[PATCH] drm/mst: Fix possible NULL pointer dereference in drm_dp_mst_process_up_req()

[José Roberto de Souza]

According to DP specification, DP_SINK_EVENT_NOTIFY is also a
broadcast message but as this function only handles
DP_CONNECTION_STATUS_NOTIFY I will only make the static
analyzer that caught this issue happy by not calling
drm_dp_get_mst_branch_device_by_guid() with a NULL guid, causing
drm_dp_mst_process_up_req() to return in the "if (!mstb)" right
bellow.

Fixes: 9408cc94eb04 ("drm/dp_mst: Handle UP requests asynchronously")
Cc: Lyude Paul <lyude at redhat.com>
Cc: Sean Paul <sean at poorly.run>
Signed-off-by: José Roberto de Souza <jose.souza at intel.com>
---
 drivers/gpu/drm/drm_dp_mst_topology.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/drm_dp_mst_topology.c b/drivers/gpu/drm/drm_dp_mst_topology.c
index 23cf46bfef74..a811247cecfe 100644
--- a/drivers/gpu/drm/drm_dp_mst_topology.c
+++ b/drivers/gpu/drm/drm_dp_mst_topology.c
@@ -3829,7 +3829,8 @@ drm_dp_mst_process_up_req(struct drm_dp_mst_topology_mgr *mgr,
 		else if (msg->req_type == DP_RESOURCE_STATUS_NOTIFY)
 			guid = msg->u.resource_stat.guid;
 
-		mstb = drm_dp_get_mst_branch_device_by_guid(mgr, guid);
+		if (guid)
+			mstb = drm_dp_get_mst_branch_device_by_guid(mgr, guid);
 	} else {
 		mstb = drm_dp_get_mst_branch_device(mgr, hdr->lct, hdr->rad);
 	}
-- 
2.25.0