linux-kernel - Re: [PATCH] drm/of: free the iterator object on failure

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Message-ID: <YOy6VQNz8Htg6Usb@pendragon.ideasonboard.com>
Date:   Tue, 13 Jul 2021 00:55:33 +0300
From:   Laurent Pinchart <laurent.pinchart@...asonboard.com>
To:     Steven Price <steven.price@....com>
Cc:     Daniel Vetter <daniel@...ll.ch>, David Airlie <airlied@...ux.ie>,
        Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
        Maxime Ripard <mripard@...nel.org>,
        Thomas Zimmermann <tzimmermann@...e.de>,
        dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org,
        Biju Das <biju.das.jz@...renesas.com>
Subject: Re: [PATCH] drm/of: free the iterator object on failure

Hi Steven,

On Mon, Jul 12, 2021 at 10:31:52PM +0100, Steven Price wrote:
> On 12/07/2021 17:50, Laurent Pinchart wrote:
> > On Mon, Jul 12, 2021 at 04:57:58PM +0100, Steven Price wrote:
> >> When bailing out due to the sanity check the iterator value needs to be
> >> freed because the early return prevents for_each_child_of_node() from
> >> doing the dereference itself.
> >>
> >> Fixes: 4ee48cc5586b ("drm: of: Fix double-free bug")
> > 
> > I don't think the Fixes tag is correct, the issue was already present
> > before 4ee48cc5586b. The fix looks right though.
> 
> I'm not sure quite what you mean by "already present". As I understand
> it the timeline was:
> 
> 1. 6529007522de drm: of: Add drm_of_lvds_get_dual_link_pixel_order
>    The function was originally added. This made the mistake twice of
>    calling of_node_put() on the wrong variable (remote_port rather than
>    endpoint).

Correct.

> 2. 4ee48cc5586b drm: of: Fix double-free bug
>    One of the of_node_put() calls was removed as it was a double-free.
>    This left the first incorrect of_node_put() in place, and the second
>    is now a straight leak.

That's right, but this commit didn't introduce the leak, it was already
there in 6529007522de (in addition to the double-free).

> 3. b557a5f8da57 drm/of: free the right object
>    This (correctly) fixes the first of_node_put() to free endpoint. And
>    the post from Daniel was what caused me to look.
> 
> 4. This patch
>    Reintroduces the of_node_put() removed in (2) but putting endpoint
>    rather than remote_port.
> 
> I've put (2) in the Fixes line as this patch is fixing the leak
> introduced by that patch, but that in itself was of course 'fixing' the
> double free of the original patch.
> 
> >> Signed-off-by: Steven Price <steven.price@....com>
> >> ---
> >>  drivers/gpu/drm/drm_of.c | 4 +++-
> >>  1 file changed, 3 insertions(+), 1 deletion(-)
> >>
> >> Daniel's email[1] made me take a look at this function and it appears
> >> that for_each_child_of_node()'s interface had caused a bad bug fix due
> >> to the hidden reference counting in the iterator.
> >>
> >> [1] https://lore.kernel.org/r/YOxQ5TbkNrqCGBDJ%40phenom.ffwll.local
> >>
> >> diff --git a/drivers/gpu/drm/drm_of.c b/drivers/gpu/drm/drm_of.c
> >> index 197c57477344..997b8827fed2 100644
> >> --- a/drivers/gpu/drm/drm_of.c
> >> +++ b/drivers/gpu/drm/drm_of.c
> >> @@ -331,8 +331,10 @@ static int drm_of_lvds_get_remote_pixels_type(
> >>  		 * configurations by passing the endpoints explicitly to
> >>  		 * drm_of_lvds_get_dual_link_pixel_order().
> >>  		 */
> >> -		if (!current_pt || pixels_type != current_pt)
> >> +		if (!current_pt || pixels_type != current_pt) {
> >> +			of_node_put(endpoint);
> >>  			return -EINVAL;
> >> +		}
> >>  	}
> >>  
> >>  	return pixels_type;

-- 
Regards,

Laurent Pinchart