linux-kernel - Re: [PATCH 5.4 106/434] media: flexcop-usb: fix NULL-ptr deref in flexcop_usb_transfer

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Date:   Fri, 3 Jan 2020 17:53:12 +0100
From:   Johan Hovold <johan@...nel.org>
To:     Sean Young <sean@...s.org>
Cc:     Johan Hovold <johan@...nel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        linux-kernel@...r.kernel.org, stable@...r.kernel.org,
        Yang Yingliang <yangyingliang@...wei.com>,
        Mauro Carvalho Chehab <mchehab+samsung@...nel.org>,
        Sasha Levin <sashal@...nel.org>
Subject: Re: [PATCH 5.4 106/434] media: flexcop-usb: fix NULL-ptr deref in
 flexcop_usb_transfer_init()

On Fri, Jan 03, 2020 at 04:12:07PM +0000, Sean Young wrote:
> On Fri, Jan 03, 2020 at 04:02:42PM +0100, Johan Hovold wrote:
> > On Sun, Dec 29, 2019 at 06:22:39PM +0100, Greg Kroah-Hartman wrote:
> > > From: Yang Yingliang <yangyingliang@...wei.com>
> > > 
> > > [ Upstream commit 649cd16c438f51d4cd777e71ca1f47f6e0c5e65d ]
> > > 
> > > If usb_set_interface() failed, iface->cur_altsetting will
> > > not be assigned and it will be used in flexcop_usb_transfer_init()
> > > It may lead a NULL pointer dereference.
> > > 
> > > Check usb_set_interface() return value in flexcop_usb_init()
> > > and return failed to avoid using this NULL pointer.
> > > 
> > > Signed-off-by: Yang Yingliang <yangyingliang@...wei.com>
> > > Signed-off-by: Sean Young <sean@...s.org>
> > > Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@...nel.org>
> > > Signed-off-by: Sasha Levin <sashal@...nel.org>
> > 
> > This commit is bogus and should be dropped from all stable queues.
> > 
> > Contrary to what the commit message claims, iface->cur_altsetting will
> > never be NULL so there's no risk for a NULL-pointer dereference here.
> 
> Yes, you are right, I can't find any path through which cur_altsetting
> will be set to NULL. The commit message is wrong. I am sorry for letting
> this slip through.
> 
> Thank you for pointing this out.
> 
> > Even though the change itself is benign, we shouldn't spread this
> > confusion further.
> 
> usb_set_interface() can fail for a number of reasons, and we should not
> continue if it fails. So, the commit message is misleading but the
> change itself is still valid.

Sure, the change itself is fine, but I wouldn't consider it stable
material even with a correct commit message as it is not a critical fix.

And the user would still see an error message in case changing
altsetting fails.

Johan