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

[Daniel Vetter]

On Mon, May 09, 2022 at 10:00:41PM +0200, Javier Martinez Canillas wrote:
> Hello Thomas,
> 
> On 5/9/22 20:32, Thomas Zimmermann wrote:
> > 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?
> > 
> 
> Andrzej mentioned intel/radeon/nouveau as example, I only looked at i915
> and the call chain is the following as far as I can tell:
> 
> struct pci_driver i915_pci_driver = {
> ...
>         .remove = i915_pci_remove,
> ...
> };
> 
> 
> i915_driver_remove
>   intel_modeset_driver_remove_noirq
>     intel_fbdev_fini
>       intel_fbdev_destroy
>         drm_fb_helper_fini
>           framebuffer_release
>               
> So my underdestanding is that if a program has the emulated fbdev device
> opened and the i915 module is removed, then a use-after-free would be
> triggered on drm_fbdev_fb_destroy() once the program closes the device:
> 
> drm_fbdev_fb_destroy
>   drm_fbdev_release(info->par); <-- info was already freed on .remove

Yeah the old drivers that haven't switched over to the drm_client based
fbdev emulations are all kinds of wrong and release it too early.

Note that they don't use the provided ->remove hook, since that would
result in double-cleanup like you point out. Those old drivers work more
like all the other fbdev drivers where all the cleanup is done in
->remove, and if it's a real hotunplug you just die in fire :-/

Switching them all over to the drm_client based fbdev helpers and
unexporting these old (dangerous!) functions would be really neat. But
it's also a loooooot of work, and generally those big drivers don't get
hotunplugged.
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch