net: dsa: don't leak tagger-owned storage on switch driver unbind

In the initial commit dc452a471d ("net: dsa: introduce tagger-owned
storage for private and shared data"), we had a call to
tag_ops->disconnect(dst) issued from dsa_tree_free(), which is called at
tree teardown time.

There were problems with connecting to a switch tree as a whole, so this
got reworked to connecting to individual switches within the tree. In
this process, tag_ops->disconnect(ds) was made to be called only from
switch.c (cross-chip notifiers emitted as a result of dynamic tag proto
changes), but the normal driver teardown code path wasn't replaced with
anything.

Solve this problem by adding a function that does the opposite of
dsa_switch_setup_tag_protocol(), which is called from the equivalent
spot in dsa_switch_teardown(). The positioning here also ensures that we
won't have any use-after-free in tagging protocol (*rcv) ops, since the
teardown sequence is as follows:

dsa_tree_teardown
-> dsa_tree_teardown_master
   -> dsa_master_teardown
      -> unsets master->dsa_ptr, making no further packets match the
         ETH_P_XDSA packet type handler
-> dsa_tree_teardown_ports
   -> dsa_port_teardown
      -> dsa_slave_destroy
         -> unregisters DSA net devices, there is even a synchronize_net()
            in unregister_netdevice_many()
-> dsa_tree_teardown_switches
   -> dsa_switch_teardown
      -> dsa_switch_teardown_tag_protocol
         -> finally frees the tagger-owned storage

Fixes: 7f2973149c ("net: dsa: make tagging protocols connect to individual switches from a tree")
Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Reviewed-by: Saeed Mahameed <saeed@kernel.org>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Link: https://lore.kernel.org/r/20221114143551.1906361-1-vladimir.oltean@nxp.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

net/dsa/dsa2.c

@@ -864,6 +864,14 @@ static int dsa_switch_setup_tag_protocol(struct dsa_switch *ds)
 	return err;
 }
 
+static void dsa_switch_teardown_tag_protocol(struct dsa_switch *ds)
+{
+	const struct dsa_device_ops *tag_ops = ds->dst->tag_ops;
+
+	if (tag_ops->disconnect)
+		tag_ops->disconnect(ds);
+}
+
 static int dsa_switch_setup(struct dsa_switch *ds)
 {
 	struct dsa_devlink_priv *dl_priv;
@@ -953,6 +961,8 @@ static void dsa_switch_teardown(struct dsa_switch *ds)
 		ds->slave_mii_bus = NULL;
 	}
 
+	dsa_switch_teardown_tag_protocol(ds);
+
 	if (ds->ops->teardown)
 		ds->ops->teardown(ds);