[PATCH 2/3] drm/display/dp_mst: Fix drm_atomic_get_mst_topology_state()

[Lyude Paul]

I noticed a rather surprising issue here while working on removing all of
the non-atomic MST code: drm_atomic_get_mst_topology_state() doesn't check
the return value of drm_atomic_get_private_obj_state() and instead just
passes it directly to to_dp_mst_topology_state(). This means that if we
hit a deadlock or something else which would return an error code pointer,
we'll likely segfault the kernel.

This is definitely another one of those fixes where I'm astonished we
somehow managed never to discover this issue until now…

Signed-off-by: Lyude Paul <lyude@xxxxxxxxxx>
Fixes: a4370c777406 ("drm/atomic: Make private objs proper objects")
Cc: Ville Syrjälä <ville.syrjala@xxxxxxxxxxxxxxx>
Cc: <stable@xxxxxxxxxxxxxxx> # v4.14+
---
 drivers/gpu/drm/display/drm_dp_mst_topology.c | 2 +-
 include/drm/display/drm_dp_mst_helper.h       | 2 ++
 2 files changed, 3 insertions(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/display/drm_dp_mst_topology.c b/drivers/gpu/drm/display/drm_dp_mst_topology.c
index d84673b3294b..d6e595b95f07 100644
--- a/drivers/gpu/drm/display/drm_dp_mst_topology.c
+++ b/drivers/gpu/drm/display/drm_dp_mst_topology.c
@@ -5468,7 +5468,7 @@ EXPORT_SYMBOL(drm_dp_mst_topology_state_funcs);
 struct drm_dp_mst_topology_state *drm_atomic_get_mst_topology_state(struct drm_atomic_state *state,
 								    struct drm_dp_mst_topology_mgr *mgr)
 {
-	return to_dp_mst_topology_state(drm_atomic_get_private_obj_state(state, &mgr->base));
+	return to_dp_mst_topology_state_safe(drm_atomic_get_private_obj_state(state, &mgr->base));
 }
 EXPORT_SYMBOL(drm_atomic_get_mst_topology_state);
 
diff --git a/include/drm/display/drm_dp_mst_helper.h b/include/drm/display/drm_dp_mst_helper.h
index 10adec068b7f..fe7577e7f305 100644
--- a/include/drm/display/drm_dp_mst_helper.h
+++ b/include/drm/display/drm_dp_mst_helper.h
@@ -541,6 +541,8 @@ struct drm_dp_payload {
 };
 
 #define to_dp_mst_topology_state(x) container_of(x, struct drm_dp_mst_topology_state, base)
+#define to_dp_mst_topology_state_safe(x) \
+	container_of_safe(x, struct drm_dp_mst_topology_state, base)
 
 struct drm_dp_vcpi_allocation {
 	struct drm_dp_mst_port *port;
-- 
2.35.3