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

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Date:   Mon, 9 May 2022 18:33:46 +0200
From:   Javier Martinez Canillas <javierm@...hat.com>
To:     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,
        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 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);
> 
> 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

-- 
Best regards,

Javier Martinez Canillas
Linux Engineering
Red Hat