linux-kernel - Re: [PATCH] drm/plane: fix error handling in __drm_universal_plane

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Date:   Wed, 6 Dec 2023 09:22:04 +0100
From:   Thomas Zimmermann <tzimmermann@...e.de>
To:     Dinghao Liu <dinghao.liu@....edu.cn>
Cc:     linux-kernel@...r.kernel.org, dri-devel@...ts.freedesktop.org,
        Maxime Ripard <mripard@...nel.org>
Subject: Re: [PATCH] drm/plane: fix error handling in
 __drm_universal_plane_init

Hi

Am 06.12.23 um 08:27 schrieb Dinghao Liu:
> __drm_universal_plane_init() frees plane->format_types and
> plane->modifiers on failure. However, sometimes its callers
> will free these two pointers again, which may lead to a
> double-free. One possible call chain is:
> 
> mdp5_plane_init
>    |-> drm_universal_plane_init
>    |     |-> __drm_universal_plane_init (first free)
>    |
>    |-> mdp5_plane_destroy
>          |-> drm_plane_cleanup (second free)
> 
> Fix this by setting the two pointers to NULL after kfree.
> 
> Signed-off-by: Dinghao Liu <dinghao.liu@....edu.cn>

Reviewed-by: Thomas Zimmermann <tzimmermann@...e.de>

to get the immediate bug fixed.

However, I don't think it's the correct way of doing things in general. 
Planes should probably not attempt to even make a copy, but use the 
supplied pointers. Lifetime of the arrays is the same as of the plane. 
That's for a different patch set, of course.  (I did not review the DRM 
code whether the internal copy is required.)

For now, maybe drm_plane_cleanup() could warn if format_types equals 
NULL. [1] It indicates that the plane has not been initialized correctly 
and the driver's memory lifetime handling is somehow broken.

Best regards
Thomas

[1] 
https://elixir.bootlin.com/linux/latest/source/drivers/gpu/drm/drm_plane.c#L542

> ---
>   drivers/gpu/drm/drm_plane.c | 3 +++
>   1 file changed, 3 insertions(+)
> 
> diff --git a/drivers/gpu/drm/drm_plane.c b/drivers/gpu/drm/drm_plane.c
> index 24e7998d1731..1331b8224920 100644
> --- a/drivers/gpu/drm/drm_plane.c
> +++ b/drivers/gpu/drm/drm_plane.c
> @@ -304,6 +304,7 @@ static int __drm_universal_plane_init(struct drm_device *dev,
>   	if (format_modifier_count && !plane->modifiers) {
>   		DRM_DEBUG_KMS("out of memory when allocating plane\n");
>   		kfree(plane->format_types);
> +		plane->format_types = NULL;
>   		drm_mode_object_unregister(dev, &plane->base);
>   		return -ENOMEM;
>   	}
> @@ -317,6 +318,8 @@ static int __drm_universal_plane_init(struct drm_device *dev,
>   	if (!plane->name) {
>   		kfree(plane->format_types);
>   		kfree(plane->modifiers);
> +		plane->format_types = NULL;
> +		plane->modifiers = NULL;
>   		drm_mode_object_unregister(dev, &plane->base);
>   		return -ENOMEM;
>   	}

-- 
Thomas Zimmermann
Graphics Driver Developer
SUSE Software Solutions Germany GmbH
Frankenstrasse 146, 90461 Nuernberg, Germany
GF: Ivo Totev, Andrew Myers, Andrew McDonald, Boudien Moerman
HRB 36809 (AG Nuernberg)

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