linux-kernel - Re: UBSAN: Undefined behaviour in linux-4.7.2/drivers/usb/core/devio.c:1713:25

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Message-ID: <Pine.LNX.4.44L0.1608221703460.6636-100000@iolanthe.rowland.org>
Date:   Mon, 22 Aug 2016 17:06:23 -0400 (EDT)
From:   Alan Stern <stern@...land.harvard.edu>
To:     Bjørn Mork <bjorn@...k.no>
cc:     Jiri Slaby <jslaby@...e.cz>, Vittorio Zecca <zeccav@...il.com>,
        <stable@...r.kernel.org>, USB list <linux-usb@...r.kernel.org>,
        Linux kernel mailing list <linux-kernel@...r.kernel.org>
Subject: Re: UBSAN: Undefined behaviour in linux-4.7.2/drivers/usb/core/devio.c:1713:25

On Mon, 22 Aug 2016, Bjørn Mork wrote:

> Alan Stern <stern@...land.harvard.edu> writes:
> 
> > On Mon, 22 Aug 2016, Bjørn Mork wrote:
> >
> >> Alan Stern <stern@...land.harvard.edu> writes:
> >> 
> >> > On Sun, 21 Aug 2016, Jiri Slaby wrote:
> >> >
> >> >> Cc: proper lists.
> >> >> 
> >> >> ep->desc.bInterval seems to be 0 here.
> >
> >> > As far as I can see, this isn't possible.  The usb_parse_endpoint()  
> >> > routine in drivers/usb/core/config.c is supposed to guarantee that
> >> > ep->desc.bInterval is never 0.
> >> 
> >> That is if it is an ISO endpoint, right?
> >
> > I can't tell; the bug report doesn't say.  However, ep->desc.bInterval 
> > is ignored for bulk and control endpoints, so it must be either 
> > isochronous or interrupt.
> 
> So what if the endpoint is not isochronous or interrupt here?

Oops.  You're right; for some reason I thought that code was executed 
only if the endpoint type was known to be isochronous or interrupt.

> > USB uses two different encodings for endpoint intervals.  The second
> > encoding above just gives the interval in frames; this is used for low-
> > and full-speed interrupt endpoints.  The first encoding above is
> > exponential (it gives n where the actual interval is 2^(n-1) frames or
> > microframes); this is used for all isochronous endpoints and for
> > high-speed (or SuperSpeed etc.) interrupt endpoints.
> 
> OK, I am still puzzled: Won't the code I quoted above do the shift for
> *any* uurb->type and endpoint type?  There doesn't seem to be any test
> for isochronous or interrupt endpoint around it?

Yes, that's the reason for the UBSAN report.  I'll write a patch to fix 
it.

Alan Stern