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

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Date:   Mon, 22 Aug 2016 22:57:51 +0200
From:   Bjørn Mork <bjorn@...k.no>
To:     Alan Stern <stern@...land.harvard.edu>
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

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?

>> Maybe I misunderstand something fundamental, but the "||" strikes me as
>> odd here:
>> 
>>         as->urb->stream_id = stream_id;
>>         if (uurb->type == USBDEVFS_URB_TYPE_ISO ||
>>                         ps->dev->speed == USB_SPEED_HIGH)
>>                 as->urb->interval = 1 << min(15, ep->desc.bInterval - 1);
>>         else
>>                 as->urb->interval = ep->desc.bInterval;
>>         as->urb->context = as;
>
> No, that's right (mostly -- we really should check for ps->dev->speed 
>>= USB_SPEED_SUPER as well as == USB_SPEED_HIGH).
>
>> Typo?
>
> 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?


Bjørn