net: dsa: existing DSA masters cannot join upper interfaces

All the traffic to/from a DSA master is supposed to be distributed among
its DSA switch upper interfaces, so we should not allow other upper
device kinds.

An exception to this is DSA_TAG_PROTO_NONE (switches with no DSA tags),
and in that case it is actually expected to create e.g. VLAN interfaces
on the master. But for those, netdev_uses_dsa(master) returns false, so
the restriction doesn't apply.

The motivation for this change is to allow LAG interfaces of DSA masters
to be DSA masters themselves. We want to restrict the user's degrees of
freedom by 1: the LAG should already have all DSA masters as lowers, and
while lower ports of the LAG can be removed, none can be added after the
fact.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

net/dsa/slave.c

@@ -2699,6 +2699,35 @@ dsa_slave_prechangeupper_sanity_check(struct net_device *dev,
 	return NOTIFY_DONE;
 }
 
+static int
+dsa_master_prechangeupper_sanity_check(struct net_device *master,
+				       struct netdev_notifier_changeupper_info *info)
+{
+	struct netlink_ext_ack *extack;
+
+	if (!netdev_uses_dsa(master))
+		return NOTIFY_DONE;
+
+	if (!info->linking)
+		return NOTIFY_DONE;
+
+	/* Allow DSA switch uppers */
+	if (dsa_slave_dev_check(info->upper_dev))
+		return NOTIFY_DONE;
+
+	/* Allow bridge uppers of DSA masters, subject to further
+	 * restrictions in dsa_bridge_prechangelower_sanity_check()
+	 */
+	if (netif_is_bridge_master(info->upper_dev))
+		return NOTIFY_DONE;
+
+	extack = netdev_notifier_info_to_extack(&info->info);
+
+	NL_SET_ERR_MSG_MOD(extack,
+			   "DSA master cannot join unknown upper interfaces");
+	return notifier_from_errno(-EBUSY);
+}
+
 /* Don't allow bridging of DSA masters, since the bridge layer rx_handler
  * prevents the DSA fake ethertype handler to be invoked, so we don't get the
  * chance to strip off and parse the DSA switch tag protocol header (the bridge
@@ -2753,6 +2782,10 @@ static int dsa_slave_netdevice_event(struct notifier_block *nb,
 		if (notifier_to_errno(err))
 			return err;
 
+		err = dsa_master_prechangeupper_sanity_check(dev, info);
+		if (notifier_to_errno(err))
+			return err;
+
 		err = dsa_bridge_prechangelower_sanity_check(dev, info);
 		if (notifier_to_errno(err))
 			return err;