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Date:   Wed, 27 Feb 2019 20:12:02 -0300
From:   Rodrigo Siqueira <rodrigosiqueiramelo@...il.com>
To:     Eric Biggers <ebiggers@...nel.org>
Cc:     dri-devel@...ts.freedesktop.org,
        syzkaller-bugs <syzkaller-bugs@...glegroups.com>,
        linux-kernel@...r.kernel.org,
        Haneen Mohammed <hamohammed.sa@...il.com>,
        Daniel Vetter <daniel.vetter@...ll.ch>,
        Chris Wilson <chris@...is-wilson.co.uk>, stable@...r.kernel.org
Subject: Re: [PATCH] drm/vkms: fix use-after-free when
 drm_gem_handle_create() fails

On 02/26, Eric Biggers wrote:
> From: Eric Biggers <ebiggers@...gle.com>
> 
> If drm_gem_handle_create() fails in vkms_gem_create(), then the
> vkms_gem_object is freed twice: once when the reference is dropped by
> drm_gem_object_put_unlocked(), and again by the extra calls to
> drm_gem_object_release() and kfree().
> 
> Fix it by skipping the second release and free.
> 
> This bug was originally found in the vgem driver by syzkaller using
> fault injection, but I noticed it's also present in the vkms driver.
> 
> Fixes: 559e50fd34d1 ("drm/vkms: Add dumb operations")
> Cc: Rodrigo Siqueira <rodrigosiqueiramelo@...il.com>
> Cc: Haneen Mohammed <hamohammed.sa@...il.com>
> Cc: Daniel Vetter <daniel.vetter@...ll.ch>
> Cc: Chris Wilson <chris@...is-wilson.co.uk>
> Cc: stable@...r.kernel.org
> Signed-off-by: Eric Biggers <ebiggers@...gle.com>
> ---
>  drivers/gpu/drm/vkms/vkms_gem.c | 5 +----
>  1 file changed, 1 insertion(+), 4 deletions(-)
> 
> diff --git a/drivers/gpu/drm/vkms/vkms_gem.c b/drivers/gpu/drm/vkms/vkms_gem.c
> index 138b0bb325cf9..69048e73377dc 100644
> --- a/drivers/gpu/drm/vkms/vkms_gem.c
> +++ b/drivers/gpu/drm/vkms/vkms_gem.c
> @@ -111,11 +111,8 @@ struct drm_gem_object *vkms_gem_create(struct drm_device *dev,
>  
>  	ret = drm_gem_handle_create(file, &obj->gem, handle);
>  	drm_gem_object_put_unlocked(&obj->gem);
> -	if (ret) {
> -		drm_gem_object_release(&obj->gem);
> -		kfree(obj);
> +	if (ret)
>  		return ERR_PTR(ret);
> -	}
>  
>  	return &obj->gem;
>  }
> -- 
> 2.21.0.rc2.261.ga7da99ff1b-goog
> 

Hi,

Thanks for your patch! :)

The patch looks good for me. I also tested it under the IGT tests on my
local VM and everything was fine.

Reviewed-by: Rodrigo Siqueira <rodrigosiqueiramelo@...il.com>

-- 
Rodrigo Siqueira
https://siqueira.tech
Graduate Student
Department of Computer Science
University of São Paulo

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