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 a14b1b42477c5ef089fcda88cbaae50d979eb8f9
  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>

diff --git a/drivers/gpu/drm/drm_crtc.c b/drivers/gpu/drm/drm_crtc.c
index 5ebc972..bff2fa9 100644 (file)
--- a/drivers/gpu/drm/drm_crtc.c
+++ b/drivers/gpu/drm/drm_crtc.c
@@ -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);