Commit 101b96f3 authored Aug 26, 2013 by David Herrmann Committed by Dave Airlie Sep 02, 2013

drm: fix DRM_IOCTL_MODE_GETFB handle-leak



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 a14b1b42
  Author: Mandeep Singh Baines <mandeep.baines@gmail.com>
  Date:   Fri Jan 20 12:11:16 2012 -0800

      drm: remove master fd restriction on mode setting getters

However, alongside width, height and stride information,
DRM_IOCTL_MODE_GETFB also passes back a handle to the underlying buffer 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 as a failing MODE_GETFB call.

v2: add capable(CAP_SYS_ADMIN) check for compatibility with i-g-t

Cc: <stable@vger.kernel.org>
Signed-off-by: David Herrmann <dh.herrmann@gmail.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Dave Airlie <airlied@redhat.com>

parent a3376e3e

drivers/gpu/drm/drm_crtc.c

+15 −3

Original line number	Diff line number	Diff line
		@@ -2641,10 +2641,22 @@ int drm_mode_getfb(struct drm_device *dev,
		r->depth = fb->depth;
		r->bpp = fb->bits_per_pixel;
		r->pitch = fb->pitches[0];
		if (fb->funcs->create_handle)
		ret = fb->funcs->create_handle(fb, file_priv, &r->handle);
		else
		if (fb->funcs->create_handle) {
		if (file_priv->is_master \|\| capable(CAP_SYS_ADMIN)) {
		ret = fb->funcs->create_handle(fb, file_priv,
		&r->handle);
		} else {
		/* GET_FB() is an unprivileged ioctl so we must not
		* return a buffer-handle to non-master processes! For
		* backwards-compatibility reasons, we cannot make
		* GET_FB() privileged, so just return an invalid handle
		* for non-masters. */
		r->handle = 0;
		ret = 0;
		}
		} else {
		ret = -ENODEV;
		}

		drm_framebuffer_unreference(fb);

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