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

[Javier Martinez Canillas]

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

-- 
Best regards,

Javier Martinez Canillas
Linux Engineering
Red Hat