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Date:   Fri, 19 Apr 2019 14:36:38 -0400 (EDT)
From:   Alan Stern <>
To:     Andrey Konovalov <>,
        Felipe Balbi <>
cc:     Greg Kroah-Hartman <>,
        "Gustavo A. R. Silva" <>,
        LKML <>,
        syzkaller-bugs <>,
        USB list <>
Subject: UDC hardware for fuzzing [was: Re: INFO: task hung in usb_kill_urb]

On Wed, 17 Apr 2019, Andrey Konovalov wrote:

> On Tue, Apr 16, 2019 at 8:25 PM Alan Stern <> wrote:
> >
> > On Tue, 16 Apr 2019, syzbot wrote:
> >
> > > Hello,
> > >
> > > syzbot has tested the proposed patch but the reproducer still triggered
> > > crash:
> > > INFO: task hung in usb_kill_urb
> >
> > Okay, I think I found the problem.  dummy-hcd doesn't check for
> > unsupported speeds until it is too late.  Andrey, what values does your
> > usb-fuzzer gadget driver set for its max_speed field?
> It's passed from userspace without any validation :( I'll fix this!
> Thanks for looking into it!
> I wonder why other people saw this hang as well, they didn't use the
> dummy hcd module for sure. I guess there are might be other reasons.

Unquestionably it would be for other reasons.  usb_kill_urb() is a 
host-side routine, not used by gadget drivers.  If it fails, the reason 
lies in host controller driver.  And if people aren't using dummy-hcd 
then they must be using a different host controller driver.

Is there any chance you could get hold of a USB device controller for 
more fuzzing tests?  With it, you could test other parts of the USB 
stack: the UDC driver for whatever hardware you get, and the host 
controller driver for whatever you plug the UDC into.

I don't know what types of UDC are readily available for the type of
computer syzkaller uses.  Perhaps Felipe or other people on the mailing
list will have some suggestions.

Alan Stern

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