net: dsa: don't assign an error value to tag_ops

Use a temporary variable to hold the return value from
dsa_tag_driver_get() instead of assigning it to dst->tag_ops. Leaving
an error value in dst->tag_ops can result in deferencing an invalid
pointer when a deferred switch configuration happens later.

Fixes: 357f203b ("net: dsa: keep a copy of the tagging protocol in the DSA switch tree")
Signed-off-by: George McCollister <george.mccollister@gmail.com>
Reviewed-by: Vladimir Oltean <olteanv@gmail.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/dsa/dsa2.c

@@ -1066,6 +1066,7 @@ static int dsa_port_parse_cpu(struct dsa_port *dp, struct net_device *master)
 {
 	struct dsa_switch *ds = dp->ds;
 	struct dsa_switch_tree *dst = ds->dst;
+	const struct dsa_device_ops *tag_ops;
 	enum dsa_tag_protocol tag_protocol;
 
 	tag_protocol = dsa_get_tag_protocol(dp, master);
@@ -1080,14 +1081,16 @@ static int dsa_port_parse_cpu(struct dsa_port *dp, struct net_device *master)
 		 * nothing to do here.
 		 */
 	} else {
-		dst->tag_ops = dsa_tag_driver_get(tag_protocol);
-		if (IS_ERR(dst->tag_ops)) {
-			if (PTR_ERR(dst->tag_ops) == -ENOPROTOOPT)
+		tag_ops = dsa_tag_driver_get(tag_protocol);
+		if (IS_ERR(tag_ops)) {
+			if (PTR_ERR(tag_ops) == -ENOPROTOOPT)
 				return -EPROBE_DEFER;
 			dev_warn(ds->dev, "No tagger for this switch\n");
 			dp->master = NULL;
-			return PTR_ERR(dst->tag_ops);
+			return PTR_ERR(tag_ops);
 		}
+
+		dst->tag_ops = tag_ops;
 	}
 
 	dp->master = master;