linux-kernel - Re: [PATCH v3 1/4] fbdev: Prevent possible use-after-free in fb

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Message-ID: <753d0350-42dc-389b-b10b-4533ddcf32ac@intel.com>
Date:   Mon, 9 May 2022 16:56:23 +0200
From:   Andrzej Hajda <andrzej.hajda@...el.com>
To:     Javier Martinez Canillas <javierm@...hat.com>,
        <linux-kernel@...r.kernel.org>
CC:     <linux-fbdev@...r.kernel.org>,
        Daniel Vetter <daniel.vetter@...ll.ch>,
        Helge Deller <deller@....de>,
        <dri-devel@...ts.freedesktop.org>,
        "Thomas Zimmermann" <tzimmermann@...e.de>,
        Daniel Vetter <daniel.vetter@...el.com>
Subject: Re: [PATCH v3 1/4] fbdev: Prevent possible use-after-free in
 fb_release()

On 06.05.2022 00:04, Javier Martinez Canillas wrote:
> From: Daniel Vetter <daniel.vetter@...ll.ch>
> 
> Most fbdev drivers have issues with the fb_info lifetime, because call to
> framebuffer_release() from their driver's .remove callback, rather than
> doing from fbops.fb_destroy callback.
> 
> Doing that will destroy the fb_info too early, while references to it may
> still exist, leading to a use-after-free error.
> 
> To prevent this, check the fb_info reference counter when attempting to
> kfree the data structure in framebuffer_release(). That will leak it but
> at least will prevent the mentioned error.
> 
> Signed-off-by: Daniel Vetter <daniel.vetter@...el.com>
> Signed-off-by: Javier Martinez Canillas <javierm@...hat.com>
> Reviewed-by: Thomas Zimmermann <tzimmermann@...e.de>
> ---
> 
> (no changes since v1)
> 
>   drivers/video/fbdev/core/fbsysfs.c | 4 ++++
>   1 file changed, 4 insertions(+)
> 
> diff --git a/drivers/video/fbdev/core/fbsysfs.c b/drivers/video/fbdev/core/fbsysfs.c
> index 8c1ee9ecec3d..c2a60b187467 100644
> --- a/drivers/video/fbdev/core/fbsysfs.c
> +++ b/drivers/video/fbdev/core/fbsysfs.c
> @@ -80,6 +80,10 @@ void framebuffer_release(struct fb_info *info)
>   {
>   	if (!info)
>   		return;
> +
> +	if (WARN_ON(refcount_read(&info->count)))
> +		return;
> +

Regarding drm:
What about drm_fb_helper_fini? It calls also framebuffer_release and is 
called often from _remove paths (checked intel/radeon/nouveau). I guess 
it should be fixed as well. Do you plan to fix it?


Regarding fb drivers, just for stats:
git grep -p framebuffer_release | grep _remove | wc -l
Suggests there is at least 70 incorrect users of this :)

Regards
Andrzej