Re: [PATCH] usb: core: Null deref in kernel with USB webcams.

[John Boero]

Thanks Alan
I just spent some more time investigating and playing with different patches,
locks, mutexes, and sleeps, and I think I see exactly what's happening now.
I now understand why it:
A) seems to happen randomly during uvc start_stream
B) affects multiple device vendors
C) has been reported in RaspberryPi and IoT threads

I put a trace on usb/core/hub.c:usb_disconnect to identify why the device was
disconnecting and it seems this is a low power issue.  An idle webcam appears
fine to usbcore but as soon as you initialize it or uvc starts a stream, it
consumes more power, might find the cable can't supply it, and then disconnects
via interrupt. In my case I can reproduce this consistently with a cheap USB
extension cable, but this issue appears common to passive hubs, and IoT or SBCs
that don't always supply clean power over USB.  My simplified patch can at least
protect usbcore from crashing on a bad device: