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Fuzzing Bitcoin Core using libFuzzer

Quickstart guide

To quickly get started fuzzing Bitcoin Core using libFuzzer:

$ git clone https://github.com/bitcoin/bitcoin
$ cd bitcoin/
$ ./autogen.sh
$ CC=clang CXX=clang++ ./configure --enable-fuzz --with-sanitizers=address,fuzzer,undefined
# macOS users: If you have problem with this step then make sure to read "macOS hints for
# libFuzzer" on https://github.com/bitcoin/bitcoin/blob/master/doc/fuzzing.md#macos-hints-for-libfuzzer
$ make
$ FUZZ=process_message src/test/fuzz/fuzz
# abort fuzzing using ctrl-c

Fuzzing harnesses, fuzzing output and fuzzing corpora

process_message is a fuzzing harness for the ProcessMessage(...) function (net_processing). The available fuzzing harnesses are found in src/test/fuzz/.

The fuzzer will output NEW every time it has created a test input that covers new areas of the code under test. For more information on how to interpret the fuzzer output, see the libFuzzer documentation.

If you specify a corpus directory then any new coverage increasing inputs will be saved there:

$ mkdir -p process_message-seeded-from-thin-air/
$ FUZZ=process_message src/test/fuzz/fuzz process_message-seeded-from-thin-air/
INFO: Seed: 840522292
INFO: Loaded 1 modules   (424174 inline 8-bit counters): 424174 [0x55e121ef9ab8, 0x55e121f613a6),
INFO: Loaded 1 PC tables (424174 PCs): 424174 [0x55e121f613a8,0x55e1225da288),
INFO:        0 files found in process_message-seeded-from-thin-air/
INFO: -max_len is not provided; libFuzzer will not generate inputs larger than 4096 bytes
INFO: A corpus is not provided, starting from an empty corpus
#2      INITED cov: 94 ft: 95 corp: 1/1b exec/s: 0 rss: 150Mb
#3      NEW    cov: 95 ft: 96 corp: 2/3b lim: 4 exec/s: 0 rss: 150Mb L: 2/2 MS: 1 InsertByte-
#4      NEW    cov: 96 ft: 98 corp: 3/7b lim: 4 exec/s: 0 rss: 150Mb L: 4/4 MS: 1 CrossOver-
#21     NEW    cov: 96 ft: 100 corp: 4/11b lim: 4 exec/s: 0 rss: 150Mb L: 4/4 MS: 2 ChangeBit-CrossOver-
#324    NEW    cov: 101 ft: 105 corp: 5/12b lim: 6 exec/s: 0 rss: 150Mb L: 6/6 MS: 5 CrossOver-ChangeBit-CopyPart-ChangeBit-ChangeBinInt-
#1239   REDUCE cov: 102 ft: 106 corp: 6/24b lim: 14 exec/s: 0 rss: 150Mb L: 13/13 MS: 5 ChangeBit-CrossOver-EraseBytes-ChangeBit-InsertRepeatedBytes-
#1272   REDUCE cov: 102 ft: 106 corp: 6/23b lim: 14 exec/s: 0 rss: 150Mb L: 12/12 MS: 3 ChangeBinInt-ChangeBit-EraseBytes-
        NEW_FUNC[1/677]: 0x55e11f456690 in std::_Function_base::~_Function_base() /usr/lib/gcc/x86_64-linux-gnu/8/../../../../include/c++/8/bits/std_function.h:255
        NEW_FUNC[2/677]: 0x55e11f465800 in CDataStream::CDataStream(std::vector<unsigned char, std::allocator<unsigned char> > const&, int, int) src/./streams.h:248
#2125   REDUCE cov: 4820 ft: 4867 corp: 7/29b lim: 21 exec/s: 0 rss: 155Mb L: 6/12 MS: 2 CopyPart-CMP- DE: "block"-
