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

[Andrzej Hajda]

On 09.05.2022 17:30, Javier Martinez Canillas wrote:
> Hello Andrzej,
>
> On 5/9/22 16:56, Andrzej Hajda wrote:
> > On 06.05.2022 00:04, Javier Martinez Canillas wrote:
> > > From: Daniel Vetter <daniel.vetter@xxxxxxxx>
> > >
> > > Most fbdev drivers have issues with the fb_info lifetime, because call to
> > > framebuffer_release() from their driver's .remove callback, rather than
> > > doing from fbops.fb_destroy callback.
> > >
> > > Doing that will destroy the fb_info too early, while references to it may
> > > still exist, leading to a use-after-free error.
> > >
> > > To prevent this, check the fb_info reference counter when attempting to
> > > kfree the data structure in framebuffer_release(). That will leak it but
> > > at least will prevent the mentioned error.
> > >
> > > Signed-off-by: Daniel Vetter <daniel.vetter@xxxxxxxxx>
> > > Signed-off-by: Javier Martinez Canillas <javierm@xxxxxxxxxx>
> > > Reviewed-by: Thomas Zimmermann <tzimmermann@xxxxxxx>
> > > ---
> > >
> > > (no changes since v1)
> > >
> > >    drivers/video/fbdev/core/fbsysfs.c | 4 ++++
> > >    1 file changed, 4 insertions(+)
> > >
> > > diff --git a/drivers/video/fbdev/core/fbsysfs.c b/drivers/video/fbdev/core/fbsysfs.c
> > > index 8c1ee9ecec3d..c2a60b187467 100644
> > > --- a/drivers/video/fbdev/core/fbsysfs.c
> > > +++ b/drivers/video/fbdev/core/fbsysfs.c
> > > @@ -80,6 +80,10 @@ void framebuffer_release(struct fb_info *info)
> > >    {
> > >    	if (!info)
> > >    		return;
> > > +
> > > +	if (WARN_ON(refcount_read(&info->count)))
> > > +		return;
> > > +
> > 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.

Regards
Andrzej

>          }
>          fb_helper->fbdev = NULL;
>   
> @@ -2112,6 +2111,7 @@ static void drm_fbdev_release(struct drm_fb_helper *fb_helper)
>   static void drm_fbdev_fb_destroy(struct fb_info *info)
>   {
>          drm_fbdev_release(info->par);
> +       framebuffer_release(info);
>   }
>   
>   static int drm_fbdev_fb_mmap(struct fb_info *info, struct vm_area_struct *vma)
>
> > Regarding fb drivers, just for stats:
> > git grep -p framebuffer_release | grep _remove | wc -l
> > Suggests there is at least 70 incorrect users of this :)
> Yes, Daniel already mentioned that most of them get this wrong but I was
> mostly interested in {simple,efi,vesa}fb since are used with "nomodeset".
>
> But given that I only touched those tree and still managed to introduce
> a regression, I won't attempt to fix the others.