linux-kernel - Re: [PATCH] fbdev: vesafb: Fix a use-after-free due early fb

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Date:   Thu, 26 May 2022 22:03:05 +0200
From:   Pascal Ernster <dri-devel@...dfalcon.net>
To:     Javier Martinez Canillas <javierm@...hat.com>,
        linux-kernel@...r.kernel.org
Cc:     Daniel Vetter <daniel.vetter@...ll.ch>,
        Helge Deller <deller@....de>,
        Thomas Zimmermann <tzimmermann@...e.de>,
        dri-devel@...ts.freedesktop.org, linux-fbdev@...r.kernel.org
Subject: Re: [PATCH] fbdev: vesafb: Fix a use-after-free due early fb_info
 cleanup

[2022-05-26 21:47:52] Javier Martinez Canillas:
> Commit b3c9a924aab6 ("fbdev: vesafb: Cleanup fb_info in .fb_destroy rather
> than .remove") fixed a use-after-free error due the vesafb driver freeing
> the fb_info in the .remove handler instead of doing it in .fb_destroy.
> 
> This can happen if the .fb_destroy callback is executed after the .remove
> callback, since the former tries to access a pointer freed by the latter.
> 
> But that change didn't take into account that another possible scenario is
> that .fb_destroy is called before the .remove callback. For example, if no
> process has the fbdev chardev opened by the time the driver is removed.
> 
> If that's the case, fb_info will be freed when unregister_framebuffer() is
> called, making the fb_info pointer accessed in vesafb_remove() after that
> to no longer be valid.
> 
> To prevent that, move the expression containing the info->par to happen
> before the unregister_framebuffer() function call.
> 
> Fixes: b3c9a924aab6 ("fbdev: vesafb: Cleanup fb_info in .fb_destroy rather than .remove")
> Reported-by: Pascal Ernster <dri-devel@...dfalcon.net>
> Signed-off-by: Javier Martinez Canillas <javierm@...hat.com>

Tested on a bare metal machine and on a test VM that had both crashed 
when booting a kernel lacking this patch.
Again, thanks a lot for the quick fix! :)

Tested-by: Pascal Ernster <dri-devel@...dfalcon.net>


> ---
> 
>   drivers/video/fbdev/vesafb.c | 5 +++--
>   1 file changed, 3 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/video/fbdev/vesafb.c b/drivers/video/fbdev/vesafb.c
> index e25e8de5ff67..929d4775cb4b 100644
> --- a/drivers/video/fbdev/vesafb.c
> +++ b/drivers/video/fbdev/vesafb.c
> @@ -490,11 +490,12 @@ 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);
>   
> +	/* vesafb_destroy takes care of info cleanup */
> +	unregister_framebuffer(info);
> +
>   	return 0;
>   }
>