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

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Date:	Tue, 10 Jul 2012 11:09:42 -0400 (EDT)
From:	Alan Stern <stern@...land.harvard.edu>
To:	Venu Byravarasu <vbyravarasu@...dia.com>
cc:	"gregkh@...uxfoundation.org" <gregkh@...uxfoundation.org>,
	"linux-usb@...r.kernel.org" <linux-usb@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v1] usb: host: Fix possible kernel crash

On Tue, 10 Jul 2012, 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.

> 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.

This seems to be a false positive from Coverity.

In any case, I'm about to submit some patches which get rid of the 
reference counting entirely.  So let's not worry about this.

Alan Stern

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