br: fix use of ->rx_handler_data in code executed on non-rx_handler path

br_stp_rcv() is reached by non-rx_handler path. That means there is no
guarantee that dev is bridge port and therefore simple NULL check of
->rx_handler_data is not enough. There is need to check if dev is really
bridge port and since only rcu read lock is held here, do it by checking
->rx_handler pointer.

Note that synchronize_net() in netdev_rx_handler_unregister() ensures
this approach as valid.

Introduced originally by:
commit f350a0a8
  "bridge: use rx_handler_data pointer to store net_bridge_port pointer"

Fixed but not in the best way by:
commit b5ed54e9
  "bridge: fix RCU races with bridge port"

Reintroduced by:
commit 716ec052
  "bridge: fix NULL pointer deref of br_port_get_rcu"

Please apply to stable trees as well. Thanks.

RH bugzilla reference: https://bugzilla.redhat.com/show_bug.cgi?id=1025770Reported-by: Laine Stump <laine@redhat.com>
Debugged-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Jiri Pirko <jiri@resnulli.us>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Acked-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/bridge/br_private.h

@@ -426,6 +426,16 @@ netdev_features_t br_features_recompute(struct net_bridge *br,
 int br_handle_frame_finish(struct sk_buff *skb);
 rx_handler_result_t br_handle_frame(struct sk_buff **pskb);
 
+static inline bool br_rx_handler_check_rcu(const struct net_device *dev)
+{
+	return rcu_dereference(dev->rx_handler) == br_handle_frame;
+}
+
+static inline struct net_bridge_port *br_port_get_check_rcu(const struct net_device *dev)
+{
+	return br_rx_handler_check_rcu(dev) ? br_port_get_rcu(dev) : NULL;
+}
+
 /* br_ioctl.c */
 int br_dev_ioctl(struct net_device *dev, struct ifreq *rq, int cmd);
 int br_ioctl_deviceless_stub(struct net *net, unsigned int cmd,

net/bridge/br_stp_bpdu.c

@@ -153,7 +153,7 @@ void br_stp_rcv(const struct stp_proto *proto, struct sk_buff *skb,
 	if (buf[0] != 0 || buf[1] != 0 || buf[2] != 0)
 		goto err;
 
-	p = br_port_get_rcu(dev);
+	p = br_port_get_check_rcu(dev);
 	if (!p)
 		goto err;