Fuzzing the Kernel using AFL Unicorn
- Install Python
- Clone afl-unicorn and follow installation instructions
pip install -r requirements.txt
Debug Kernel Setup (Skip this if you know how this works)
- Create a qemu-img and install your preferred OS on there through qemu
- An easy way to get a working userspace up and running in QEMU is to follow the steps described by syzkaller, namely create-image.sh
- For kernel customization you might want to clone your preferred kernel version and compile it on the host. This way you can also compile your own kernel modules (e.g. example_module).
- In order to find out the address of a loaded module in the guest OS you can use
cat /proc/modulesto find out the base address of the module location. Use this as the offset for the function where you want to break. If you specify MODULE and BREAKOFFSET in the
config.py, it should use
./get_mod_addr.shto start it automated.
- You can compile the kernel with debug info. When you have compiled the linux kernel you can start gdb from the kernel folder with
gdb vmlinux. After having loaded other modules you can use the
lx-symbolscommand in gdb to load the symbols for the other modules (make sure the .ko files of the modules are in your kernel folder). This way you can just use something like
break function_to_breakto set breakpoints for the required functions.
- In order to compile a custom kernel for Arch, download the current Arch kernel and set the .config to the Arch default. Then set
KASAN_EXTRA=y. For convenience, we added a working example_config that can be place to the linux dir.
- To only get necessary kernel modules boot the current system and execute
lsmod > mylsmodand copy the mylsmod file to your host system into the linux kernel folder that you downloaded. Then you can use
make LSMOD=mylsmod localmodconfigto only make the kernel modules that are actually needed by the guest system. Then you can compile the kernel like normal with
make. Then mount the guest file system to
make modules_install INSTALL_MOD_PATH=/mnt. At last you have to create a new initramfs, which apparently has to be done on the guest system. Here use
mkinitcpio -k <folder in /lib/modules/...> -g <where to put initramfs>. Then you just need to copy that back to the host and let qemu know where your kernel and the initramfs are located.
- Setting breakpoints anywhere else is possible. For this, set
- ensure a target gdbserver is reachable, for example via
- provide the target's gdbserver network address in the config to the probe wrapper
- provide the target's target function to the probe wrapper and harness
- make the harness put AFL's input to the desired memory location by adopting the
- add all EXITs
./probe_wrapper.py, it will (try to) connect to gdb.
- make the target execute the target function
Putting afl's input to the correct location must be coded invididually for most targets. However with modern binary analysis frameworks like IDA or Ghidra it's possible to find the desired location's address.
place_input method places at the data section of
# read input into param xyz here: rdx = uc.reg_read(UC_X86_REG_RDX) util.map_page_blocking(uc, rdx) # ensure sk_buf is mapped bufferPtr = struct.unpack("<Q",uc.mem_read(rdx + 0xd8, 8)) util.map_page_blocking(uc, bufferPtr) # ensure the buffer is mapped uc.mem_write(rdx, input) # insert afl input uc.mem_write(rdx + 0xc4, b"\xdc\x05") # fix tail
A few things to consider.
FS_BASE and GS_BASE
Unicorn does not offer a way to directly set model specific registers. Since kernels use them to store pointers to process- or cpu-specific memory, Unicorefuzz makes unicorn execute
wrmsr to set the
gs registers to the appropriate values.
To do this, you'll have to provide a non-used
SCRATCH_ADDR in the
Improve Fuzzing Speed
Right now, the Unicorefuzz
probe_wrapper.py needs to be manually restarted after an amount of pages has been allocated. Allocated pages don't propagate back to the forkserver parent automatically and would need be reloaded from disk for each iteration.
Unicorn does not handle CMPXCHNG16b correctly.
Until that is addressed, we needed to hook each cmpxchng16b with a python reimplementation.
This is super hacky and should only be used if really needed.
Instead, try to start fuzzing after the instruction or do anything else.
If you do need it, add all occurrences you want to replace to the