linux-kernel - Re: [PATCH] Usb: host - Fix possible NULL derefrence.

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Date:   Mon, 30 Jan 2017 08:03:23 +0100
From:   Thierry Reding <thierry.reding@...il.com>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     Shailendra Verma <shailendra.v@...sung.com>,
        Mathias Nyman <mathias.nyman@...el.com>,
        Stephen Warren <swarren@...dotorg.org>,
        Alexandre Courbot <gnurou@...il.com>,
        linux-usb@...r.kernel.org, linux-tegra@...r.kernel.org,
        linux-kernel@...r.kernel.org, p.shailesh@...sung.com,
        ashish.kalra@...sung.com,
        Shailendra Verma <shailendra.capricorn@...il.com>
Subject: Re: [PATCH] Usb: host - Fix possible NULL derefrence.

On Mon, Jan 30, 2017 at 07:45:21AM +0100, Greg Kroah-Hartman wrote:
> On Mon, Jan 30, 2017 at 10:36:29AM +0530, Shailendra Verma wrote:
> > of_device_get_match_data could return NULL, and so can cause
> > a NULL pointer dereference later.
> > 
> > Signed-off-by: Shailendra Verma <shailendra.v@...sung.com>
> > ---
> >  drivers/usb/host/xhci-tegra.c |    4 ++++
> >  1 file changed, 4 insertions(+)
> > 
> > diff --git a/drivers/usb/host/xhci-tegra.c b/drivers/usb/host/xhci-tegra.c
> > index a59fafb..890c778 100644
> > --- a/drivers/usb/host/xhci-tegra.c
> > +++ b/drivers/usb/host/xhci-tegra.c
> > @@ -903,6 +903,10 @@ static int tegra_xusb_probe(struct platform_device *pdev)
> >  		return -ENOMEM;
> >  
> >  	tegra->soc = of_device_get_match_data(&pdev->dev);
> > +	if (!tegra->soc) {
> 
> How would the driver be loaded and the probe function called if this
> returns NULL?
> 
> Is this ever possible?

No, it isn't. I've been NAK'ing this kind of patch for a while now.
There are two variants of this patch going about:

  1) checking the return value of of_match_device()
  2) checking the return value of of_device_get_match_data()

The same may also apply to of_match_node(), but I haven't seen that used
very much lately.

For of_match_device() the problem could technically occur if used in non
OF setups, because the device could be instantiated by hand in board
setup code. Tegra has been OF-only for a couple of years now, so there
is no way this can happen today.

of_device_get_match_data() is somewhat more complicated because it could
still return NULL if the OF table entry had its .data field set to NULL.
However in all drivers that I know that would be considered a bug, so
might as well let things crash at this point to make it immediately
obvious.

I had once been tempted to write a checkpatch rule for this, but I'm not
sure it's as easy as just warning if there's a check, because there are
some legitimate cases, even if they're very rare.

Thierry

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