drm/dp_mst: Ensure mst_primary pointer is valid in drm_dp_mst_handle_up_req()

While receiving an MST up request message from one thread in
drm_dp_mst_handle_up_req(), the MST topology could be removed from
another thread via drm_dp_mst_topology_mgr_set_mst(false), freeing
mst_primary and setting drm_dp_mst_topology_mgr::mst_primary to NULL.
This could lead to a NULL deref/use-after-free of mst_primary in
drm_dp_mst_handle_up_req().

Avoid the above by holding a reference for mst_primary in
drm_dp_mst_handle_up_req() while it's used.

v2: Fix kfreeing the request if getting an mst_primary reference fails.

Cc: Lyude Paul <lyude@redhat.com>
Reviewed-by: Lyude Paul <lyude@redhat.com> (v1)
Signed-off-by: Imre Deak <imre.deak@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20241204132007.3132494-1-imre.deak@intel.com

Author: ideak

drivers/gpu/drm/display/drm_dp_mst_topology.c

@@ -4109,9 +4109,10 @@ static void drm_dp_mst_up_req_work(struct work_struct *work)
 static int drm_dp_mst_handle_up_req(struct drm_dp_mst_topology_mgr *mgr)
 {
 	struct drm_dp_pending_up_req *up_req;
+	struct drm_dp_mst_branch *mst_primary;
 
 	if (!drm_dp_get_one_sb_msg(mgr, true, NULL))
-		goto out;
+		goto out_clear_reply;
 
 	if (!mgr->up_req_recv.have_eomt)
 		return 0;
@@ -4129,10 +4130,19 @@ static int drm_dp_mst_handle_up_req(struct drm_dp_mst_topology_mgr *mgr)
 		drm_dbg_kms(mgr->dev, "Received unknown up req type, ignoring: %x\n",
 			    up_req->msg.req_type);
 		kfree(up_req);
-		goto out;
+		goto out_clear_reply;
+	}
+
+	mutex_lock(&mgr->lock);
+	mst_primary = mgr->mst_primary;
+	if (!mst_primary || !drm_dp_mst_topology_try_get_mstb(mst_primary)) {
+		mutex_unlock(&mgr->lock);
+		kfree(up_req);
+		goto out_clear_reply;
 	}
+	mutex_unlock(&mgr->lock);
 
-	drm_dp_send_up_ack_reply(mgr, mgr->mst_primary, up_req->msg.req_type,
+	drm_dp_send_up_ack_reply(mgr, mst_primary, up_req->msg.req_type,
 				 false);
 
 	if (up_req->msg.req_type == DP_CONNECTION_STATUS_NOTIFY) {
@@ -4149,13 +4159,13 @@ static int drm_dp_mst_handle_up_req(struct drm_dp_mst_topology_mgr *mgr)
 			    conn_stat->peer_device_type);
 
 		mutex_lock(&mgr->probe_lock);
-		handle_csn = mgr->mst_primary->link_address_sent;
+		handle_csn = mst_primary->link_address_sent;
 		mutex_unlock(&mgr->probe_lock);
 
 		if (!handle_csn) {
 			drm_dbg_kms(mgr->dev, "Got CSN before finish topology probing. Skip it.");
 			kfree(up_req);
-			goto out;
+			goto out_put_primary;
 		}
 	} else if (up_req->msg.req_type == DP_RESOURCE_STATUS_NOTIFY) {
 		const struct drm_dp_resource_status_notify *res_stat =
@@ -4172,7 +4182,9 @@ static int drm_dp_mst_handle_up_req(struct drm_dp_mst_topology_mgr *mgr)
 	mutex_unlock(&mgr->up_req_lock);
 	queue_work(system_long_wq, &mgr->up_req_work);
 
-out:
+out_put_primary:
+	drm_dp_mst_topology_put_mstb(mst_primary);
+out_clear_reply:
 	memset(&mgr->up_req_recv, 0, sizeof(struct drm_dp_sideband_msg_rx));
 	return 0;
 }