drm/mst: Avoid dereferencing a NULL mstb in drm_dp_mst_handle_up_req()

In case of an unknown broadcast message is sent mstb will remain unset,
so check for this.

Cc: Dave Airlie <airlied@redhat.com>
Cc: Lyude <lyude@redhat.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Lyude <lyude@redhat.com>
Cc: stable@vger.kernel.org
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20170719114330.26540-3-imre.deak@intel.com

1 files changed, 3 insertions, 1 deletions

diff --git a/drivers/gpu/drm/drm_dp_mst_topology.c b/drivers/gpu/drm/drm_dp_mst_topology.c
index ce0e58442939..7a6201133eb9 100644
--- a/drivers/gpu/drm/drm_dp_mst_topology.c
+++ b/drivers/gpu/drm/drm_dp_mst_topology.c
@@ -2324,7 +2324,9 @@ static int drm_dp_mst_handle_up_req(struct drm_dp_mst_topology_mgr *mgr)
                         DRM_DEBUG_KMS("Got RSN: pn: %d avail_pbn %d\n", msg.u.resource_stat.port_number, msg.u.resource_stat.available_pbn);
                 }
 
-                drm_dp_put_mst_branch_device(mstb);
+                if (mstb)
+                        drm_dp_put_mst_branch_device(mstb);
+
                 memset(&mgr->up_req_recv, 0, sizeof(struct drm_dp_sideband_msg_rx));
         }
         return ret;