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

[Andrzej Hajda <andrzej.hajda@intel.com>]

On 10.05.2022 00:42, Javier Martinez Canillas wrote:
> On 5/10/22 00:22, Andrzej Hajda wrote:
>
> [snip]
>
>>>    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.
>>
> Indeed, you are correct again. [0] is the final version of the patch I've
> but don't have an i915 test machine to give it a try. I'll test tomorrow
> on my test systems to verify that it doesn't cause any regressions since
> with other DRM drivers.
>
> I think that besides this patch, drivers shouldn't need to call to the
> drm_fb_helper_fini() function directly. Since that would be called during
> drm_fbdev_fb_destroy() anyways.
>
> We should probably remove that call in all drivers and make this helper
> function static and just private to drm_fb_helper functions.
>
> Or am I missing something here ?

This is question for experts :)
I do not know what are user API/ABI expectations regarding removal of 
fbdev driver, I wonder if they are documented somewhere :)
Apparently we have some process of 'zombification'  here - we need to 
remove the driver without waiting for userspace closing framebuffer(???) 
(to unbind ops-es and remove references to driver related things), but 
we need to leave some structures to fool userspace, 'info' seems to be 
one of them.
So I guess there should be something called on driver's _remove path, 
and sth on destroy path.

Regards
Andrzej

>
> [0]:
>  From 5170cafcf2936da8f1c53231e3baa7d7a2b16c61 Mon Sep 17 00:00:00 2001
> From: Javier Martinez Canillas <javierm@redhat.com>
> Date: Tue May 10 00:39:55 2022 +0200
> Subject: [RFT PATCH] drm/fb-helper: Don't deallocate fb colormap and free fb info
>   too early
>
> Currently these are done in drm_fb_helper_fini() but this helper is called
> by drivers in their .remove callback, which could lead to a use-after-free
> if a process has opened the emulated fbdev node while a driver is removed.
>
> For example, in i915 driver the call chain during remove is the following:
>
> struct pci_driver i915_pci_driver = {
> ...
>          .remove = i915_pci_remove,
> ...
> };
>
> i915_pci_remove
>    i915_driver_remove
>      intel_modeset_driver_remove_noirq
>        intel_fbdev_fini
>          intel_fbdev_destroy
>            drm_fb_helper_fini
>              framebuffer_release
>
> Later the process will close the fbdev node file descriptor leading to the
> mentioned use-after-free bug in drm_fbdev_fb_destroy(), due the following:
>
> drm_fbdev_fb_destroy
>    drm_fbdev_release(info->par); <-- info was already freed on .remove
>
> To prevent that, let's move the framebuffer_release() call to the end of
> the drm_fbdev_fb_destroy() function.
>
> Also, the call to fb_dealloc_cmap() in drm_fb_helper_fini() is too early
> and is more correct to do it in drm_fbdev_fb_destroy() as well. After a
> call to drm_fbdev_release() has been made.
>
> Reported-by: Andrzej Hajda <andrzej.hajda@intel.com>
> Signed-off-by: Javier Martinez Canillas <javierm@redhat.com>
> ---
>   drivers/gpu/drm/drm_fb_helper.c | 9 +++------
>   1 file changed, 3 insertions(+), 6 deletions(-)
>
> diff --git a/drivers/gpu/drm/drm_fb_helper.c b/drivers/gpu/drm/drm_fb_helper.c
> index d265a73313c9..7288fbd26bcc 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);
> @@ -2112,6 +2106,9 @@ 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);
> +	if (info->cmap.len)
> +		fb_dealloc_cmap(&info->cmap);
> +	framebuffer_release(info);
>   }
>   
>   static int drm_fbdev_fb_mmap(struct fb_info *info, struct vm_area_struct *vma)