Re: [PATCH v2 2/2] drm: allow render capable master with DRM_AUTH ioctls

[Daniel Vetter <daniel@ffwll.ch>]

On Mon, Jan 14, 2019 at 08:54:08AM +0000, Emil Velikov wrote:
> From: Emil Velikov <emil.velikov@collabora.com>
> 
> There are cases (in mesa and applications) where one would open the
> primary node without properly authenticating the client.
> 
> Sometimes we don't check if the authentication succeeds, but there's
> also cases we simply forget to do it.
> 
> The former was a case for Mesa where it did not not check the return
> value of drmGetMagic() [1]. That was fixed recently although, there's
> the question of older drivers or other apps that exbibit this behaviour.
> 
> While omitting the call results in issues as seen in [2] and [3].
> 
> In the libva case, libva itself doesn't authenticate the DRM client and
> the vaGetDisplayDRM documentation doesn't mention if the app should
> either.
> 
> As of today, the official vainfo utility doesn't authenticate.
> 
> To workaround issues like these, some users resort to running their apps
> under sudo. Which admittedly isn't always a good idea.
> 
> Since any DRIVER_RENDER driver has sufficient isolation between clients,
> we can use that, for unauthenticated [primary node] ioctls that require
> DRM_AUTH. But only if the respective ioctl is tagged as DRM_RENDER_ALLOW.
> 
> v2:
> - Rework/simplify if check (Daniel V)
> - Add examples to commit messages, elaborate. (Daniel V)
> 
> [1] https://gitlab.freedesktop.org/mesa/mesa/blob/2bc1f5c2e70fe3b4d41f060af9859bc2a94c5b62/src/egl/drivers/dri2/platform_wayland.c#L1136
> [2] https://lists.freedesktop.org/archives/libva/2016-July/004185.html
> [3] https://gitlab.freedesktop.org/mesa/kmscube/issues/1
> Testcase: igt/core_unauth_vs_render
> Cc: intel-gfx@lists.freedesktop.org
> Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
> ---
> Daniel, the if conditionals did not work exactly as you put them.
> This is the closest thing that I can think of.
> ---
>  drivers/gpu/drm/drm_ioctl.c | 20 ++++++++++++++++----
>  1 file changed, 16 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/gpu/drm/drm_ioctl.c b/drivers/gpu/drm/drm_ioctl.c
> index 94bd872d56c4..08a0b4cc3a76 100644
> --- a/drivers/gpu/drm/drm_ioctl.c
> +++ b/drivers/gpu/drm/drm_ioctl.c
> @@ -507,6 +507,13 @@ int drm_version(struct drm_device *dev, void *data,
>  	return err;
>  }
>  
> +static inline bool
> +drm_render_driver_and_ioctl(const struct drm_device *dev, u32 flags)
> +{
> +	return drm_core_check_feature(dev, DRIVER_RENDER) &&
> +		(flags & DRM_RENDER_ALLOW);
> +}
> +
>  /**
>   * drm_ioctl_permit - Check ioctl permissions against caller
>   *
> @@ -521,14 +528,19 @@ int drm_version(struct drm_device *dev, void *data,
>   */
>  int drm_ioctl_permit(u32 flags, struct drm_file *file_priv)
>  {
> +	const struct drm_device *dev = file_priv->minor->dev;
> +
>  	/* ROOT_ONLY is only for CAP_SYS_ADMIN */
>  	if (unlikely((flags & DRM_ROOT_ONLY) && !capable(CAP_SYS_ADMIN)))
>  		return -EACCES;
>  
> -	/* AUTH is only for authenticated or render client */
> -	if (unlikely((flags & DRM_AUTH) && !drm_is_render_client(file_priv) &&
> -		     !file_priv->authenticated))
> -		return -EACCES;
> +	/* AUTH is only for master ... */
> +	if ((flags & DRM_AUTH) && drm_is_primary_client(file_priv)) {
> +		/* authenticated ones, or render capable on DRM_RENDER_ALLOW. */
> +		if (unlikely(!file_priv->authenticated) &&
> +		    unlikely(!drm_render_driver_and_ioctl(dev, flags)))

The double-unlikely looks a bit strange, I'd move it out so there's only
one. But this is correct too (because unlikely() && unlikely == unlikely(
&& )). Either way:

Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>

> +			return -EACCES;
> +	}
>  
>  	/* MASTER is only for master or control clients */
>  	if (unlikely((flags & DRM_MASTER) &&
> -- 
> 2.20.1
> 
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-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch
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