Re: [PATCH v1] usb: host: Fix possible kernel crash

[Venu Byravarasu <vbyravarasu@xxxxxxxxxx>]

Thanks Alan for your comments.

On Monday 09 July 2012 08:04 PM, Alan Stern wrote:
> On Mon, 9 Jul 2012, Venu Byravarasu wrote:
>
> > In functions itd_complete &  sitd_complete, a pointer
> > by name stream may get dereferenced after freeing it, when
> > iso_stream_put is called with stream->refcount = 2.
> I don't understand the problem.  Did you actually see this happen or is
> it only theoretical?

Yes it is a theoretical problem, as complained by Coverity.

>   	/* for each uframe with a packet */
>   	for (uframe = 0; uframe < 8; uframe++) {
> @@ -1783,7 +1784,8 @@ itd_complete (
>   			dev->devpath, stream->bEndpointAddress & 0x0f,
>   			(stream->bEndpointAddress & USB_DIR_IN) ? "in" : "out");
>   	}
> -	iso_stream_put (ehci, stream);
> +	stream_ref_count = stream->refcount;
> +	iso_stream_put(ehci, stream);
> This iso_stream_put removes the reference held by the URB.  Before it
> is called, stream->refcount must be >= 3:
>
> 	refcount is set to 1 when the stream is created;
>
> 	each active URB holds a reference;
>
> 	each itd holds a reference.
>
> So after the call, the refcount value must be >= 2 and the stream could
> not have been deallocated.
>
> >   done:
> >   	itd->urb = NULL;
> > @@ -1797,7 +1799,7 @@ done:
> >   		 * Move it to a safe place until a new frame starts.
> >   		 */
> >   		list_move(&itd->itd_list, &ehci->cached_itd_list);
> > -		if (stream->refcount == 2) {
> > +		if (stream_ref_count == 3) {
> Therefore this seems unnecessary.

As per the logic you explained above, this change is not needed.
However coverity was complaining as below:

/kernel/drivers/usb/host/ehci-sched.c 1777 USE_AFTER_FREE Dereferencing 
freed pointer "stream"

Hence to pacify coverity, this change is done.
Please let me know if you see any other better way to handle it.

> >   			/* If iso_stream_put() were called here, stream
> >   			 * would be freed.  Instead, just prevent reuse.
> >   			 */
> Alan Stern

Thanks,
Venu
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