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Date:   Thu, 5 May 2022 15:02:21 +0200
From:   Daniel Vetter <daniel@ffwll.ch>
To:     Javier Martinez Canillas <javierm@redhat.com>
Cc:     linux-kernel@vger.kernel.org,
        Daniel Vetter <daniel.vetter@ffwll.ch>,
        Helge Deller <deller@gmx.de>, dri-devel@lists.freedesktop.org,
        linux-fbdev@vger.kernel.org
Subject: Re: [PATCH v2 4/4] fbdev: vesafb: Cleanup fb_info in .fb_destroy
 rather than .remove
Message-ID: <YnPK3RLPdtvFze+8@phenom.ffwll.local>
Mail-Followup-To: Javier Martinez Canillas <javierm@redhat.com>,
        linux-kernel@vger.kernel.org, Helge Deller <deller@gmx.de>,
        dri-devel@lists.freedesktop.org, linux-fbdev@vger.kernel.org
References: <20220505113128.264963-1-javierm@redhat.com>
 <20220505113128.264963-5-javierm@redhat.com>
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On Thu, May 05, 2022 at 01:31:27PM +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@ffwll.ch>
> Signed-off-by: Javier Martinez Canillas <javierm@redhat.com>
> ---
> 
> Changes in v2:
> - Also do the change for vesafb (Thomas Zimmermann).
> 
>  drivers/video/fbdev/vesafb.c | 14 +++++++++++---
>  1 file changed, 11 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/video/fbdev/vesafb.c b/drivers/video/fbdev/vesafb.c
> index df6de5a9dd4c..1f03a449e505 100644
> --- a/drivers/video/fbdev/vesafb.c
> +++ b/drivers/video/fbdev/vesafb.c
> @@ -179,6 +179,10 @@ static int vesafb_setcolreg(unsigned regno, unsigned red, unsigned green,
>  	return err;
>  }
>  
> +/*
> + * 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 vesafb_destroy(struct fb_info *info)
>  {
>  	struct vesafb_par *par = info->par;
> @@ -187,7 +191,13 @@ static void vesafb_destroy(struct fb_info *info)
>  	arch_phys_wc_del(par->wc_cookie);
>  	if (info->screen_base)
>  		iounmap(info->screen_base);
> +
> +	if (((struct vesafb_par *)(info->par))->region)
> +		release_region(0x3c0, 32);

This move seems rather iffy, so maybe justify it with "makes the code
exactly as busted before 27599aacbaef ("fbdev: Hot-unplug firmware fb
devices on forced removal")"

Also same comments as on v1 about adding more details about what/how this
fixes, with that: Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>

> +
>  	release_mem_region(info->apertures->ranges[0].base, info->apertures->ranges[0].size);
> +
> +	framebuffer_release(info);
>  }
>  
>  static struct fb_ops vesafb_ops = {
> @@ -484,10 +494,8 @@ static int vesafb_remove(struct platform_device *pdev)
>  {
>  	struct fb_info *info = platform_get_drvdata(pdev);
>  
> +	/* vesafb_destroy takes care of info cleanup */
>  	unregister_framebuffer(info);
> -	if (((struct vesafb_par *)(info->par))->region)
> -		release_region(0x3c0, 32);
> -	framebuffer_release(info);
>  
>  	return 0;
>  }
> -- 
> 2.35.1
> 

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