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131 lines
4.6 KiB
Markdown
131 lines
4.6 KiB
Markdown
# I Can Haz Fuzz?
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LibFuzzer
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=========
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Or, 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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Use Chrome's handy recent build of clang. Older versions may also work.
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$ sudo apt-get install git
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$ mkdir git-work
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$ git clone https://chromium.googlesource.com/chromium/src/tools/clang
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$ clang/scripts/update.py
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You may want to git pull and re-run the update from time to time.
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Update your path:
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$ PATH=~/third_party/llvm-build/Release+Asserts/bin/:$PATH
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Get and build libFuzzer (there is a git mirror at
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https://github.com/llvm-mirror/llvm/tree/master/lib/Fuzzer if you prefer):
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$ cd
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$ sudo apt-get install subversion
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$ mkdir svn-work
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$ cd svn-work
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$ svn co https://llvm.org/svn/llvm-project/compiler-rt/trunk/lib/fuzzer Fuzzer
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$ cd Fuzzer
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$ clang++ -c -g -O2 -std=c++11 *.cpp
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$ ar r libFuzzer.a *.o
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$ ranlib libFuzzer.a
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Configure for fuzzing:
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$ CC=clang ./config enable-fuzz-libfuzzer \
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--with-fuzzer-include=../../svn-work/Fuzzer \
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--with-fuzzer-lib=../../svn-work/Fuzzer/libFuzzer.a \
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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-coverage=trace-pc-guard,indirect-calls,trace-cmp \
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enable-ec_nistp_64_gcc_128 -fno-sanitize=alignment enable-tls1_3 \
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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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$ sudo apt-get install make
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$ LDCMD=clang++ make -j
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$ fuzz/helper.py $FUZZER
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Where $FUZZER is one of the executables in `fuzz/`.
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If you get a crash, you should find a corresponding input file in
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`fuzz/corpora/$FUZZER-crash/`.
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AFL
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===
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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 -DPEDANTIC \
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enable-tls1_3 enable-weak-ssl-ciphers enable-rc5 enable-md2 \
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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
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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 $FUZZER is one of the executables in `fuzz/`.
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Reproducing issues
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==================
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If a fuzzer generates a reproducible error, you can reproduce the problem using
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the fuzz/*-test binaries and the file generated by the fuzzer. They binaries
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don't need to be build for fuzzing, there is no need to set CC or the call
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config with enable-fuzz-* or -fsanitize-coverage, but some of the other options
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above might be needed. For instance the enable-asan or enable-ubsan option might
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be useful to show you when the problem happens. For the client and server fuzzer
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it might be needed to use -DFUZZING_BUILD_MODE_UNSAFE_FOR_PRODUCTION to
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reproduce the generated random numbers.
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To reproduce the crash you can run:
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$ fuzz/$FUZZER-test $file
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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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