        NEW_FUNC[1/9]: 0x55e11f64d790 in std::_Rb_tree<uint256, std::pair<uint256 const, std::chrono::duration<long, std::ratio<1l, 1000000l> > >, std::_Select1st<std::pair<uint256 const, std::chrono::duration<long, std::ratio<1l, 1000000l> > > >, std::less<uint256>, std::allocator<std::pair<uint256 const, std::chrono::duration<long, std::ratio<1l, 1000000l> > > > >::~_Rb_tree() /usr/lib/gcc/x86_64-linux-gnu/8/../../../../include/c++/8/bits/stl_tree.h:972
        NEW_FUNC[2/9]: 0x55e11f64d870 in std::_Rb_tree<uint256, std::pair<uint256 const, std::chrono::duration<long, std::ratio<1l, 1000000l> > >, std::_Select1st<std::pair<uint256 const, std::chrono::duration<long, std::ratio<1l, 1000000l> > > >, std::less<uint256>, std::allocator<std::pair<uint256 const, std::chrono::duration<long, std::ratio<1l, 1000000l> > > > >::_M_erase(std::_Rb_tree_node<std::pair<uint256 const, std::chrono::duration<long, std::ratio<1l, 1000000l> > > >*) /usr/lib/gcc/x86_64-linux-gnu/8/../../../../include/c++/8/bits/stl_tree.h:1875
#2228   NEW    cov: 4898 ft: 4971 corp: 8/35b lim: 21 exec/s: 0 rss: 156Mb L: 6/12 MS: 3 EraseBytes-CopyPart-PersAutoDict- DE: "block"-
        NEW_FUNC[1/5]: 0x55e11f46df70 in std::enable_if<__and_<std::allocator_traits<zero_after_free_allocator<char> >::__construct_helper<char, unsigned char const&>::type>::value, void>::type std::allocator_traits<zero_after_free_allocator<char> >::_S_construct<char, unsigned char const&>(zero_after_free_allocator<char>&, char*, unsigned char const&) /usr/lib/gcc/x86_64-linux-gnu/8/../../../../include/c++/8/bits/alloc_traits.h:243
        NEW_FUNC[2/5]: 0x55e11f477390 in std::vector<unsigned char, std::allocator<unsigned char> >::data() /usr/lib/gcc/x86_64-linux-gnu/8/../../../../include/c++/8/bits/stl_vector.h:1056
#2456   NEW    cov: 4933 ft: 5042 corp: 9/55b lim: 21 exec/s: 0 rss: 160Mb L: 20/20 MS: 3 ChangeByte-InsertRepeatedBytes-PersAutoDict- DE: "block"-
#2467   NEW    cov: 4933 ft: 5043 corp: 10/76b lim: 21 exec/s: 0 rss: 161Mb L: 21/21 MS: 1 InsertByte-
#4215   NEW    cov: 4941 ft: 5129 corp: 17/205b lim: 29 exec/s: 4215 rss: 350Mb L: 29/29 MS: 5 InsertByte-ChangeBit-CopyPart-InsertRepeatedBytes-CrossOver-
#4567   REDUCE cov: 4941 ft: 5129 corp: 17/204b lim: 29 exec/s: 4567 rss: 404Mb L: 24/29 MS: 2 ChangeByte-EraseBytes-
#6642   NEW    cov: 4941 ft: 5138 corp: 18/244b lim: 43 exec/s: 2214 rss: 450Mb L: 43/43 MS: 3 CopyPart-CMP-CrossOver- DE: "verack"-
# abort fuzzing using ctrl-c
$ ls process_message-seeded-from-thin-air/
349ac589fc66a09abc0b72bb4ae445a7a19e2cd8 4df479f1f421f2ea64b383cd4919a272604087a7
a640312c98dcc55d6744730c33e41c5168c55f09 b135de16e4709558c0797c15f86046d31c5d86d7
c000f7b41b05139de8b63f4cbf7d1ad4c6e2aa7f fc52cc00ec1eb1c08470e69f809ae4993fa70082
$ cat --show-nonprinting process_message-seeded-from-thin-air/349ac589fc66a09abc0b72bb4ae445a7a19e2cd8
block^@M-^?M-^?M-^?M-^?M-^?nM-^?M-^?

In this case the fuzzer managed to create a block message which when passed to ProcessMessage(...) increased coverage.

The project's collection of seed corpora is found in the bitcoin-core/qa-assets repo.

