Re: [PATCH v2 11/12] 8021q: Fix data race when publishing vlan net_device pointers

[Jakub Kicinski <kuba@kernel.org>]

On Tue, 1 Sep 2026 10:42:33 +0800 Jinjie Ruan 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 <nb@tipi-net.de>
> Cc: Dan Aloni <da-x@monatomic.org>
> Cc: Jeff Garzik <jeff@garzik.org>
> Fixes: 5c15bdec5c38 ("[VLAN]: Avoid a 4-order allocation.")
> Link: https://sashiko.dev/#/patchset/20260825095422.3166067-1-ruanjinjie%40huawei.com
> Signed-off-by: Jinjie Ruan <ruanjinjie@huawei.com>

make C=1 says:

net/8021q/vlan.c: note: in included file:
net/8021q/vlan.h:91:9: error: incompatible types in comparison expression (different address spaces):
net/8021q/vlan.h:91:9:    struct net_device [noderef] __rcu *
net/8021q/vlan.h:91:9:    struct net_device *
net/8021q/vlan.h:91:9: error: incompatible types in comparison expression (different address spaces):
net/8021q/vlan.h:91:9:    struct net_device [noderef] __rcu *
net/8021q/vlan.h:91:9:    struct net_device *