docs: add USB fuzzing docs

docs/linux/external_fuzzing_usb.md

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+External USB fuzzing for Linux kernel
+=====================================
+
+# USB fuzzing with syzkaller
+
+This page describes the current state of external USB fuzzing support in syzkaller.
+Note, that it's still in development and things might change.
+
+This allowed to find over [80 bugs](/docs/linux/found_bugs_usb.md) in the Linux kernel USB stack so far.
+
+How to set this up:
+
+1. Checkout the `usb-fuzzer` branch from https://github.com/google/kasan
+
+2. Configure and build the kernel. You need to enable `CONFIG_USB_FUZZER=y`, `CONFIG_USB_DUMMY_HCD=y` and all the USB drivers you're interested in fuzzing:
+
+   ```
+   menu config -> Device Drivers -> USB Support ->
+     -> USB Gadget Support (enable) ->
+       -> USB Peripheral Controller -> Dummy HCD (enable)
+       -> USB Gadget Fuzzer (enable)
+   ```
+
+3. Update syzkaller descriptions by extracting USB device info using the instructions below.
+
+4. Enable `syz_usb_connect`, `syz_usb_disconnect`, `syz_usb_control_io` and `syz_usb_ep_write` syscalls in the manager config.
+
+5. Set `sandbox` to `none` in the manager config.
+
+6. Pass `dummy_hcd.num=8` to the kernel command line in the maganer config.
+
+7. Run.
+
+Syzkaller descriptions for USB fuzzing can be found here: [1](/sys/linux/vusb.txt), [2](/sys/linux/init_vusb.go) and [3](/sys/linux/init_vusb_ids.go).
+
+
+## Updating syzkaller USB descriptions
+
+1. Apply [this](/tools/syz-usbgen/usb_ids.patch) kernel patch.
+
+2. Build and boot the kernel.
+
+3. Connect some USB device to it (e.g. with `syz-exeprog usb.log`, where `usb.log` is some program that utilizes the `syz_usb_connect` syzcall).
+
+4. Use [syz-usbgen](/tools/syz-usbgen/usbgen.go) script to update [syzkaller descriptions](/sys/linux/init_vusb_ids.go).