[snap] Chromium can enumerate USB devices without the raw-usb plug

[Alistair Buxton]

Version:

chromium 87.0.4280.66 1411

Steps to reproduce:

1. Have Chromium snap, with raw-usb plug disconnected.
2. Open Chromium and try to use a webpage that has webusb.

Result:

USB.getDevices() and USB.requestDevice() will succeed but USBDevice.open() will fail, emitting "DOMException: Permission Denied" on the browser console. Or, put another way, you can select a USB device that you want the webpage to use, but it is impossible for the webpage to actually use it.

Why this happens:

Chromium enumerates USB devices by querying udev. It does not require elevated permissions beyond the snap defaults in order to do this. So it can retrieve a list of USB devices even without raw-usb.

Impact:

It is impossible to access the USB devices until the raw-usb plug is connected. However, to a normal user, it appears that Chromium already has USB permissions because it can enumerate devices. The webusb API is in a confusing, partially-working state, and there is nothing to indicate that Chromium needs additional permissions in order for it to fully work.