Objective
Learn fuzzing methodology and discover memory corruption vulnerabilities
Lab Overview
In this bonus lab, you'll learn how to fuzz a complex application (a GameBoy emulator) and discover real memory corruption vulnerabilities. This lab is based on research that found 11 unique crashes in the VBA-M emulator.
What You'll Learn
- Building fuzzing harnesses for complex applications
- Using AFL++ with LLVM mode and persistent fuzzing
- Analyzing memory corruption vulnerabilities with AddressSanitizer
- Understanding emulator attack surfaces
- Triaging and exploiting buffer overflows
Target Analysis: VBA-M Emulator
VBA-M is a fork of VisualBoyAdvance that emulates GameBoy and GameBoy Advance systems. It has a large attack surface due to:
Attack Surface Components
- ROM Parsing: GameBoy/GBA ROM file format parsing
- Save States: Game save file handling
- Cheating Support: Built-in cheat code functionality
- Audio/Video I/O: Complex multimedia processing
- Memory Mappers: Special cartridge hardware emulation
Target Selection Rationale
We'll focus on GameBoy ROM files because:
- ROMs are the primary input format
- Complex parsing logic with multiple code paths
- Memory mappers introduce additional complexity
- Historical vulnerabilities in similar parsers
Lab Setup Instructions
Step 1: Environment Preparation
Your instance should have the following pre-installed:
- AFL++ with LLVM support
- VBA-M source code
- AddressSanitizer (ASAN)
- GDB with debugging symbols
- Sample GameBoy ROM files
Step 2: Building the Fuzzing Harness
The key to effective fuzzing is creating a minimal harness that focuses on the target functionality while removing unnecessary components. Based on the provided patch, the following modifications transform VBA-M into an efficient fuzzing target:
- CMake Integration: Configure AFL++ compilers directly in the build system
- AFL Initialization: Set up proper AFL fuzzing infrastructure
- Persistent Mode: Implement AFL's persistent fuzzing for performance
- Frame Limiting: Add execution bounds to prevent infinite loops
- Feature Removal: Disable GUI, rendering, and non-essential components
- Target Focus: Concentrate on GameBoy ROM parsing only
Configure AFL++ compilers and enable SDL port while disabling wxWidgets.
Step 3: AFL Integration and Persistent Mode Setup
The main function must be modified to support AFL's persistent fuzzing mode, which significantly improves performance by reusing the same process for multiple test cases. This implementation also removes unnecessary initialization routines that slow down fuzzing.
Key Changes
- Initialize AFL fuzzing infrastructure with __AFL_FUZZ_INIT()
- Wrap main logic in __AFL_LOOP() for persistent mode
- Remove input mapping initialization (not needed for fuzzing)
- Maintain essential emulator initialization
Fuzzing Process
This section covers the execution workflow for the fuzzing campaign.
Step 4: Input Corpus Creation
Create minimal test cases:
- Use GBStudio to create minimal GameBoy ROMs
- Manually minimize with hex editor
- Verify ROMs still load correctly
- Place in AFL input directory
You can also get any ROM...
Step 5: Frame Counter Implementation
To prevent infinite execution and ensure the fuzzer can process multiple test cases efficiently, a frame counter is added to the main emulation loop. This counter limits each test case to a fixed number of emulation cycles.
Implementation Details
- Counter set to 1000 frames per test case
- Debug output shows remaining frames
- Clean exit when counter reaches zero
- Placed in gbEmulate() function for GameBoy emulation
Step 6: Rendering Function Disabling
Graphics and text rendering functions are significant performance bottlenecks during fuzzing. These functions are disabled by adding early return statements, preserving the original code for potential restoration while eliminating rendering overhead.
Functions Modified
- gbSgbRenderBorder(): Super GameBoy border rendering
- systemDrawScreen(): Main screen drawing function
- drawText(): Text overlay rendering
Step 7: Target Focus - GameBoy Only
To maximize fuzzing effectiveness, this implementation focuses exclusively on GameBoy ROM parsing by removing GameBoy Advance support. This reduces code complexity and increases coverage depth on the target functionality.
Rationale
- GameBoy ROMs have simpler format and parsing logic
- Concentrating on one target increases bug discovery rates
- Removes GBA-specific initialization and memory allocation
- Simplifies state management between fuzzing iterations