linux-kernel - Re: [PATCH v4 3/4] usb: ffs: check quirk to pad epout buf size when not aligned to maxpacketsize

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Date:	Tue, 5 Nov 2013 13:24:56 -0500 (EST)
From:	Alan Stern <stern@...land.harvard.edu>
To:	David Cohen <david.a.cohen@...ux.intel.com>,
	Michal Nazarewicz <mina86@...a86.com>
cc:	Felipe Balbi <balbi@...com>, <gregkh@...uxfoundation.org>,
	USB list <linux-usb@...r.kernel.org>,
	Kernel development list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v4 3/4] usb: ffs: check quirk to pad epout buf size when
 not aligned to maxpacketsize

On Tue, 5 Nov 2013, David Cohen wrote:

> Hi Alan,
> 
> On 11/05/2013 07:38 AM, Alan Stern wrote:
> > On Tue, 5 Nov 2013, David Cohen wrote:
> > 
> >>>> +		/*
> >>>> +		 * Controller requires buffer size to be aligned to
> >>>> +		 * maxpacketsize of an out endpoint.
> >>>> +		 */
> >>>> +		if (gadget->quirk_ep_out_aligned_size && read) {
> >>>> +			/*
> >>>> +			 * We pass 'orig_len' to usp_ep_align_maxpacketsize()
> >>>> +			 * due to we're in a loop and 'len' may have been
> >>>> +			 * changed.
> >>>> +			 */
> >>>> +			len = usb_ep_align_maxpacketsize(ep->ep, orig_len);
> >>>> +			if (data && len > data_len) {
> >>>> +				kfree(data);
> >>>> +				data = NULL;
> >>>> +				data_len = 0;
> >>>> +			}
> >>>> +		}
> >>>
> >>> Since the value of orig_len never changes, there's no point calling 
> >>> usb_ep_align_maxpacketsize() inside the loop.  You should call it only 
> >>> once, before the loop starts.  Once you do that, you won't need 
> >>> orig_len at all.
> >>
> >> orig_len doesn't change but ep->ep does. If USB specs say max packet
> >> size won't change even if ep does, than we can call it from outside the
> >> loop.
> > 
> > I'm not too familiar with this driver.  It looks like the only way 
> > ep->ep can change is if the endpoint gets enabled while you're sitting 
> > inside the wait_event_interruptible() call.
> > 
> > In fact, the whole structure of that loop looks peculiar.  Why not
> > acquire the mutex first and then do everything else?
> 
> I'm not 100% familiar with this driver too. I'd keep this change to
> another patch.
> 
> > 
> > Does it even make sense for ep to change?  Would this change be visible
> > to the host?  What if the host changes the alternate setting while this
> > loop is running -- does it make sense for the userspace program to
> > start a read or write under one altsetting but then have the read/write
> > take place under a different altsetting?
> 
> It doesn't make sense to do so, but gadget driver allows it. If we just
> ignore, it would be a security or instability issue possible to xploit
> (for DWC3 and any other controller which may depend on this quirk).

Maybe Michal can enlighten us.

Alan Stern

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