Fuzzer for libgsm decoder

Plugin Design Considerations

The fuzzer plugin for GSM is designed based on the understanding of the codec and tries to achieve the following:

Maximize code coverage

The configuration parameters are not hardcoded, but instead selected based on incoming data. This ensures more code paths are reached by the fuzzer.

GSM supports the following options:

1. Verbosity (parameter name: GSM_VERBOSE)
2. Fastness (parameter name: GSM_OPT_FAST)

This also ensures that the plugin is always deterministic for any given input.

Maximize utilization of input data

The plugin feeds the entire input data to the codec using a loop.

• If the size of media file is less than 65 bytes, (65 - size) bytes are padded and then sent to the decoder so that decode happens for all the input files, regardless of their size. GSM decoder consumes alternating frames of 33 and 32 bytes, so 65 bytes of input are needed.
• If the decode operation was successful, the input is advanced by the frame size
• If the decode operation was un-successful, the input is still advanced by frame size so that the fuzzer can proceed to feed the next frame.

This ensures that the plugin tolerates any kind of input (empty, huge, malformed, etc) and doesnt exit() on any input and thereby increasing the chance of identifying vulnerabilities.

Build

This describes steps to build gsm_dec_fuzzer binary.

Android

Steps to build

Build the fuzzer

  $ mm -j$(nproc) gsm_dec_fuzzer

Steps to run

Create a directory CORPUS_DIR and copy some gsm files to that folder Push this directory to device.

To run on device

  $ adb sync data
  $ adb shell /data/fuzz/arm64/gsm_dec_fuzzer/gsm_dec_fuzzer CORPUS_DIR

To run on host

  $ $ANDROID_HOST_OUT/fuzz/x86_64/gsm_dec_fuzzer/gsm_dec_fuzzer CORPUS_DIR

References:

• http://llvm.org/docs/LibFuzzer.html
• https://github.com/google/oss-fuzz