Re: [PATCH net] net: fix NULL pointer dereference in l3mdev_l3_rcv

[Simon Horman]

On Thu, Mar 13, 2025 at 09:27:13AM +0800, Wang Liang wrote:
> When delete l3s ipvlan:
> 
>     ip link del link eth0 ipvlan1 type ipvlan mode l3s
> 
> This may cause a null pointer dereference:
> 
>     Call trace:
>      ip_rcv_finish+0x48/0xd0
>      ip_rcv+0x5c/0x100
>      __netif_receive_skb_one_core+0x64/0xb0
>      __netif_receive_skb+0x20/0x80
>      process_backlog+0xb4/0x204
>      napi_poll+0xe8/0x294
>      net_rx_action+0xd8/0x22c
>      __do_softirq+0x12c/0x354
> 
> This is because l3mdev_l3_rcv() visit dev->l3mdev_ops after
> ipvlan_l3s_unregister() assign the dev->l3mdev_ops to NULL. The process
> like this:
> 
>     (CPU1)                     | (CPU2)
>     l3mdev_l3_rcv()            |
>       check dev->priv_flags:   |
>         master = skb->dev;     |
>                                |
>                                | ipvlan_l3s_unregister()
>                                |   set dev->priv_flags
>                                |   dev->l3mdev_ops = NULL;
>                                |
>       visit master->l3mdev_ops |
> 
> Add lock for dev->priv_flags and dev->l3mdev_ops is too expensive. Resolve
> this issue by add check for master->l3mdev_ops.

Hi Wang Liang,

It seems to me that checking master->l3mdev_ops like this is racy.

> 
> Fixes: c675e06a98a4 ("ipvlan: decouple l3s mode dependencies from other modes")
> Signed-off-by: Wang Liang <wangliang74@xxxxxxxxxx>
> ---
>  include/net/l3mdev.h | 9 +++++++--
>  1 file changed, 7 insertions(+), 2 deletions(-)
> 
> diff --git a/include/net/l3mdev.h b/include/net/l3mdev.h
> index f7fe796e8429..b5af87a35a9f 100644
> --- a/include/net/l3mdev.h
> +++ b/include/net/l3mdev.h
> @@ -165,6 +165,7 @@ static inline
>  struct sk_buff *l3mdev_l3_rcv(struct sk_buff *skb, u16 proto)
>  {
>  	struct net_device *master = NULL;
> +	const struct l3mdev_ops *l3mdev_ops;

If you end up respinning for some other reason, please rearrange the lines
above in reverse xmas tree order - longest line to shortest.

>  
>  	if (netif_is_l3_slave(skb->dev))
>  		master = netdev_master_upper_dev_get_rcu(skb->dev);
> @@ -172,8 +173,12 @@ struct sk_buff *l3mdev_l3_rcv(struct sk_buff *skb, u16 proto)
>  		 netif_has_l3_rx_handler(skb->dev))
>  		master = skb->dev;
>  
> -	if (master && master->l3mdev_ops->l3mdev_l3_rcv)
> -		skb = master->l3mdev_ops->l3mdev_l3_rcv(master, skb, proto);
> +	if (!master)
> +		return skb;
> +
> +	l3mdev_ops = master->l3mdev_ops;
> +	if (l3mdev_ops && l3mdev_ops->l3mdev_l3_rcv)
> +		skb = l3mdev_ops->l3mdev_l3_rcv(master, skb, proto);
>  
>  	return skb;
>  }
> -- 
> 2.34.1
>