To fuzz process_message using the bitcoin-core/qa-assets seed corpus:

$ git clone https://github.com/bitcoin-core/qa-assets
$ FUZZ=process_message src/test/fuzz/fuzz qa-assets/fuzz_seed_corpus/process_message/
INFO: Seed: 1346407872
INFO: Loaded 1 modules   (424174 inline 8-bit counters): 424174 [0x55d8a9004ab8, 0x55d8a906c3a6),
INFO: Loaded 1 PC tables (424174 PCs): 424174 [0x55d8a906c3a8,0x55d8a96e5288),
INFO:      991 files found in qa-assets/fuzz_seed_corpus/process_message/
INFO: -max_len is not provided; libFuzzer will not generate inputs larger than 4096 bytes
INFO: seed corpus: files: 991 min: 1b max: 1858b total: 288291b rss: 150Mb
#993    INITED cov: 7063 ft: 8236 corp: 25/3821b exec/s: 0 rss: 181Mb

If you find coverage increasing inputs when fuzzing you are highly encouraged to submit them for inclusion in the bitcoin-core/qa-assets repo.

Every single pull request submitted against the Bitcoin Core repo is automatically tested against all inputs in the bitcoin-core/qa-assets repo. Contributing new coverage increasing inputs is an easy way to help make Bitcoin Core more robust.

macOS hints for libFuzzer

The default Clang/LLVM version supplied by Apple on macOS does not include fuzzing libraries, so macOS users will need to install a full version, for example using brew install llvm.

Should you run into problems with the address sanitizer, it is possible you may need to run ./configure with --disable-asm to avoid errors with certain assembly code from Bitcoin Core's code. See developer notes on sanitizers for more information.

You may also need to take care of giving the correct path for clang and clang++, like CC=/path/to/clang CXX=/path/to/clang++ if the non-systems clang does not come first in your path.

Full configure that was tested on macOS Catalina with brew installed llvm:

./configure --enable-fuzz --with-sanitizers=fuzzer,address,undefined CC=/usr/local/opt/llvm/bin/clang CXX=/usr/local/opt/llvm/bin/clang++ --disable-asm

Read the libFuzzer documentation for more information. This libFuzzer tutorial might also be of interest.

Fuzzing Bitcoin Core using american fuzzy lop (afl-fuzz)

Quickstart guide

To quickly get started fuzzing Bitcoin Core using afl-fuzz:

$ git clone https://github.com/bitcoin/bitcoin
$ cd bitcoin/
$ git clone https://github.com/google/afl
$ make -C afl/
$ make -C afl/llvm_mode/
$ ./autogen.sh
# It is possible to compile with afl-gcc and afl-g++ instead of afl-clang. However, running afl-fuzz
# may require more memory via the -m flag.
$ CC=$(pwd)/afl/afl-clang-fast CXX=$(pwd)/afl/afl-clang-fast++ ./configure --enable-fuzz
$ make
# For macOS you may need to ignore x86 compilation checks when running "make". If so,
# try compiling using: AFL_NO_X86=1 make
$ mkdir -p inputs/ outputs/
$ echo A > inputs/thin-air-input
$ FUZZ=bech32 afl/afl-fuzz -i inputs/ -o outputs/ -- src/test/fuzz/fuzz
# You may have to change a few kernel parameters to test optimally - afl-fuzz
# will print an error and suggestion if so.

Read the afl-fuzz documentation for more information.

Fuzzing Bitcoin Core using Honggfuzz

Quickstart guide

To quickly get started fuzzing Bitcoin Core using Honggfuzz:

$ git clone https://github.com/bitcoin/bitcoin
$ cd bitcoin/
$ ./autogen.sh
$ git clone https://github.com/google/honggfuzz
$ cd honggfuzz/
$ make
$ cd ..
$ CC=$(pwd)/honggfuzz/hfuzz_cc/hfuzz-clang CXX=$(pwd)/honggfuzz/hfuzz_cc/hfuzz-clang++ ./configure --enable-fuzz --with-sanitizers=address,undefined
$ make
$ mkdir -p inputs/
$ FUZZ=process_message honggfuzz/honggfuzz -i inputs/ -- src/test/fuzz/fuzz

Read the Honggfuzz documentation for more information.

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