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

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Date:   Mon, 9 May 2022 22:03:46 +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:     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()

On 5/9/22 20:12, Thomas Zimmermann wrote:

[snip]

>> 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.
> 
> The cmap data structure is software state that can be accessed via icotl 
> as long as the devfile is open. Drivers update the hardware from it. See 
> [1].  Moving that cleanup into fb_destroy seems appropriate to me.
> 

I see, that makes sense. Then something like the following instead?

diff --git a/drivers/gpu/drm/drm_fb_helper.c b/drivers/gpu/drm/drm_fb_helper.c
index d265a73313c9..ce0d89c49e42 100644
--- a/drivers/gpu/drm/drm_fb_helper.c
+++ b/drivers/gpu/drm/drm_fb_helper.c
@@ -627,12 +627,6 @@ void drm_fb_helper_fini(struct drm_fb_helper *fb_helper)
        cancel_work_sync(&fb_helper->resume_work);
        cancel_work_sync(&fb_helper->damage_work);
 
-       info = fb_helper->fbdev;
-       if (info) {
-               if (info->cmap.len)
-                       fb_dealloc_cmap(&info->cmap);
-               framebuffer_release(info);
-       }
        fb_helper->fbdev = NULL;
 
        mutex_lock(&kernel_fb_helper_lock);
@@ -2111,7 +2105,11 @@ static void drm_fbdev_release(struct drm_fb_helper *fb_helper)
  */
 static void drm_fbdev_fb_destroy(struct fb_info *info)
 {
+       if (info->cmap.len)
+               fb_dealloc_cmap(&info->cmap);
+
        drm_fbdev_release(info->par);
+       framebuffer_release(info);
 }
 
 static int drm_fbdev_fb_mmap(struct fb_info *info, struct vm_area_struct *vma)

-- 
Best regards,

Javier Martinez Canillas
Linux Engineering
Red Hat