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Fuzzing the Kernel Using AFL-Unicorn
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README.md

Unicorefuzz

Fuzzing the Kernel using AFL Unicorn

Unicorefuzz Setup

  • 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/modules to 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.sh to 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-symbols command 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_break to 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 DEBUG_KERNEL=y, DEBUG_INFO=y, GDB_SCRIPTS=y (for convenience), KASAN=y, 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 > mylsmod and copy the mylsmod file to your host system into the linux kernel folder that you downloaded. Then you can use make LSMOD=mylsmod localmodconfig to 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 /mnt and use 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 BREAKADDR in the config.py instead.

Run

  • ensure a target gdbserver is reachable, for example via ./startvm.sh
  • adopt config.py:
  • 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 place_input func config.py
  • add all EXITs
  • start ./probe_wrapper.py, it will (try to) connect to gdb.
  • make the target execute the target function
  • run ./start_afl.py

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.

The following place_input method places at the data section of sk_buff in key_extract:

    # 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))[0]
    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

Gotchas

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 fs and gs registers to the appropriate values. To do this, you'll have to provide a non-used SCRATCH_ADDR in the config.py.

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.

CMPXCHNG16b

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 config.py

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