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

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Message-ID: <c148afe9-f10a-8751-d761-2480c05a137a@redhat.com>
Date:   Tue, 10 May 2022 11:44:49 +0200
From:   Javier Martinez Canillas <javierm@...hat.com>
To:     Thomas Zimmermann <tzimmermann@...e.de>,
        Andrzej Hajda <andrzej.hajda@...el.com>,
        linux-kernel@...r.kernel.org
Cc:     Daniel Vetter <daniel.vetter@...ll.ch>,
        linux-fbdev@...r.kernel.org, Helge Deller <deller@....de>,
        dri-devel@...ts.freedesktop.org,
        Daniel Vetter <daniel.vetter@...el.com>
Subject: Re: [PATCH v3 1/4] fbdev: Prevent possible use-after-free in
 fb_release()

On 5/10/22 11:39, Thomas Zimmermann wrote:

[snip]

>>
>> 3) Set .fb_destroy to drm_fbdev_fb_destroy() if isn't set by drivers when
>>     they call drm_fb_helper_initial_config() or drm_fb_helper_fill_info().
>>
>> I'm leaning towards option (3). Then the fb_info release will be automatic
>> whether drivers are using the generic setup or a custom one.
> 
> IMHO this would just be another glitch to paper over all the broken 
> code. And if you follow through drm_fbdev_fb_helper(), [1] it'll call 
> _fini at some point and probably blow up in some other way. Instances of 
> struct fb_ops are also usually const.
> 
> The only reliable way AFAICT is to do what generic fbdev does: use 
> unregister_framebuffer and do the software cleanup somewhere within 
> fb_destroy. And then fix all drivers to use that pattern.
> 

Right. We can't really abstract this away from drivers that are not
using the generic fbdev helpers. So then they will have to provide
their own .fb_destroy() callback and do the cleanup.

-- 
Best regards,

Javier Martinez Canillas
Linux Engineering
Red Hat