1 ======================================================== 2 LibFuzzer -- a library for coverage-guided fuzz testing. 3 ======================================================== 4 .. contents:: 5 :local: 6 :depth: 4 7 8 Introduction 9 ============ 10 11 This library is intended primarily for in-process coverage-guided fuzz testing 12 (fuzzing) of other libraries. The typical workflow looks like this: 13 14 * Build the Fuzzer library as a static archive (or just a set of .o files). 15 Note that the Fuzzer contains the main() function. 16 Preferably do *not* use sanitizers while building the Fuzzer. 17 * Build the library you are going to test with -fsanitize-coverage=[234] 18 and one of the sanitizers. We recommend to build the library in several 19 different modes (e.g. asan, msan, lsan, ubsan, etc) and even using different 20 optimizations options (e.g. -O0, -O1, -O2) to diversify testing. 21 * Build a test driver using the same options as the library. 22 The test driver is a C/C++ file containing interesting calls to the library 23 inside a single function ``extern "C" void TestOneInput(const uint8_t *Data, size_t Size);`` 24 * Link the Fuzzer, the library and the driver together into an executable 25 using the same sanitizer options as for the library. 26 * Collect the initial corpus of inputs for the 27 fuzzer (a directory with test inputs, one file per input). 28 The better your inputs are the faster you will find something interesting. 29 Also try to keep your inputs small, otherwise the Fuzzer will run too slow. 30 * Run the fuzzer with the test corpus. As new interesting test cases are 31 discovered they will be added to the corpus. If a bug is discovered by 32 the sanitizer (asan, etc) it will be reported as usual and the reproducer 33 will be written to disk. 34 Each Fuzzer process is single-threaded (unless the library starts its own 35 threads). You can run the Fuzzer on the same corpus in multiple processes. 36 in parallel. For run-time options run the Fuzzer binary with '-help=1'. 37 38 39 The Fuzzer is similar in concept to AFL_, 40 but uses in-process Fuzzing, which is more fragile, more restrictive, but 41 potentially much faster as it has no overhead for process start-up. 42 It uses LLVM's SanitizerCoverage_ instrumentation to get in-process 43 coverage-feedback 44 45 The code resides in the LLVM repository, requires the fresh Clang compiler to build 46 and is used to fuzz various parts of LLVM, 47 but the Fuzzer itself does not (and should not) depend on any 48 part of LLVM and can be used for other projects w/o requiring the rest of LLVM. 49 50 Usage examples 51 ============== 52 53 Toy example 54 ----------- 55 56 A simple function that does something interesting if it receives the input "HI!":: 57 58 cat << EOF >> test_fuzzer.cc 59 extern "C" void TestOneInput(const unsigned char *data, unsigned long size) { 60 if (size > 0 && data[0] == 'H') 61 if (size > 1 && data[1] == 'I') 62 if (size > 2 && data[2] == '!') 63 __builtin_trap(); 64 } 65 EOF 66 # Get lib/Fuzzer. Assuming that you already have fresh clang in PATH. 67 svn co http://llvm.org/svn/llvm-project/llvm/trunk/lib/Fuzzer 68 # Build lib/Fuzzer files. 69 clang -c -g -O2 -std=c++11 Fuzzer/*.cpp -IFuzzer 70 # Build test_fuzzer.cc with asan and link against lib/Fuzzer. 71 clang++ -fsanitize=address -fsanitize-coverage=3 test_fuzzer.cc Fuzzer*.o 72 # Run the fuzzer with no corpus. 73 ./a.out 74 75 You should get ``Illegal instruction (core dumped)`` pretty quickly. 76 77 PCRE2 78 ----- 79 80 Here we show how to use lib/Fuzzer on something real, yet simple: pcre2_:: 81 82 COV_FLAGS=" -fsanitize-coverage=4 -mllvm -sanitizer-coverage-8bit-counters=1" 83 # Get PCRE2 84 svn co svn://vcs.exim.org/pcre2/code/trunk pcre 85 # Get lib/Fuzzer. Assuming that you already have fresh clang in PATH. 86 svn co http://llvm.org/svn/llvm-project/llvm/trunk/lib/Fuzzer 87 # Build PCRE2 with AddressSanitizer and coverage. 88 (cd pcre; ./autogen.sh; CC="clang -fsanitize=address $COV_FLAGS" ./configure --prefix=`pwd`/../inst && make -j && make install) 89 # Build lib/Fuzzer files. 