[PATCH] drm: fix DRM_IOCTL_MODE_GETFB handle-leak

[Chris Wilson]

On Sun, Aug 25, 2013 at 11:12:39PM +0200, David Herrmann wrote:
> DRM_IOCTL_MODE_GETFB is used to retrieve information about a given
> framebuffer ID. It is a read-only helper and was thus declassified for
> unprivileged access in:
> 
>   commit a14b1b42477c5ef089fcda88cbaae50d979eb8f9
>   Author: Mandeep Singh Baines <mandeep.baines at gmail.com>
>   Date:   Fri Jan 20 12:11:16 2012 -0800
> 
>       drm: remove master fd restriction on mode setting getters
> 
> However, beside width, height and stride information,
> DRM_IOCTL_MODE_GETFB also passes back a handle to the underlying gem-bo of
> the framebuffer. This handle allows users to mmap() it and read or write
> into it. Obviously, this should be restricted to DRM-Master.
> 
> With the current setup, *any* process with access to /dev/dri/card0 (which
> means any process with access to hardware-accelerated rendering) can
> access the current screen framebuffer and modify it ad libitum.
> 
> For backwards-compatibility reasons we want to keep the
> DRM_IOCTL_MODE_GETFB call unprivileged. Besides, it provides quite useful
> information regarding screen setup. So we simply test whether the caller
> is the current DRM-Master and if not, we return 0 as handle, which is
> always invalid. A following DRM_IOCTL_GEM_CLOSE on this handle will fail
> with EINVAL, but we accept this. Users shouldn't test for errors during
> GEM_CLOSE, anyway. And it is still better than a failing MODE_GETFB call.

Hmm, I do have userspace that takes advantage of this already for reads. So
if you can make it a DRM_MASTER || cappable(SYS_ADMID) privileged
operation instead, I can live with that.
-Chris

-- 
Chris Wilson, Intel Open Source Technology Centre