linux-kernel - Re: [PATCH 2/3] fbdev/simplefb: Cleanup fb_info in .fb

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Date:   Thu, 5 May 2022 14:52:46 +0200
From:   Daniel Vetter <daniel@...ll.ch>
To:     Javier Martinez Canillas <javierm@...hat.com>
Cc:     linux-kernel@...r.kernel.org, dri-devel@...ts.freedesktop.org,
        linux-fbdev@...r.kernel.org,
        Daniel Vetter <daniel.vetter@...ll.ch>,
        Helge Deller <deller@....de>,
        Hans de Goede <hdegoede@...hat.com>
Subject: Re: [PATCH 2/3] fbdev/simplefb: Cleanup fb_info in .fb_destroy
 rather than .remove

On Wed, May 04, 2022 at 11:57:22PM +0200, 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.
> 
> 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@...ll.ch>
> Signed-off-by: Javier Martinez Canillas <javierm@...hat.com>

I think this should have a Fixes: line for the patch from Thomas which
changed the remove_conflicting_fb code:

27599aacbaef ("fbdev: Hot-unplug firmware fb devices on forced removal")

I think we should also mention that strictly speaking the code flow is now
wrong, because hw cleanup (like iounmap) should be done from ->remove
while sw cleanup (like calling framebuffer_release()) is the only thing
that should be done from ->fb_destroy. But the current code matches what
was happening before 27599aacbaef so more minimal "fix"

With those details added Reviewed-by: Daniel Vetter <daniel.vetter@...ll.ch>

Same for the next patch.
-Daniel

> ---
> 
>  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));
>  }
> @@ -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;
>  }
> -- 
> 2.35.1
> 

-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch