linux-kernel - Re: [PATCH v3 1/4] fbdev: Prevent possible use-after-free in fb

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Date:   Tue, 10 May 2022 10:04:32 +0200
From:   Thomas Zimmermann <tzimmermann@...e.de>
To:     Javier Martinez Canillas <javierm@...hat.com>,
        Andrzej Hajda <andrzej.hajda@...el.com>,
        linux-kernel@...r.kernel.org
Cc:     linux-fbdev@...r.kernel.org,
        Daniel Vetter <daniel.vetter@...ll.ch>,
        Helge Deller <deller@....de>, dri-devel@...ts.freedesktop.org,
        Daniel Vetter <daniel.vetter@...el.com>
Subject: Re: [PATCH v3 1/4] fbdev: Prevent possible use-after-free in
 fb_release()

Hi

Am 10.05.22 um 00:42 schrieb Javier Martinez Canillas:
> 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.

You have to go through all DRM drivers that call drm_fb_helper_fini() 
and make sure that they free fb_info. For example armada appears to be 
leaking now. [1]

Best regards
Thomas

[1] 
https://elixir.bootlin.com/linux/v5.17.6/source/drivers/gpu/drm/armada/armada_fbdev.c#L152

> 
> 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 ?
> 
> [0]:
>  From 5170cafcf2936da8f1c53231e3baa7d7a2b16c61 Mon Sep 17 00:00:00 2001
> From: Javier Martinez Canillas <javierm@...hat.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@...el.com>
> Signed-off-by: Javier Martinez Canillas <javierm@...hat.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)

-- 
Thomas Zimmermann
Graphics Driver Developer
SUSE Software Solutions Germany GmbH
Maxfeldstr. 5, 90409 Nürnberg, Germany
(HRB 36809, AG Nürnberg)
Geschäftsführer: Ivo Totev

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