net: bridge: br_vlan_get_pvid_rcu() should dereference the VLAN group under RCU

When calling the RCU brother of br_vlan_get_pvid(), lockdep warns:

=============================
WARNING: suspicious RCU usage
5.9.0-rc3-01631-g13c17acb8e38-dirty #814 Not tainted
-----------------------------
net/bridge/br_private.h:1054 suspicious rcu_dereference_protected() usage!

Call trace:
 lockdep_rcu_suspicious+0xd4/0xf8
 __br_vlan_get_pvid+0xc0/0x100
 br_vlan_get_pvid_rcu+0x78/0x108

The warning is because br_vlan_get_pvid_rcu() calls nbp_vlan_group()
which calls rtnl_dereference() instead of rcu_dereference(). In turn,
rtnl_dereference() calls rcu_dereference_protected() which assumes
operation under an RCU write-side critical section, which obviously is
not the case here. So, when the incorrect primitive is used to access
the RCU-protected VLAN group pointer, READ_ONCE() is not used, which may
cause various unexpected problems.

I'm sad to say that br_vlan_get_pvid() and br_vlan_get_pvid_rcu() cannot
share the same implementation. So fix the bug by splitting the 2
functions, and making br_vlan_get_pvid_rcu() retrieve the VLAN groups
under proper locking annotations.

Fixes: 7582f5b7 ("bridge: add br_vlan_get_pvid_rcu()")
Signed-off-by: NVladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/bridge/br_vlan.c

@@ -1288,11 +1288,13 @@ void br_vlan_get_stats(const struct net_bridge_vlan *v,
 	}
 }
 
-static int __br_vlan_get_pvid(const struct net_device *dev,
-			      struct net_bridge_port *p, u16 *p_pvid)
+int br_vlan_get_pvid(const struct net_device *dev, u16 *p_pvid)
 {
 	struct net_bridge_vlan_group *vg;
+	struct net_bridge_port *p;
 
+	ASSERT_RTNL();
+	p = br_port_get_check_rtnl(dev);
 	if (p)
 		vg = nbp_vlan_group(p);
 	else if (netif_is_bridge_master(dev))
@@ -1303,18 +1305,23 @@ static int __br_vlan_get_pvid(const struct net_device *dev,
 	*p_pvid = br_get_pvid(vg);
 	return 0;
 }
-
-int br_vlan_get_pvid(const struct net_device *dev, u16 *p_pvid)
-{
-	ASSERT_RTNL();
-
-	return __br_vlan_get_pvid(dev, br_port_get_check_rtnl(dev), p_pvid);
-}
 EXPORT_SYMBOL_GPL(br_vlan_get_pvid);
 
 int br_vlan_get_pvid_rcu(const struct net_device *dev, u16 *p_pvid)
 {
-	return __br_vlan_get_pvid(dev, br_port_get_check_rcu(dev), p_pvid);
+	struct net_bridge_vlan_group *vg;
+	struct net_bridge_port *p;
+
+	p = br_port_get_check_rcu(dev);
+	if (p)
+		vg = nbp_vlan_group_rcu(p);
+	else if (netif_is_bridge_master(dev))
+		vg = br_vlan_group_rcu(netdev_priv(dev));
+	else
+		return -EINVAL;
+
+	*p_pvid = br_get_pvid(vg);
+	return 0;
 }
 EXPORT_SYMBOL_GPL(br_vlan_get_pvid_rcu);