diff --git a/net/mctp/route.c b/net/mctp/route.c
index 0381377ab760..59ad60b88563 100644
--- a/net/mctp/route.c
+++ b/net/mctp/route.c
@@ -359,6 +359,7 @@ static void mctp_flow_prepare_output(struct sk_buff *skb, struct mctp_dev *dev)
 {
 	struct mctp_sk_key *key;
 	struct mctp_flow *flow;
+	unsigned long flags;
 
 	flow = skb_ext_find(skb, SKB_EXT_MCTP);
 	if (!flow)
@@ -366,12 +367,14 @@ static void mctp_flow_prepare_output(struct sk_buff *skb, struct mctp_dev *dev)
 
 	key = flow->key;
 
-	if (key->dev) {
-		WARN_ON(key->dev != dev);
-		return;
-	}
+	spin_lock_irqsave(&key->lock, flags);
 
-	mctp_dev_set_key(dev, key);
+	if (!key->dev)
+		mctp_dev_set_key(dev, key);
+	else
+		WARN_ON(key->dev != dev);
+
+	spin_unlock_irqrestore(&key->lock, flags);
 }
 #else
 static void mctp_skb_set_flow(struct sk_buff *skb, struct mctp_sk_key *key) {}