[PATCH] bridge: Fix possible races between assigning rx_handler_data and setting IFF_BRIDGE_PORT bit

From: Zhang Zhengming <redacted>

There is a crash in the function br_get_link_af_size_filtered,
as the port_exists(dev) is true and the rx_handler_data of dev is NULL.
But the rx_handler_data of dev is correct saved in vmcore.

The oops looks something like:
 ...
 pc : br_get_link_af_size_filtered+0x28/0x1c8 [bridge]
 ...
 Call trace:
  br_get_link_af_size_filtered+0x28/0x1c8 [bridge]
  if_nlmsg_size+0x180/0x1b0
  rtnl_calcit.isra.12+0xf8/0x148
  rtnetlink_rcv_msg+0x334/0x370
  netlink_rcv_skb+0x64/0x130
  rtnetlink_rcv+0x28/0x38
  netlink_unicast+0x1f0/0x250
  netlink_sendmsg+0x310/0x378
  sock_sendmsg+0x4c/0x70
  __sys_sendto+0x120/0x150
  __arm64_sys_sendto+0x30/0x40
  el0_svc_common+0x78/0x130
  el0_svc_handler+0x38/0x78
  el0_svc+0x8/0xc

In br_add_if(), we found there is no guarantee that
assigning rx_handler_data to dev->rx_handler_data
will before setting the IFF_BRIDGE_PORT bit of priv_flags.
So there is a possible data competition:

CPU 0:                                                        CPU 1:
(RCU read lock)                                               (RTNL lock)
rtnl_calcit()                                                 br_add_slave()
  if_nlmsg_size()                                               br_add_if()
    br_get_link_af_size_filtered()                              -> netdev_rx_handler_register
                                                                    ...
                                                                    // The order is not guaranteed
      ...                                                           -> dev->priv_flags |= IFF_BRIDGE_PORT;
      // The IFF_BRIDGE_PORT bit of priv_flags has been set
      -> if (br_port_exists(dev)) {
        // The dev->rx_handler_data has NOT been assigned
        -> p = br_port_get_rcu(dev);
        ....
                                                                    -> rcu_assign_pointer(dev->rx_handler_data, rx_handler_data);
                                                                     ...

Fix this by adding memory barrier instruction to ensure the order.

Signed-off-by: Zhang Zhengming <redacted>
Reviewed-by: Zhao Lei <redacted>
Reviewed-by: Wang Xiaogang <redacted>
---
 net/bridge/br_if.c      | 6 ++++++
 net/bridge/br_netlink.c | 1 +
 2 files changed, 7 insertions(+)

diff --git a/net/bridge/br_if.c b/net/bridge/br_if.c
index f7d2f47..42895be 100644
--- a/net/bridge/br_if.c
+++ b/net/bridge/br_if.c

@@ -636,6 +636,12 @@ int br_add_if(struct net_bridge *br, struct net_device *dev,
 	err = netdev_rx_handler_register(dev, br_get_rx_handler(dev), p);
 	if (err)
 		goto err4;
+
+	/* Make sure dev->rx_handler_data is written in netdev_rx_handler_register
+	 * before the bit IFF_BRIDGE_PORT of dev->priv_flags is set.
+	 * coupled with smp_rmb() in br_get_link_af_size_filtered.
+	 */
+	smp_wmb();
 
 	dev->priv_flags |= IFF_BRIDGE_PORT;
 

diff --git a/net/bridge/br_netlink.c b/net/bridge/br_netlink.c
index f2b1343..ccc1fd7 100644
--- a/net/bridge/br_netlink.c
+++ b/net/bridge/br_netlink.c

@@ -103,6 +103,7 @@ static size_t br_get_link_af_size_filtered(const struct net_device *dev,
 
 	rcu_read_lock();
 	if (netif_is_bridge_port(dev)) {
+		smp_rmb(); /* coupled with smp_wmb() in br_add_if() */
 		p = br_port_get_rcu(dev);
 		vg = nbp_vlan_group_rcu(p);
 	} else if (dev->priv_flags & IFF_EBRIDGE) {

-- 
2.7.4