Re: [PATCH v3 1/4] fbdev: Prevent possible use-after-free in fb_release()

[Andrzej Hajda]

On 09.05.2022 22:03, Javier Martinez Canillas wrote:
> 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);

I would put drm_fbdev_release at the beginning - it cancels workers 
which could expect cmap to be still valid.

Regards
Andrzej


>   }
>   
>   static int drm_fbdev_fb_mmap(struct fb_info *info, struct vm_area_struct *vma)