drm: Check for lessee in DROP_MASTER ioctl

Don't let a lessee control what the current DRM master is set to;
that's the job of the "real" master. Otherwise, the lessee would
disable all access to master operations for the owner and all lessees
under it.

This matches the same check made in the SET_MASTER ioctl.

Signed-off-by: Keith Packard <keithp@keithp.com>
Fixes: 2ed077e467ee ("drm: Add drm_object lease infrastructure [v5]")
Cc: <stable@vger.kernel.org> # v4.15+
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20180119015159.1606-1-keithp@keithp.com

1 files changed, 6 insertions, 0 deletions

diff --git a/drivers/gpu/drm/drm_auth.c b/drivers/gpu/drm/drm_auth.c
index aad468d170a7..d9c0f7573905 100644
--- a/drivers/gpu/drm/drm_auth.c
+++ b/drivers/gpu/drm/drm_auth.c
@@ -230,6 +230,12 @@ int drm_dropmaster_ioctl(struct drm_device *dev, void *data,
         if (!dev->master)
                 goto out_unlock;
 
+        if (file_priv->master->lessor != NULL) {
+                DRM_DEBUG_LEASE("Attempt to drop lessee %d as master\n", file_priv->master->lessee_id);
+                ret = -EINVAL;
+                goto out_unlock;
+        }
+
         ret = 0;
         drm_drop_master(dev, file_priv);
 out_unlock: