1# Fuzzer for libstagefright_amrwbdec decoder 2 3## Plugin Design Considerations 4The fuzzer plugin for AMR-WB is designed based on the understanding of the 5codec and tries to achieve the following: 6 7##### Maximize code coverage 8The configuration parameters are not hardcoded, but instead selected based on 9incoming data. This ensures more code paths are reached by the fuzzer. 10 11AMR-WB supports the following parameters: 121. Quality (parameter name: `quality`) 132. Mode (parameter name: `mode`) 14 15| Parameter| Valid Values| Configured Value| 16|------------- |-------------| ----- | 17| `quality` | 0. `Bad Quality` 1. `Good quality` | Bit 0 (LSB) of 1st byte of data. | 18| `mode` | 0. `MODE_7k` 1. `MODE_9k` 2. `MODE_12k` 3. `MODE_14k` 4. `MODE_16k ` 5. `MODE_18k` 6. `MODE_20k` 7. `MODE_23k` 8. `MODE_24k` 9. `MRDTX` | Bits 3, 4, 5 and 6 of 1st byte of data. | 19 20This also ensures that the plugin is always deterministic for any given input. 21 22##### Maximize utilization of input data 23The plugin feeds the entire input data to the codec using a loop. 24If the decode operation was successful, the input is advanced by the frame size 25which is based on `mode` and `quality` selected. 26If the decode operation was un-successful, the input is still advanced by frame size so 27that the fuzzer can proceed to feed the next frame. 28 29This ensures that the plugin tolerates any kind of input (empty, huge, 30malformed, etc) and doesnt `exit()` on any input and thereby increasing the 31chance of identifying vulnerabilities. 32 33## Build 34 35This describes steps to build amrwb_dec_fuzzer binary. 36 37### Android 38 39#### Steps to build 40Build the fuzzer 41``` 42 $ mm -j$(nproc) amrwb_dec_fuzzer 43``` 44 45#### Steps to run 46Create a directory CORPUS_DIR and copy some amrwb files to that folder 47Push this directory to device. 48 49To run on device 50``` 51 $ adb sync data 52 $ adb shell /data/fuzz/arm64/amrwb_dec_fuzzer/amrwb_dec_fuzzer CORPUS_DIR 53``` 54To run on host 55``` 56 $ $ANDROID_HOST_OUT/fuzz/x86_64/amrwb_dec_fuzzer/amrwb_dec_fuzzer CORPUS_DIR 57``` 58 59## References: 60 * http://llvm.org/docs/LibFuzzer.html 61 * https://github.com/google/oss-fuzz 62