corpus
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---------------------
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When you have gathered corpus data from more than one fuzzer run
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or for any other reason want to minimize the data
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in some corpus subdirectory `fuzz/corpora/DIR` this can be done as follows:
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mkdir fuzz/corpora/NEWDIR
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fuzz/$FUZZER -merge=1 fuzz/corpora/NEWDIR fuzz/corpora/DIR
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