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Continuous Fuzzing for C/C++ Example

This is an example of how to integrate your libfuzzer targets with the Fuzzit Continuous Fuzzing Platform.

This example will show the following steps:

• Building and running a simple libFuzzer target locally
• Integrate the libFuzzer target with Fuzzit via Travis-CI

Result:

• Fuzzit will run the fuzz targets continuously on a daily basis with the latest release.
• Fuzzit will run regression tests on every pull-request with the generated corpus and crashes to catch bugs early on.

Fuzzing for C/C++ can help find both complex bugs, as well as critical security bugs as both c/c++ are unsafe languages.

This tutorial focuses less on how to build libFuzzer targets and more on how to integrate the targets with Fuzzit. A lot of great information is available at the libFuzzer tutorial.

Building libFuzzer Target

Prerequisite

This tutorial was tested on Ubuntu 18, though it should work on any Unix environment.

The required packages are cmake and clang > 6.0

apt update && apt install -y git clang cmake 

Getting the code

git clone https://github.com/fuzzitdev/example-c

Compiling

# you might need to export CXX=<path_to_clang++> CC=<path_to_clang>

cd example-c
mkdir build && cd build
export CC=`which clang`
export CXX=`which clang++`
cmake .. -DCMAKE_BUILD_TYPE=AddressSanitizer
make

Understanding the code

This code consist of a library and a command line program.

The code is a dummy code (just for the sake of the example) that trigger an heap out-of-bound access when it is passed the correct "secret" flag.

int test_string(const char *src, size_t len)
{
    if (len < strlen(FLAG)) {
        return 0;
    } else {
        if (strncmp(src, FLAG, strlen(FLAG)) == 0) {
            // TRIGGER HEAP OVERFLOW READ
            if (src[len] == 0) {
                return -2;
            }
            return -1;
        } else {
            return 0;
        }
    }

}

when the secret flag is passed, the code accesses an off-by-one in the src array.

Fuzzing

we will use the libFuzzer binary located under fuzz/ to find the "secret" flag and trigger the bug.

./fuzz/fuzz_test_string -exact_artifact_path=crash

Because it's a very simple code libFuzzer takes under 1 second (Although even in complex code, libFuzzer can sometimes find complicated bugs quickly, such as Heartbleed).

The output should look something like this:

INFO: Seed: 432631686
INFO: Loaded 1 modules   (1 inline 8-bit counters): 1 [0x76bf40, 0x76bf41), 
INFO: Loaded 1 PC tables (1 PCs): 1 [0x545248,0x545258), 
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: 1 ft: 2 corp: 1/1b lim: 4 exec/s: 0 rss: 26Mb
=================================================================
==15057==ERROR: AddressSanitizer: heap-buffer-overflow on address 0x603000009da4 at pc 0x0000005299ba bp 0x7fffddf93d80 sp 0x7fffddf93d78
READ of size 1 at 0x603000009da4 thread T0
    #0 0x5299b9 in test_string(char const*, unsigned long) /home/jeka/fuzzitdev/continuous-fuzzing-example/src/test_string.cpp:15:17
    #1 0x5298f0 in LLVMFuzzerTestOneInput /home/jeka/fuzzitdev/continuous-fuzzing-example/fuzz/fuzz_test_string.cpp:13:5
    #2 0x42ef2a in fuzzer::Fuzzer::ExecuteCallback(unsigned char const*, unsigned long) (/home/jeka/fuzzitdev/continuous-fuzzing-example/build/fuzz/fuzz_test_string+0x42ef2a)
    #3 0x42e4d5 in fuzzer::Fuzzer::RunOne(unsigned char const*, unsigned long, bool, fuzzer::InputInfo*, bool*) (/home/jeka/fuzzitdev/continuous-fuzzing-example/build/fuzz/fuzz_test_string+0x42e4d5)
    #4 0x430a4e in fuzzer::Fuzzer::MutateAndTestOne() (/home/jeka/fuzzitdev/continuous-fuzzing-example/build/fuzz/fuzz_test_string+0x430a4e)
    #5 0x431595 in fuzzer::Fuzzer::Loop(std::vector<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >, fuzzer::fuzzer_allocator<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > > > const&) (/home/jeka/fuzzitdev/continuous-fuzzing-example/build/fuzz/fuzz_test_string+0x431595)
    #6 0x424c53 in fuzzer::FuzzerDriver(int*, char***, int (*)(unsigned char const*, unsigned long)) (/home/jeka/fuzzitdev/continuous-fuzzing-example/build/fuzz/fuzz_test_string+0x424c53)
    #7 0x44d472 in main (/home/jeka/fuzzitdev/continuous-fuzzing-example/build/fuzz/fuzz_test_string+0x44d472)
    #8 0x7fb0633e282f in __libc_start_main /build/glibc-LK5gWL/glibc-2.23/csu/../csu/libc-start.c:291
    #9 0x41d9c8 in _start (/home/jeka/fuzzitdev/continuous-fuzzing-example/build/fuzz/fuzz_test_string+0x41d9c8)

0x603000009da4 is located 0 bytes to the right of 20-byte region [0x603000009d90,0x603000009da4)
allocated by thread T0 here:
    #0 0x526b52 in operator new[](unsigned long) (/home/jeka/fuzzitdev/continuous-fuzzing-example/build/fuzz/fuzz_test_string+0x526b52)
    #1 0x42ee11 in fuzzer::Fuzzer::ExecuteCallback(unsigned char const*, unsigned long) (/home/jeka/fuzzitdev/continuous-fuzzing-example/build/fuzz/fuzz_test_string+0x42ee11)
    #2 0x42e4d5 in fuzzer::Fuzzer::RunOne(unsigned char const*, unsigned long, bool, fuzzer::InputInfo*, bool*) (/home/jeka/fuzzitdev/continuous-fuzzing-example/build/fuzz/fuzz_test_string+0x42e4d5)
    #3 0x430a4e in fuzzer::Fuzzer::MutateAndTestOne() (/home/jeka/fuzzitdev/continuous-fuzzing-example/build/fuzz/fuzz_test_string+0x430a4e)
    #4 0x431595 in fuzzer::Fuzzer::Loop(std::vector<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >, fuzzer::fuzzer_allocator<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > > > const&) (/home/jeka/fuzzitdev/continuous-fuzzing-example/build/fuzz/fuzz_test_string+0x431595)
    #5 0x424c53 in fuzzer::FuzzerDriver(int*, char***, int (*)(unsigned char const*, unsigned long)) (/home/jeka/fuzzitdev/continuous-fuzzing-example/build/fuzz/fuzz_test_string+0x424c53)
    #6 0x44d472 in main (/home/jeka/fuzzitdev/continuous-fuzzing-example/build/fuzz/fuzz_test_string+0x44d472)
    #7 0x7fb0633e282f in __libc_start_main /build/glibc-LK5gWL/glibc-2.23/csu/../csu/libc-start.c:291

SUMMARY: AddressSanitizer: heap-buffer-overflow /home/jeka/fuzzitdev/continuous-fuzzing-example/src/test_string.cpp:15:17 in test_string(char const*, unsigned long)
Shadow bytes around the buggy address:
  0x0c067fff9360: fd fa fa fa fd fd fd fa fa fa fd fd fd fa fa fa
  0x0c067fff9370: fd fd fd fa fa fa fd fd fd fa fa fa fd fd fd fa
  0x0c067fff9380: fa fa fd fd fd fa fa fa fd fd fd fa fa fa fd fd
  0x0c067fff9390: fd fa fa fa fd fd fd fa fa fa fd fd fd fa fa fa
  0x0c067fff93a0: fd fd fd fa fa fa fd fd fd fa fa fa fd fd fd fa
=>0x0c067fff93b0: fa fa 00 00[04]fa fa fa fa fa fa fa fa fa fa fa
  0x0c067fff93c0: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
  0x0c067fff93d0: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
  0x0c067fff93e0: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
  0x0c067fff93f0: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
  0x0c067fff9400: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
Shadow byte legend (one shadow byte represents 8 application bytes):
  Addressable:           00
  Partially addressable: 01 02 03 04 05 06 07 
  Heap left redzone:       fa
  Freed heap region:       fd
  Stack left redzone:      f1
  Stack mid redzone:       f2
  Stack right redzone:     f3
  Stack after return:      f5
  Stack use after scope:   f8
  Global redzone:          f9
  Global init order:       f6
  Poisoned by user:        f7
  Container overflow:      fc
  Array cookie:            ac
  Intra object redzone:    bb
  ASan internal:           fe
  Left alloca redzone:     ca
  Right alloca redzone:    cb
  Shadow gap:              cc
==15057==ABORTING
MS: 2 InsertRepeatedBytes-CMP- DE: "FUZZING IS IMPORTANT"-; base unit: adc83b19e793491b1c6ea0fd8b46cd9f32e592fc
0x46,0x55,0x5a,0x5a,0x49,0x4e,0x47,0x20,0x49,0x53,0x20,0x49,0x4d,0x50,0x4f,0x52,0x54,0x41,0x4e,0x54,
FUZZING IS IMPORTANT
artifact_prefix='./'; Test unit written to crash
Base64: RlVaWklORyBJUyBJTVBPUlRBTlQ=

We can see clearly the heap-buffer-overflow READ 1 byte memory bug and the exact line that triggers it. We also see the data that triggered the bug, which is FUZZING IS IMPORTANT.

The data is also saved to ./crash and we can double test it with the main command line

cat ./crash
# FUZZING IS IMPORTANT
./src/test_string "FUZZING IS IMPORTANT"
# you got it:)

Integrating with Fuzzit from CI

The best way to integrate with Fuzzit is by adding a two stages in your Continuous Build system (like Travis CI or Circle CI).

Fuzzing stage:

• Build a fuzzing target
• Download fuzzit cli
• Authenticate via passing FUZZIT_API_KEY environment variable
• Create a fuzzing job by uploading the fuzzing target

Regression stage

• Build a fuzzing target
• Download fuzzit cli
• Authenticate via passing FUZZIT_API_KEY environment variable OR defining the corpus as public. This way No authentication would be require and regression can be used for forked PRs as well
• Create a local regression fuzzing job - This will pull all the generated corpuses and run them through the fuzzing binary. If new bugs are introduced this will fail the CI and alert

Here is the relevant snippet from the fuzzit.sh which is being run by .travis.yml

## Install fuzzit latest version:
wget -q -O fuzzit https://github.com/fuzzitdev/fuzzit/releases/latest/download/fuzzit_Linux_x86_64
chmod a+x fuzzit

## upload fuzz target for long fuzz testing on fuzzit.dev server or run locally for regression
./fuzzit create job --type ${FUZZING_TYPE} fuzzitdev/fuzz-test-string ./fuzz/fuzz_test_string

In production it is advised to download a pinned version of the CLI like in the example. In development you can use the latest version: https://github.com/fuzzitdev/fuzzit/releases/latest/download/fuzzit_${OS}_${ARCH}. Valid values for ${OS} are: Linux, Darwin, Windows. Valid values for ${ARCH} are: x86_64 and i386.

More Examples

You can look in the following open-source examples integrating Fuzzit with different CIs and different build systems:

• systemd - travis, case-study.
• radare2 - travis
• pdfgen - circle