trezor-firmware/crypto/fuzzer

Fuzz Testing trezor-crypto

Selected functions can be fuzzed via specific libFuzzer harnesses for increased test coverage and issue detection.

Note: the following commands are relative to the trezor-crypto main directory.

Build

A modern C compiler with built-in libFuzzer support is required. The build process will use clang by default. Set the CC= environment variable if you want to use a special compiler variant.

make clean
FUZZER=1 make fuzzer -j$(nproc)

Sanitizers

Recommended: ASAN / UBSAN / MSAN flags for error detection can be specified via the special SANFLAGS.

Examples:

• SANFLAGS="-fsanitize=address,undefined"
• SANFLAGS="-fsanitize=memory -fsanitize-memory-track-origins"

Optimizations

Override OPTFLAGS to test the library at different optimization levels or simplify the debugging of detected issues.

Examples:

• OPTFLAGS="-O0 -ggdb3"
• OPTFLAGS="-O3 -march=native -fno-omit-frame-pointer -gline-tables-only"

To be determined:

• semi-automatic use of -fsanitize-ignorelist to reduce sanitizer overhead on hot functions
• -flto and -flto=thin link time optimization

Advanced usage:

• Profile guided optimization

Fuzzer-specific Configuration Flags

• -DFUZZ_ALLOW_SLOW to enable optional fuzzing targets of slow functions
• select a specific fuzz testing harness with -DFUZZ_EXCLUSIVE_TARGET= to disable the use of all other targets

Other Flags

To be determined:

• -DNDEBUG
• -DUSE_BIP39_CACHE=0 -DUSE_BIP32_CACHE=0 to explicitly disable the cache, a workaround for automatic cache clearing is used otherwise
• -D_FORTIFY_SOURCE=2 together with optimization flag -O2 or above
• -fstack-protector-strong or -fstack-protector-all
• -m32 to closer emulate the 32-bit environment present on microcontroller platforms
  • manually adjust Makefile DSECP256K1_CONTEXT_SIZE= for 32-bit values, see legacy/firmware/Makefile
  • this flag requires 32-bit build support for gcc-multilib, libc and others
  • switching from 64-bit to 32-bit has some effects on sanitizer internals such as Address Sanitizer
• -DSHA2_UNROLL_TRANSFORM SHA2 optimization flags
• -fsanitize-coverage=edge,trace-cmp,trace-div,indirect-calls,trace-gep,no-prune to add program counter granularity
• starting with clang-15, the additional trace-loads and trace-stores sanitizer coverage options are also available

Architecture-specific Configuration

• aarch64 supports Hardware-assisted AddressSanitizer via -fsanitize=hwaddress
  • initial testing suggests that this is usable but slower than Address Sanitizer on some systems
  • use lld instead of the default linker

Operation

See the libFuzzer documentation for valid options and usage. Detailed fuzzer usage and relevant considerations are out of scope of this document.

Warning: fuzzing is resource-intensive and can have a negative impact on your system stability.

Basic fuzzer call:

./fuzzer/fuzzer

Here is a more sophisticated multithreading example with a persistent input corpus and other optimizations:

mkdir fuzzer/fuzzer_corpus
./fuzzer/fuzzer -max_len=2048 -use_value_profile=1 -workers=16 -jobs=16 -timeout=1 -reload=5 -print_pcs=1 -print_funcs=42  fuzzer/fuzzer_corpus

Hint: for more permanent setups, consider invoking the fuzzer from outside of the source directory to avoid cluttering it with logfiles and crash inputs. Similarly, it is recommended to store the fuzzer corpus in another location.

Automated Fuzzer Dictionary Generation

Dictionaries are a useful mechanism to augment the capabilities of the fuzzer. Specify them via the -dict= flag.

Collect Interesting Strings From Unit Tests

cd fuzzer
./extract_fuzzer_dictionary.sh fuzzer_crypto_tests_strings_dictionary1.txt

The resulting file can be used as a fuzzer dictionary.

Evaluate Source Coverage

1. build the fuzzer binary with CFLAGS="-fprofile-instr-generate -fcoverage-mapping"
2. run with suitable -runs= or -max_total_time= limits
3. convert the recorded data llvm-profdata merge -output=default.profdata -instr default.profraw
4. render the data llvm-cov show fuzzer/fuzzer -instr-profile=default.profdata -format=html -output-dir=coverage-report
5. analyze report at coverage-report/index.html
6. (optional) remove artifacts with rm default.profraw default.profdata && rm -r coverage-report

Using Honggfuzz Fuzzer

Although this code is designed primarily for libFuzzer, it can also be used with Honggfuzz in a hybrid Honggfuzz/libFuzzer mode. The setup and usage details are mostly out of scope of this document, but here are some helpful pointers:

• compile with a clang-based hfuzz_cc Honggfuzz compiler variant
• set -DFUZZ_HONGGFUZZ=1 to suppress compatibility issues