90 clang -c -g -O2 -std=c++11 Fuzzer/*.cpp -IFuzzer 91 # Build the the actual function that does something interesting with PCRE2. 92 cat << EOF > pcre_fuzzer.cc 93 #include <string.h> 94 #include "pcre2posix.h" 95 extern "C" void TestOneInput(const unsigned char *data, size_t size) { 96 if (size < 1) return; 97 char *str = new char[size+1]; 98 memcpy(str, data, size); 99 str[size] = 0; 100 regex_t preg; 101 if (0 == regcomp(&preg, str, 0)) { 102 regexec(&preg, str, 0, 0, 0); 103 regfree(&preg); 104 } 105 delete [] str; 106 } 107 EOF 108 clang++ -g -fsanitize=address $COV_FLAGS -c -std=c++11 -I inst/include/ pcre_fuzzer.cc 109 # Link. 110 clang++ -g -fsanitize=address -Wl,--whole-archive inst/lib/*.a -Wl,-no-whole-archive Fuzzer*.o pcre_fuzzer.o -o pcre_fuzzer 111 112 This will give you a binary of the fuzzer, called ``pcre_fuzzer``. 113 Now, create a directory that will hold the test corpus:: 114 115 mkdir -p CORPUS 116 117 For simple input languages like regular expressions this is all you need. 118 For more complicated inputs populate the directory with some input samples. 119 Now run the fuzzer with the corpus dir as the only parameter:: 120 121 ./pcre_fuzzer ./CORPUS 122 123 You will see output like this:: 124 125 Seed: 1876794929 126 #0 READ cov 0 bits 0 units 1 exec/s 0 127 #1 pulse cov 3 bits 0 units 1 exec/s 0 128 #1 INITED cov 3 bits 0 units 1 exec/s 0 129 #2 pulse cov 208 bits 0 units 1 exec/s 0 130 #2 NEW cov 208 bits 0 units 2 exec/s 0 L: 64 131 #3 NEW cov 217 bits 0 units 3 exec/s 0 L: 63 132 #4 pulse cov 217 bits 0 units 3 exec/s 0 133 134 * The ``Seed:`` line shows you the current random seed (you can change it with ``-seed=N`` flag). 135 * The ``READ`` line shows you how many input files were read (since you passed an empty dir there were inputs, but one dummy input was synthesised). 136 * The ``INITED`` line shows you that how many inputs will be fuzzed. 137 * The ``NEW`` lines appear with the fuzzer finds a new interesting input, which is saved to the CORPUS dir. If multiple corpus dirs are given, the first one is used. 138 * The ``pulse`` lines appear periodically to show the current status. 139 140 Now, interrupt the fuzzer and run it again the same way. You will see:: 141 142 Seed: 1879995378 143 #0 READ cov 0 bits 0 units 564 exec/s 0 144 #1 pulse cov 502 bits 0 units 564 exec/s 0 145 ... 146 #512 pulse cov 2933 bits 0 units 564 exec/s 512 147 #564 INITED cov 2991 bits 0 units 344 exec/s 564 148 #1024 pulse cov 2991 bits 0 units 344 exec/s 1024 149 #1455 NEW cov 2995 bits 0 units 345 exec/s 1455 L: 49 150 151 This time you were running the fuzzer with a non-empty input corpus (564 items). 152 As the first step, the fuzzer minimized the set to produce 344 interesting items (the ``INITED`` line) 153 154 You may run ``N`` independent fuzzer jobs in parallel on ``M`` CPUs:: 155 156 N=100; M=4; ./pcre_fuzzer ./CORPUS -jobs=$N -workers=$M 157 158 This is useful when you already have an exhaustive test corpus. 159 If you've just started fuzzing with no good corpus running independent 160 jobs will create a corpus with too many duplicates. 161 One way to avoid this and still use all of your CPUs is to use the flag ``-exit_on_first=1`` 162 which will cause the fuzzer to exit on the first new synthesised input:: 163 164 N=100; M=4; ./pcre_fuzzer ./CORPUS -jobs=$N -workers=$M -exit_on_first=1 165 166 Heartbleed 167 ---------- 168 Remember Heartbleed_? 169 As it was recently `shown <https://blog.hboeck.de/archives/868-How-Heartbleed-couldve-been-found.html>`_, 170 fuzzing with AddressSanitizer can find Heartbleed. Indeed, here are the step-by-step instructions 171 to find Heartbleed with LibFuzzer:: 172 173 wget https://www.openssl.org/source/openssl-1.0.1f.tar.gz 174 tar xf openssl-1.0.1f.tar.gz 175 COV_FLAGS="-fsanitize-coverage=4" # -mllvm -sanitizer-coverage-8bit-counters=1" 176 (cd openssl-1.0.1f/ && ./config && 177 make -j 32 CC="clang -g -fsanitize=address $COV_FLAGS") 178 # Get and build LibFuzzer 179 svn co http://llvm.org/svn/llvm-project/llvm/trunk/lib/Fuzzer 180 clang -c -g -O2 -std=c++11 Fuzzer/*.cpp -IFuzzer 181 # Get examples of key/pem files. 182 git clone https://github.com/hannob/selftls 183 cp selftls/server* . -v 184 cat << EOF > handshake-fuzz.cc 185 #include <openssl/ssl.h> 186 #include <openssl/err.h> 187 #include <assert.h> 188 SSL_CTX *sctx; 189 int Init() { 190 SSL_library_init(); 191 SSL_load_error_strings(); 192 ERR_load_BIO_strings(); 193 OpenSSL_add_all_algorithms(); 194 assert (sctx = SSL_CTX_new(TLSv1_method())); 195 assert (SSL_CTX_use_certificate_file(sctx, "server.pem", SSL_FILETYPE_PEM)); 196 assert (SSL_CTX_use_PrivateKey_file(sctx, "server.key", SSL_FILETYPE_PEM)); 197 return 0; 198 } 199 extern "C" void TestOneInput(unsigned char *Data, size_t Size) { 200 static int unused = Init(); 201 SSL *server = SSL_new(sctx); 202 BIO *sinbio = BIO_new(BIO_s_mem()); 203 BIO *soutbio = BIO_new(BIO_s_mem()); 204 SSL_set_bio(server, sinbio, soutbio); 205 SSL_set_accept_state(server); 206 BIO_write(sinbio, Data, Size); 207 SSL_do_handshake(server); 208 SSL_free(server); 209 } 210 EOF 211 # Build the fuzzer. 212 clang++ -g handshake-fuzz.cc -fsanitize=address \ 213 openssl-1.0.1f/libssl.a openssl-1.0.1f/libcrypto.a Fuzzer*.o 214 # Run 20 independent fuzzer jobs. 215 ./a.out -jobs=20 -workers=20 216 217 Voila:: 218 219 #1048576 pulse cov 3424 bits 0 units 9 exec/s 24385 220 ================================================================= 221 ==17488==ERROR: AddressSanitizer: heap-buffer-overflow on address 0x629000004748 at pc 0x00000048c979 bp 0x7fffe3e864f0 sp 0x7fffe3e85ca8 222 READ of size 60731 at 0x629000004748 thread T0 223 #0 0x48c978 in __asan_memcpy 224 #1 0x4db504 in tls1_process_heartbeat openssl-1.0.1f/ssl/t1_lib.c:2586:3 225 #2 0x580be3 in ssl3_read_bytes openssl-1.0.1f/ssl/s3_pkt.c:1092:4 226 227 Advanced features 228 ================= 229 230 Tokens 231 ------ 232 233 By default, the fuzzer is not aware of complexities of the input language 234 and when fuzzing e.g. a C++ parser it will mostly stress the lexer. 235 It is very hard for the fuzzer to come up with something like ``reinterpret_cast<int>`` 236 from a test corpus that doesn't have it. 237 See a detailed discussion of this topic at 238 http://lcamtuf.blogspot.com/2015/01/afl-fuzz-making-up-grammar-with.html. 239 240 lib/Fuzzer implements a simple technique that allows to fuzz input languages with 241 long tokens. All you need is to prepare a text file containing up to 253 tokens, one token per line, 242 and pass it to the fuzzer as ``-tokens=TOKENS_FILE.txt``. 243 Three implicit tokens are added: ``" "``, ``"\t"``, and ``"\n"``. 244 The fuzzer itself will still be mutating a string of bytes 245 but before passing this input to the target library it will replace every byte ``b`` with the ``b``-th token. 246 If there are less than ``b`` tokens, a space will be added instead. 247 248 AFL compatibility 249 ----------------- 250 LibFuzzer can be used in parallel with AFL_ on the same test corpus. 251 Both fuzzers expect the test corpus to reside in a directory, one file per input. 252 You can run both fuzzers on the same corpus in parallel:: 253 254 ./afl-fuzz -i testcase_dir -o findings_dir /path/to/program -r @@ 255 ./llvm-fuzz testcase_dir findings_dir # Will write new tests to testcase_dir 256 257 Periodically restart both fuzzers so that they can use each other's findings. 258 259 How good is my fuzzer? 260 ---------------------- 261 262 Once you implement your target function ``TestOneInput`` and fuzz it to death, 263 you will want to know whether the function or the corpus can be improved further. 264 One easy to use metric is, of course, code coverage. 265 You can get the coverage for your corpus like this:: 266 267 ASAN_OPTIONS=coverage_pcs=1 ./fuzzer CORPUS_DIR -runs=0 268 269 This will run all the tests in the CORPUS_DIR but will not generate any new tests 270 and dump covered PCs to disk before exiting. 271 Then you can subtract the set of covered PCs from the set of all instrumented PCs in the binary, 272 see SanitizerCoverage_ for details. 