linux-kernel - crash by cdc_acm driver in kernels 4.8-rc1/5

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Date:   Thu, 8 Sep 2016 13:58:03 +0200
From:   Wim Osterholt <wim@....tudelft.nl>
To:     oliver@...kum.org
Cc:     oneukum@...e.com, linux-usb@...r.kernel.org,
        linux-kernel@...r.kernel.org, Wim Osterholt <wim@....tudelft.nl>
Subject: crash by cdc_acm driver in kernels 4.8-rc1/5

L.S.

up to vanilla kernel 4.7.3 I've seen no problems.
On two different dekstops running vanilla kernel-4.8 I can force a crash by
inserting an USB-modem that requires module cdc_acm.ko .
(Strangely enough that doesn't happen on my laptop.)

The moment that cdc_acm loads (manually or automatically - the hardware needs
to be present) a kernel oops occurs.
The system remains responsive for a while, but a reboot is necessairy.
If you try a neat shutdown, the system will hang forever after 'remounting fs
read-only'. You'll need a power-down.

The oops tells things that I didn't all write down, but it says
null pointer dereference at 00000246
...
failed while handling devices/pci0000:00/0000:00:1d.3/usb7/7-1/7-1:1d  etc.
...
udevd .. is taking too long..

Could someone please explain and repair the magic that is happening here?

Thanks in advance, Wim Osterholt.

----- wim@....tudelft.nl -----