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107 lines
4.0 KiB
Markdown
107 lines
4.0 KiB
Markdown
# Fuzzers
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These are fuzzers designed for use with `libFuzzer` or `afl`. They can
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be used to run on Google's OSS-Fuzz (https://github.com/google/oss-fuzz/).
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The convention used here is that the initial values for each parser fuzzer
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are taken from the $NAME.in directory ($NAME is the name of the fuzzer, e.g.
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'libwget_xml_parse_buffer_fuzzer').
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Crash reproducers from OSS-Fuzz are put into $NAME.repro directory for
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regression testing with top dir 'make check' or 'make check-valgrind'.
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The script `get_ossfuzz_corpora` downloads the corpora from OSS-Fuzz for
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the given fuzzer. It puts those files together with the local ones and performs
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a 'merge' step to remove superfluous corpora. The next step would be to add
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changed/new corpora to the git repository.
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Example:
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```
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./get_ossfuzz_corpora libwget_xml_parse_buffer_fuzzer
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git add libwget_xml_parse_buffer_fuzzer.in/*
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git commit -a -m "Update OSS-Fuzz corpora"
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(create a branch and push if something changed)
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(create a MR)
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```
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Since there are quite a few fuzzers now, you can update all their corpora
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in one step with `./get_all_corpora`. Do this from time to time to stay
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in sync with OSS-Fuzz. Whenever library code or fuzzers change, there might
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me new corpora after 1-2 days.
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# Running a fuzzer using clang and libFuzzer
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Use the following commands on top dir:
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```
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export CC=clang
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#export CFLAGS="-O1 -g -fno-omit-frame-pointer -gline-tables-only -DFUZZING_BUILD_MODE_UNSAFE_FOR_PRODUCTION -fsanitize=undefined,integer,nullability,bool,alignment,null,enum,address,leak,nonnull-attribute -fsanitize=address -fsanitize-address-use-after-scope -fsanitize-coverage=trace-pc-guard,trace-cmp"
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export CFLAGS="-O1 -fno-omit-frame-pointer -gline-tables-only -DFUZZING_BUILD_MODE_UNSAFE_FOR_PRODUCTION -fsanitize=bool,array-bounds,float-divide-by-zero,function,integer-divide-by-zero,return,shift,signed-integer-overflow,vla-bound,vptr -fno-sanitize-recover=bool,array-bounds,float-divide-by-zero,function,integer-divide-by-zero,return,shift,signed-integer-overflow,vla-bound,vptr -fsanitize=fuzzer-no-link"
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export LIB_FUZZING_ENGINE="-lFuzzer -lstdc++"
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./configure --enable-fuzzing --disable-doc --disable-manywarnings
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make clean
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make -j$(nproc)
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cd fuzz
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# run libwget_xml_parse_buffer_fuzzer
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export UBSAN_OPTIONS=print_stacktrace=1:report_error_type=1
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export ASAN_SYMBOLIZER_PATH=/usr/bin/llvm-symbolizer
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./run-clang.sh libwget_xml_parse_buffer_fuzzer
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```
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If you see a crash, then a crash corpora is written that can be used for further
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investigation. E.g.
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```
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==2410==ERROR: AddressSanitizer: heap-use-after-free on address 0x602000004e90 at pc 0x00000049cf9c bp 0x7fffb5543f70 sp 0x7fffb5543720
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...
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Test unit written to ./crash-adc83b19e793491b1c6ea0fd8b46cd9f32e592fc
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```
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To reproduce the crash:
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```
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./libwget_xml_parse_buffer_fuzzer < ./crash-adc83b19e793491b1c6ea0fd8b46cd9f32e592fc
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```
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You can also copy/move that file into libwget_xml_parse_buffer_fuzzer.repro/
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and re-build the project without fuzzing for a valgrind run, if you like that better.
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Just a `./configure` (maybe with sanitizers enabled) and a `make check-valgrind` should reproduce it.
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# Running a fuzzer using AFL
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Use the following commands on top dir:
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```
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$ CC=afl-clang-fast ./configure --disable-doc
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$ make -j$(nproc) clean all
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$ cd fuzz
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$ ./run-afl.sh libwget_xml_parse_buffer_fuzzer
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```
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# Fuzz code coverage using the corpus directories *.in/
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Code coverage reports currently work best with gcc+lcov+genhtml.
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In the top directory:
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```
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CC=gcc CFLAGS="-O0 -g" ./configure --disable-doc --disable-manywarnings
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make fuzz-coverage
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xdg-open lcov/index.html
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```
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Each fuzzer target has it's own files/functions to cover, e.g.
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`libwget_xml_parse_buffer` covers libwget/xml.c (except
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wget_xml_parse_file() and wget_html_parse_file()).
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To work on corpora for better coverage, `cd fuzz` and use e.g.
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`./view-coverage.sh libwget_xml_parse_buffer_fuzzer`.
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# Creating wget_options_fuzzer.dict
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```
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for i in `../src/wget2 --help|tr ' ' '\n'|grep ^--|cut -c 3-|sort`;do echo \"$i\"; done >wget_options_fuzzer.dict
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```
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