Patch "bridge: Fix possible races between assigning rx_handler_data and setting IFF_BRIDGE_PORT bit" has been added to the 5.10-stable tree

[Sasha Levin <sashal@xxxxxxxxxx>]

This is a note to let you know that I've just added the patch titled

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

to the 5.10-stable tree which can be found at:
    http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
     bridge-fix-possible-races-between-assigning-rx_handl.patch
and it can be found in the queue-5.10 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@xxxxxxxxxxxxxxx> know about it.



commit d022da91cea68f044284a61b6b2cb6558ef8db89
Author: Zhang Zhengming <zhangzhengming@xxxxxxxxxx>
Date:   Wed Apr 28 22:38:14 2021 +0800

    bridge: Fix possible races between assigning rx_handler_data and setting IFF_BRIDGE_PORT bit
    
    [ Upstream commit 59259ff7a81b9eb6213891c6451221e567f8f22f ]
    
    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 it in br_get_link_af_size_filtered, using br_port_get_check_rcu() and checking the return value.
    
    Signed-off-by: Zhang Zhengming <zhangzhengming@xxxxxxxxxx>
    Reviewed-by: Zhao Lei <zhaolei69@xxxxxxxxxx>
    Reviewed-by: Wang Xiaogang <wangxiaogang3@xxxxxxxxxx>
    Suggested-by: Nikolay Aleksandrov <nikolay@xxxxxxxxxx>
    Signed-off-by: David S. Miller <davem@xxxxxxxxxxxxx>
    Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>

diff --git a/net/bridge/br_netlink.c b/net/bridge/br_netlink.c
index 92d64abffa87..73f71c22f4c0 100644
--- a/net/bridge/br_netlink.c
+++ b/net/bridge/br_netlink.c
@@ -99,8 +99,9 @@ static size_t br_get_link_af_size_filtered(const struct net_device *dev,
 
 	rcu_read_lock();
 	if (netif_is_bridge_port(dev)) {
-		p = br_port_get_rcu(dev);
-		vg = nbp_vlan_group_rcu(p);
+		p = br_port_get_check_rcu(dev);
+		if (p)
+			vg = nbp_vlan_group_rcu(p);
 	} else if (dev->priv_flags & IFF_EBRIDGE) {
 		br = netdev_priv(dev);
 		vg = br_vlan_group_rcu(br);