linux-kernel - Re: [PATCH v2] USB: EHCI: fix dereference of ERR

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Date:	Fri, 18 Sep 2015 11:18:19 +0530
From:	Sudip Mukherjee <sudipm.mukherjee@...il.com>
To:	Alan Stern <stern@...land.harvard.edu>
Cc:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	linux-kernel@...r.kernel.org, linux-usb@...r.kernel.org,
	Fabio Estevam <festevam@...il.com>,
	Sergei Shtylyov <sergei.shtylyov@...entembedded.com>
Subject: Re: [PATCH v2] USB: EHCI: fix dereference of ERR_PTR

On Wed, Sep 16, 2015 at 12:54:03PM -0400, Alan Stern wrote:
> On Wed, 16 Sep 2015, Sudip Mukherjee wrote:
> 
> > On error find_tt() returns either a NULL pointer or the error value in
> > ERR_PTR. But we were dereferencing it directly without even checking if
> > find_tt() returned a valid pointer or not.
> > 
> > Signed-off-by: Sudip Mukherjee <sudip@...torindia.org>
> > ---
<snip>
> > @@ -1373,6 +1375,8 @@ static void reserve_release_iso_bandwidth(struct ehci_hcd *ehci,
> >  		}
> >  
> >  		tt = find_tt(stream->ps.udev);
> > +		if (IS_ERR_OR_NULL(tt))
> > +			return;
> >  		if (sign > 0)
> >  			list_add_tail(&stream->ps.ps_list, &tt->ps_list);
> >  		else
> 
> This patch isn't needed.  In both reserve_release_intr_bandwidth() and 
> reserve_release_iso_bandwidth() it is known that find_tt() will return 
> a valid pointer.
> 
> This is because each of those functions is called from only one place.  
> For example, reserve_release_intr_bandwidth() is called only at the end
> of qh_schedule().  But near the start of qh_schedule() there is earlier
> call to tt_find(), and there we do test for error pointers.  If the
> first call doesn't return an error then the second call won't either.
> 
> The same sort of thing happens in reserve_release_iso_bandwidth().
Yes, I should have looked more before sending. Sorry for the noise.
But in those checkes for find_tt() only IS_ERR is checked, shouldn't we
check for IS_ERR_OR_NULL as find_tt() can return NULL also?

regards
sudip
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