External USB fuzzing for Linux kernel

This is in a prototype stage, there's still a lot of things to fix and implement.

syzkaller has support for external fuzzing of the USB stack. Initially it was based on gadgetfs, but later switched to another gadgetfs-like driver written from scratch. It allows to connect virtual USB devices from a userspace application as if they are being plugged into a USB port.

This allowed to find over 80 bugs in the Linux kernel USB stack so far.

How to set this up:

  1. Checkout upstream Linux kernel (all the patches linked below are based on 4.15-rc3).

  2. Apply two patches to the kernel: 1 and 2.

    Patch #1 allows to synchonously proccess events on a USB hub. This is required by syzkaller for proper coverage collection, since kcov can only collect coverage of the current thread. Patch #2 add another kernel interface (/sys/kernel/debug/usb-fuzzer) for emulating USB devices.

  3. Configure and build the kernel. You need to enable CONFIG_USB_FUZZER=y and CONFIG_USB_DUMMY_HCD=y:

    menu config -> Device Drivers -> USB Support ->
      -> USB Gadget Support (enable) -> 
        -> USB Peripheral Controller -> Dummy HCD (enable)
        -> USB Gadget Fuzzer (enable)

    This is the config I used for testing the upstream kernel.

  4. Optionally update syzkaller descriptions by extracting USB device info by using the instructions below.

  5. Checkout syzkaller usb-fuzzer branch, build syzkaller.

  6. Enable syz_usb_connect and syz_usb_disconnect syscalls in the manager config.

  7. Set sandbox to none and procs to 1 in the manager config.

  8. Run.

Syzkaller descriptions for USB fuzzing can be found here: 1 and 2.

Extracting USB device ids

Part of the syzkaller descriptions for USB fuzzing are generated based on the drivers enabled in a particular kernel with a particular kernel config. These descritions help syzkaller to discover enabled USB drivers. The current descriptions are generated for upstream kernel (4.15-rc3) using this config. If you wish to generate descriptions for your kernel with your config follow the instructions below.

  1. Apply this patch on top of the two described above.

    When a new device is being probed the kernel goes over a list of all USB device drivers it has loaded and tries to find the matching one. This patch makes kernel print info about all USB devices it tries to the kernel log.

  2. Compile and run this program.

    This program connects a virtual USB device, that doesn't have a matching driver. As a result the kernel will print info about all device drivers it had.

  3. Collect USB device info:

    cat kernel.log | grep -E '] [0-9a-f]{48}' | cut -c 16- | sort | uniq > usb_ids_dump

    For reference, the resulting usb_ids_dump for upstream kernel can be found here.

  4. Generate syzkaller descriptions based on the USB devices info using this script:

    ./usb_ids_parse.py ./usb_ids_dump > /syzkaller/sys/linux/vusb_ids.txt
  5. Run make generate.

  6. Rebuild and run syzkaller.