Re: [PATCH net-next v3 1/2] 8021q: Fix data race when publishing vlan net_device pointers

On Wed, Sep 2, 2026 at 12:32 AM Jinjie Ruan [off-list ref] wrote:

A plain C read and assignment of the net_device pointer
in the vlan_devices_arrays leaf entries lack proper atomicity
and ordering barriers. A concurrent lockless reader on the packet
receive fast-path could observe a torn or partially initialized
net_device pointer, leading to a potential out-of-bounds read or kernel
panic.

The data race occurs between the netlink/ioctl configuration paths
(holding the per-netns rtnl_nets_lock or RTNL lock) and the softirq
receive fast-path (holding rcu_read_lock()):

  CPU 0 (Writer, rtnl_nets_lock/RTNL)      CPU 1 (Reader, rcu_read_lock())
  -----------------------------------      -------------------------------
   rtnetlink_rcv_msg()
     // RTM_NEWLINK handler with RTNL_FLAG_DOIT_PERNET
     rtnl_newlink()
       ops->newlink() == vlan_newlink()
   OR
   vlan_ioctl_handler()
     [ADD_VLAN_CMD] -> register_vlan_device()

     register_vlan_dev()
       vlan_group_set_device()
                                      netif_receive_skb_core()
                                          vlan_do_receive()
                                              vlan_find_dev()
                                                 __vlan_group_get_device()
                                                  // Speculative / torn read
                                                  [Loads bad net_device *]
           [Plain C store]
           array[vlan_id] = dev;
                                                  // Dereferences bad pointer
                                                  // during device status check
                                                  vlan_dev->flags (PANIC!)

Fix this by using rcu_assign_pointer() in vlan_group_set_device()
and rcu_dereference_raw() in __vlan_group_get_device() to enforce
proper ordering and memory atomicity for the leaf entry traversal.

Cc: stable@vger.kernel.org
Cc: "David S. Miller" <davem@davemloft.net>
Cc: Eric Dumazet <edumazet@google.com>
Cc: Jakub Kicinski <kuba@kernel.org>
Cc: Paolo Abeni <pabeni@redhat.com>
Cc: Simon Horman <horms@kernel.org>
Cc: Stanislav Fomichev <sdf@fomichev.me>
Cc: Kuniyuki Iwashima <kuniyu@google.com>
Cc: Nicolai Buchwitz <redacted>
Cc: Dan Aloni <redacted>
Cc: Jeff Garzik <redacted>
Fixes: 5c15bdec5c38 ("[VLAN]: Avoid a 4-order allocation.")
Link: https://sashiko.dev/#/patchset/20260825095422.3166067-1-ruanjinjie%40huawei.com
Signed-off-by: Jinjie Ruan <redacted>

Reviewed-by: Kuniyuki Iwashima <kuniyu@google.com>