Re: [PATCH 2/3] fbdev/simplefb: Cleanup fb_info in .fb_destroy rather than .remove

[Thomas Zimmermann]

Hi

Am 04.05.22 um 23:57 schrieb Javier Martinez Canillas:
> 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.
>
> The correct thing to do is to only unregister the framebuffer in the
> driver's .remove callback, but do any cleanup in the fb_ops.fb_destroy.
>
> Suggested-by: Daniel Vetter <daniel.vetter@xxxxxxxx>
> Signed-off-by: Javier Martinez Canillas <javierm@xxxxxxxxxx>

Reviewed-by: Thomas Zimmermann <tzimmermann@xxxxxxx>

Please see my question below.

> ---
>
>   drivers/video/fbdev/simplefb.c | 8 +++++++-
>   1 file changed, 7 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/video/fbdev/simplefb.c b/drivers/video/fbdev/simplefb.c
> index 94fc9c6d0411..2c198561c338 100644
> --- a/drivers/video/fbdev/simplefb.c
> +++ b/drivers/video/fbdev/simplefb.c
> @@ -84,6 +84,10 @@ struct simplefb_par {
>   static void simplefb_clocks_destroy(struct simplefb_par *par);
>   static void simplefb_regulators_destroy(struct simplefb_par *par);
>   
> +/*
> + * 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 simplefb_destroy(struct fb_info *info)
>   {
>   	struct simplefb_par *par = info->par;
> @@ -94,6 +98,8 @@ static void simplefb_destroy(struct fb_info *info)
>   	if (info->screen_base)
>   		iounmap(info->screen_base);
>   
> +	framebuffer_release(info);
> +
>   	if (mem)
>   		release_mem_region(mem->start, resource_size(mem));

The original problem with fbdev hot-unplug was that vmwgfx needed the 
framebuffer region to be released. If we release it only after userspace 
closed it's final file descriptor, vmwgfx could have already failed.

I still don't fully get why this code apparently works or at least 
doesn't blow up occasionally. Any ideas?

Best regards
Thomas

>   }
> @@ -545,8 +551,8 @@ static int simplefb_remove(struct platform_device *pdev)
>   {
>   	struct fb_info *info = platform_get_drvdata(pdev);
>   
> +	/* simplefb_destroy takes care of info cleanup */
>   	unregister_framebuffer(info);
> -	framebuffer_release(info);
>   
>   	return 0;
>   }

--
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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