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Date:   Thu, 24 Oct 2019 15:17:18 -0400 (EDT)
From:   Alan Stern <stern@rowland.harvard.edu>
To:     Andrey Konovalov <andreyknvl@google.com>
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        "Jacky . Cao @ sony . com" <Jacky.Cao@sony.com>,
        Felipe Balbi <balbi@kernel.org>,
        Chunfeng Yun <chunfeng.yun@mediatek.com>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        LKML <linux-kernel@vger.kernel.org>,
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Subject: Re: divide error in dummy_timer
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On Thu, 24 Oct 2019, Andrey Konovalov wrote:

> On Thu, Oct 24, 2019 at 7:57 PM Alan Stern <stern@rowland.harvard.edu> wrote:
> >
> > On Thu, 24 Oct 2019, Andrey Konovalov wrote:
> >
> > > > Is this really the sort of thing we need to catch?  It isn't a bug in
> > > > any existing kernel code, as far as I know.  Maybe only gadgetfs and
> > > > configfs need to worry about it.
> > >
> > > Hi Alan,
> > >
> > > Do you mean that the gadget driver must ensure that the max packet
> > > size in the endpoint descriptor is not zero? Do HCDs rely on that? I
> > > can add this check into the driver we use for USB fuzzing.
> >
> > Well, if there are any gadget drivers in the kernel which do set an
> > endpoint's maxpacket size to 0, they should be fixed.  I'm not aware of
> > any.
> >
> > Of course, gadget drivers in userspace are always suspect.  That's why
> > I suggested having gadgetfs and configfs perform this check.  Even so
> > it's not really a _security_ risk, because only the superuser is
> > allowed to run a userspace gadget driver.  (Although obviously it is
> > better to have a clean failure than to crash the system when a buggy
> > program runs with superuser privileges.)
> >
> > Yes, HCDs do depend on endpoints having reasonable maxpacket values.  I
> > suppose the core should check for this.  Currently we check for values
> > that are too large or invalid in other ways (like high-speed bulk
> > endpoints with maxpacket != 512), but we don't check for 0.
> 
> Oh, I think I've confused the terms here. I meant to ask about UDCs.
> The question is whether it's OK to try and emulate a gadget with
> maxpacket = 0 on a board with a hardware UDC? Or can it cause issues?
> The fact that HCDs must ensure correct maxpacket values of course
> makes sense.

It doesn't make any sense to have an endpoint with maxpacket = 0 --
either real or emulated.  The USB spec doesn't prohibit them (probably
an oversight), but such endpoints would be useless since it would not
be possible to transfer any data to/from them.

And as you surmised, it wouldn't be at all surprising for UDC drivers
to crash (much like dummy-hcd does) when faced with an endpoint having
maxpacket = 0.  Best to rule out the possibility entirely.

Alan Stern