linux-kernel - Re: [PATCH v3 3/4] fbdev: efifb: Cleanup fb_info in .fb

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Date:   Fri, 6 May 2022 15:07:32 +0200
From:   Andrzej Hajda <andrzej.hajda@...el.com>
To:     Javier Martinez Canillas <javierm@...hat.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>, Peter Jones <pjones@...hat.com>,
        Thomas Zimmermann <tzimmermann@...e.de>
Subject: Re: [PATCH v3 3/4] fbdev: efifb: Cleanup fb_info in .fb_destroy
 rather than .remove

On 06.05.2022 00:05, Javier Martinez Canillas wrote:
> The driver is calling framebuffer_release() in its .remove callback, but
> this will cause the struct fb_info to be freed too early. Since it could
> be that a reference is still hold to it if user-space opened the fbdev.
> 
> This would lead to a use-after-free error if the framebuffer device was
> unregistered but later a user-space process tries to close the fbdev fd.
> 
> To prevent this, move the framebuffer_release() call to fb_ops.fb_destroy
> instead of doing it in the driver's .remove callback.
> 
> Strictly speaking, the code flow in the driver is still wrong because all
> the hardware cleanupd (i.e: iounmap) should be done in .remove while the
> software cleanup (i.e: releasing the framebuffer) should be done in the
> .fb_destroy handler. But this at least makes to match the behavior before
> commit 27599aacbaef ("fbdev: Hot-unplug firmware fb devices on forced removal").
> 
> Fixes: 27599aacbaef ("fbdev: Hot-unplug firmware fb devices on forced removal")
> Suggested-by: Daniel Vetter <daniel.vetter@...ll.ch>
> Signed-off-by: Javier Martinez Canillas <javierm@...hat.com>
> Reviewed-by: Thomas Zimmermann <tzimmermann@...e.de>
> Reviewed-by: Daniel Vetter <daniel.vetter@...ll.ch>
> ---
> 
> (no changes since v1)
> 
>   drivers/video/fbdev/efifb.c | 9 ++++++++-
>   1 file changed, 8 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/video/fbdev/efifb.c b/drivers/video/fbdev/efifb.c
> index ea42ba6445b2..cfa3dc0b4eee 100644
> --- a/drivers/video/fbdev/efifb.c
> +++ b/drivers/video/fbdev/efifb.c
> @@ -243,6 +243,10 @@ static void efifb_show_boot_graphics(struct fb_info *info)
>   static inline void efifb_show_boot_graphics(struct fb_info *info) {}
>   #endif
>   
> +/*
> + * fb_ops.fb_destroy is called by the last put_fb_info() call at the end
> + * of unregister_framebuffer() or fb_release(). Do any cleanup here.
> + */
>   static void efifb_destroy(struct fb_info *info)
>   {
>   	if (efifb_pci_dev)
> @@ -254,6 +258,9 @@ static void efifb_destroy(struct fb_info *info)
>   		else
>   			memunmap(info->screen_base);
>   	}
> +
> +	framebuffer_release(info);
> +
>   	if (request_mem_succeeded)
>   		release_mem_region(info->apertures->ranges[0].base,
>   				   info->apertures->ranges[0].size);

You are releasing info, then you are using it.

I suspect it is responsible for multiple failures of Intel CI [1].

[1]: 
http://gfx-ci.fi.intel.com/tree/drm-tip/CI_DRM_11615/fi-adl-ddr5/boot0.txt

Regards
Andrzej


> @@ -620,9 +627,9 @@ static int efifb_remove(struct platform_device *pdev)
>   {
>   	struct fb_info *info = platform_get_drvdata(pdev);
>   
> +	/* efifb_destroy takes care of info cleanup */
>   	unregister_framebuffer(info);
>   	sysfs_remove_groups(&pdev->dev.kobj, efifb_groups);
> -	framebuffer_release(info);
>   
>   	return 0;
>   }