[PATCH 5.5 41/80] drm/mst: Fix possible NULL pointer dereference in drm_dp_mst_process_up_req()

[Greg Kroah-Hartman]

From: Josà Roberto de Souza <jose.souza@xxxxxxxxx>

commit 8ccb5bf7619c6523e7a4384a84b72e7be804298c upstream.

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@xxxxxxxxxx>
Cc: Sean Paul <sean@xxxxxxxxxx>
Cc: <stable@xxxxxxxxxxxxxxx> # v5.5+
Signed-off-by: Josà Roberto de Souza <jose.souza@xxxxxxxxx>
[added cc to stable]
Signed-off-by: Lyude Paul <lyude@xxxxxxxxxx>
Link: https://patchwork.freedesktop.org/patch/msgid/20200129232448.84704-1-jose.souza@xxxxxxxxx
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>

---
 drivers/gpu/drm/drm_dp_mst_topology.c |    3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

--- a/drivers/gpu/drm/drm_dp_mst_topology.c
+++ b/drivers/gpu/drm/drm_dp_mst_topology.c
@@ -3772,7 +3772,8 @@ drm_dp_mst_process_up_req(struct drm_dp_
 		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);
 	}