273 274 Fuzzing components of LLVM 275 ========================== 276 277 clang-format-fuzzer 278 ------------------- 279 The inputs are random pieces of C++-like text. 280 281 Build (make sure to use fresh clang as the host compiler):: 282 283 cmake -GNinja -DCMAKE_C_COMPILER=clang -DCMAKE_CXX_COMPILER=clang++ -DLLVM_USE_SANITIZER=Address -DLLVM_USE_SANITIZE_COVERAGE=YES -DCMAKE_BUILD_TYPE=Release /path/to/llvm 284 ninja clang-format-fuzzer 285 mkdir CORPUS_DIR 286 ./bin/clang-format-fuzzer CORPUS_DIR 287 288 Optionally build other kinds of binaries (asan+Debug, msan, ubsan, etc). 289 290 TODO: commit the pre-fuzzed corpus to svn (?). 291 292 Tracking bug: https://llvm.org/bugs/show_bug.cgi?id=23052 293 294 clang-fuzzer 295 ------------ 296 297 The default behavior is very similar to ``clang-format-fuzzer``. 298 Clang can also be fuzzed with Tokens_ using ``-tokens=$LLVM/lib/Fuzzer/cxx_fuzzer_tokens.txt`` option. 299 300 Tracking bug: https://llvm.org/bugs/show_bug.cgi?id=23057 301 302 FAQ 303 ========================= 304 305 Q. Why Fuzzer does not use any of the LLVM support? 306 --------------------------------------------------- 307 308 There are two reasons. 309 310 First, we want this library to be used outside of the LLVM w/o users having to 311 build the rest of LLVM. This may sound unconvincing for many LLVM folks, 312 but in practice the need for building the whole LLVM frightens many potential 313 users -- and we want more users to use this code. 314 315 Second, there is a subtle technical reason not to rely on the rest of LLVM, or 316 any other large body of code (maybe not even STL). When coverage instrumentation 317 is enabled, it will also instrument the LLVM support code which will blow up the 318 coverage set of the process (since the fuzzer is in-process). In other words, by 319 using more external dependencies we will slow down the fuzzer while the main 320 reason for it to exist is extreme speed. 321 322 Q. What about Windows then? The Fuzzer contains code that does not build on Windows. 323 ------------------------------------------------------------------------------------ 324 325 The sanitizer coverage support does not work on Windows either as of 01/2015. 326 Once it's there, we'll need to re-implement OS-specific parts (I/O, signals). 327 328 Q. When this Fuzzer is not a good solution for a problem? 329 --------------------------------------------------------- 330 331 * If the test inputs are validated by the target library and the validator 332 asserts/crashes on invalid inputs, the in-process fuzzer is not applicable 333 (we could use fork() w/o exec, but it comes with extra overhead). 334 * Bugs in the target library may accumulate w/o being detected. E.g. a memory 335 corruption that goes undetected at first and then leads to a crash while 336 testing another input. This is why it is highly recommended to run this 337 in-process fuzzer with all sanitizers to detect most bugs on the spot. 338 * It is harder to protect the in-process fuzzer from excessive memory 339 consumption and infinite loops in the target library (still possible). 340 * The target library should not have significant global state that is not 341 reset between the runs. 342 * Many interesting target libs are not designed in a way that supports 343 the in-process fuzzer interface (e.g. require a file path instead of a 344 byte array). 345 * If a single test run takes a considerable fraction of a second (or 346 more) the speed benefit from the in-process fuzzer is negligible. 347 * If the target library runs persistent threads (that outlive 348 execution of one test) the fuzzing results will be unreliable. 349 350 Q. So, what exactly this Fuzzer is good for? 351 -------------------------------------------- 352 353 This Fuzzer might be a good choice for testing libraries that have relatively 354 small inputs, each input takes < 1ms to run, and the library code is not expected 355 to crash on invalid inputs. 356 Examples: regular expression matchers, text or binary format parsers. 357 358 .. _pcre2: http://www.pcre.org/ 359 360 .. _AFL: http://lcamtuf.coredump.cx/afl/ 361 362 .. _SanitizerCoverage: https://code.google.com/p/address-sanitizer/wiki/AsanCoverage 363 364 .. _Heartbleed: http://en.wikipedia.org/wiki/Heartbleed 365
