Re: [PATCH] drm: hold gem reference until object is no longer accessed

[Sam Ravnborg <sam@ravnborg.org>]

Hi Steve and others.

On Fri, Jul 10, 2020 at 06:40:26PM -0400, Steve Cohen wrote:
> BUG: KASAN: use-after-free in drm_gem_open_ioctl
> 
> There is potential for use-after-free here if the GEM object
> handle is closed between the idr lookup and retrieving the size
> from the object since a local reference is not being held at that
> point. Hold the local reference while the object can still be
> accessed to resolve this.
> 
> Signed-off-by: Steve Cohen <cohens@codeaurora.org>
> ---
>  drivers/gpu/drm/drm_gem.c | 7 ++++---
>  1 file changed, 4 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/gpu/drm/drm_gem.c b/drivers/gpu/drm/drm_gem.c
> index 7bf628e..4b2891c 100644
> --- a/drivers/gpu/drm/drm_gem.c
> +++ b/drivers/gpu/drm/drm_gem.c
> @@ -898,14 +898,15 @@ drm_gem_open_ioctl(struct drm_device *dev, void *data,
>  
>  	/* drm_gem_handle_create_tail unlocks dev->object_name_lock. */
>  	ret = drm_gem_handle_create_tail(file_priv, obj, &handle);
> -	drm_gem_object_put_unlocked(obj);
>  	if (ret)
> -		return ret;
> +		goto out;
>  
>  	args->handle = handle;
>  	args->size = obj->size;
>  
> -	return 0;
> +out:
> +	drm_gem_object_put_unlocked(obj);
> +	return ret;

Lookign at drm_gem_flink_ioctl() that is implmented just above this
functions there are two things that I noted.

1) In drm_gem_flink_ioctl() the label is named "err:" - and my OCD likes
that similar labels have the same name.

2) The function takes the object_name_lock but fails to release it in
the error situation.

Danile Vetter updated the locking in
20228c447846da9399ead53fdbbc8ab69b47788a ("drm/gem: completely close gem_open vs. gem_close races")

but I failed to follow it all.

	Sam

>  }
>  
>  /**
> -- 
> The Qualcomm Innovation Center, Inc. is a member of the Code Aurora Forum,
> a Linux Foundation Collaborative Project
> 
> _______________________________________________
> dri-devel mailing list
> dri-devel@lists.freedesktop.org
> https://lists.freedesktop.org/mailman/listinfo/dri-devel