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

[Venu Byravarasu]

On Tuesday 10 July 2012 08:15 PM, gregkh@xxxxxxxxxxxxxxxxxxx wrote:
> On Tue, Jul 10, 2012 at 09:56:39AM +0530, Venu Byravarasu wrote:
> > 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.
> Why are you trying to "pacify" coverity, when the tool is wrong in this
> case?  Go poke the owners of that tool to get it to stop emitting this
> false warning.  Don't paper over it in the kernel.  Especially for a
> tool that none of us can run on our own.
>
> greg k-h
Thanks Greg for your comments.
In fact coverity team also mentioned this as one of the false positives.
Also as Alan mentioned that he'll be taking care of it in a different 
way, will stop working on this patch.

Thanks,
Venu

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