[PATCH net] net: bridge: mst: move switchdev call outside rcu

[Nikolay Aleksandrov <razor@blackwall.org>]

This is a follow-up of one of sashiko's pre-existing bug reports.
br_mst_set_state() calls switchdev_port_attr_set() for nonzero MSTIs
while holding rcu_read_lock() which invokes the blocking switchdev
notifier chain and may sleep. Nonzero MSTI changes come from netlink
with rtnl held. Move the switchdev call before entering the rcu section and
assert that rtnl is held.

The call cannot be deferred because netlink needs its error and extack.
Also DSA reads the old bridge MST state during the callback and checks it.
A deferred callback will be late and will see the updated state.

Fixes: 7ae9147f4312 ("net: bridge: mst: Notify switchdev drivers of MST state changes")
Signed-off-by: Nikolay Aleksandrov <razor@blackwall.org>
---
 net/bridge/br_mst.c | 20 ++++++++++++--------
 1 file changed, 12 insertions(+), 8 deletions(-)

diff --git a/net/bridge/br_mst.c b/net/bridge/br_mst.c
index 43a300ae6bfa..ce537cb71fe8 100644
--- a/net/bridge/br_mst.c
+++ b/net/bridge/br_mst.c
@@ -107,21 +107,24 @@ int br_mst_set_state(struct net_bridge_port *p, u16 msti, u8 state,
 	struct net_bridge_vlan *v;
 	int err = 0;
 
-	rcu_read_lock();
-	vg = nbp_vlan_group_rcu(p);
-	if (!vg)
-		goto out;
-
 	/* MSTI 0 (CST) state changes are notified via the regular
-	 * SWITCHDEV_ATTR_ID_PORT_STP_STATE.
+	 * SWITCHDEV_ATTR_ID_PORT_STP_STATE. All other MSTIs are handled via
+	 * netlink with RTNL held
 	 */
 	if (msti) {
+		ASSERT_RTNL();
+
 		err = switchdev_port_attr_set(p->dev, &attr, extack);
 		if (err && err != -EOPNOTSUPP)
 			goto out;
+		err = 0;
 	}
 
-	err = 0;
+	rcu_read_lock();
+	vg = nbp_vlan_group_rcu(p);
+	if (!vg)
+		goto out_rcu_unlock;
+
 	list_for_each_entry_rcu(v, &vg->vlan_list, vlist) {
 		if (v->brvlan->msti != msti)
 			continue;
@@ -129,8 +132,9 @@ int br_mst_set_state(struct net_bridge_port *p, u16 msti, u8 state,
 		br_mst_vlan_set_state(vg, v, state);
 	}
 
-out:
+out_rcu_unlock:
 	rcu_read_unlock();
+out:
 	return err;
 }
 
-- 
2.47.3