A small fuzzer written to test motherboards / system BIOS for MBR-based hosts

This fuzzer will test Port IO using the x86 in and out instructions. It uses a BIOS Service Timer to generate random word values and then supply those random word values as operands to the in and out instructions.

There are also testing modes for BIOS Services provided by int 0x13 - specifically disk read/write.

If you are able to run this on actual hardware, please open an issue and let us know your system specs and your results.

You can assemble bootfuzz.asm with NASM thusly:

nasm -f bin -o bootfuzz.img bootfuzz.asm

Video of Bootfuzz running on physical hardware

After you have assembled the fuzzer into bootfuzz.img, you can run the fuzzer in qemu thusly:

qemu-system-i386 -fda bootfuzz.img -nographic -accel kvm

In VirtualBox, you will need to add a "Floppy" controller in the VM settings and then add a floppy drive. After the floppy drive is created, you can point it at bootfuzz.img and then boot up.

You can also use the provided bootfuzz.img in this repository as a precompiled mbr for fuzzing.

I have seen repeatable crashes in QEMU, VirtualBox, and VMWare Workstation Player already, but I do not have the time or interest in triaging them. I'm more interested in getting it running on physical hardware.

Crashing Testcase 1: QEMU and VirtualBox

• QEMU emulator version 8.0.4 (Debian 1:8.0.4+dfsg-1ubuntu3.23.10.3)
• VirtualBox Version 7.0.14 r161095

org 0x7c00

start:
    mov dx, 0x03ff
    in ax, dx

times 510-($-$$) db 0
db 0x55, 0xaa 

Crashing Testcase 2: VMWare Player

• VMWare Workstation Player Version 17.5.1 build-23298084

org 0x7c00

start:
    mov dx, 0x04ba
    mov ax, 0x00a8
    out dx, ax

times 510-($-$$) db 0
db 0x55, 0xaa 

Crashing Testcase 3: VMWare Player

• VMWare Workstation Player Version 17.5.1 build-23298084

org 0x7c00

start:
    mov ax, 0x163e
    mov dx, 0x043e
    in ax, dx

times 510-($-$$) db 0
db 0x55, 0xaa 

If you find bugs using this fuzzer I would appreciate a shout out or a link back to this project.

Here is a list of instructions that will cause problems at least with QEMU:

mov ax, 0x9090 (Any 16bit word)
mov dx, 0x0cfe ; 0x03fb
out dx, ax