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

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Message-ID: <51254d3d-af8d-61b3-e8a2-8fd0e583e783@suse.de>
Date:   Mon, 9 May 2022 20:32:53 +0200
From:   Thomas Zimmermann <tzimmermann@...e.de>
To:     Javier Martinez Canillas <javierm@...hat.com>,
        Andrzej Hajda <andrzej.hajda@...el.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,
        Daniel Vetter <daniel.vetter@...el.com>
Subject: Re: [PATCH v3 1/4] fbdev: Prevent possible use-after-free in
 fb_release()

Hi

Am 09.05.22 um 18:33 schrieb Javier Martinez Canillas:
> On 5/9/22 17:51, Andrzej Hajda wrote:
> 
> [snip]
> 
>>>>> +
>>>> 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?
>>>>
>>> I think you are correct. Maybe we need something like the following?
>>>
>>> diff --git a/drivers/gpu/drm/drm_fb_helper.c b/drivers/gpu/drm/drm_fb_helper.c
>>> index d265a73313c9..b09598f7af28 100644
>>> --- a/drivers/gpu/drm/drm_fb_helper.c
>>> +++ b/drivers/gpu/drm/drm_fb_helper.c
>>> @@ -631,7 +631,6 @@ void drm_fb_helper_fini(struct drm_fb_helper *fb_helper)
>>>           if (info) {
>>>                   if (info->cmap.len)
>>>                           fb_dealloc_cmap(&info->cmap);
>>> -               framebuffer_release(info);

After reviewing that code,  drm_fb_helper_fini() appears to be called 
from .fb_destroy (see drm_fbdev_release).  The code is hard to follow 
though.  If there another way of releasing the framebuffer here?

Best regards
Thomas

>>
>> What about cmap? I am not an fb expert, but IMO cmap can be accessed
>> from userspace as well.
>>
> 
> I actually thought about the same but then remembered what Daniel said in [0]
> (AFAIU at least) that these should be done in .remove() so the current code
> looks like matches that and only framebuffer_release() should be moved.
> 
> For vesafb a previous patch proposed to also move a release_region() call to
> .fb_destroy() and Daniel also said that it was iffy and shouldn't be done [1].
> 
> But I'm also not fb expert so happy to move fb_dealloc_cmap() as well if that
> is the correct thing to do.
> 
> [0]: https://www.spinics.net/lists/dri-devel/msg346257.html
> [1]: https://www.spinics.net/lists/dri-devel/msg346261.html
> 

-- 
Thomas Zimmermann
Graphics Driver Developer
SUSE Software Solutions Germany GmbH
Maxfeldstr. 5, 90409 Nürnberg, Germany
(HRB 36809, AG Nürnberg)
Geschäftsführer: Ivo Totev

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