Subject: Re: [PATCH v2] netfilter: nft_fwd_netdev: use recursion counter in neigh egress path

From: Pablo Neira Ayuso <[email-masked]>
Date: Thu, 9 Apr 2026 13:05:59 +0200

On Thu, Apr 09, 2026 at 06:49:12PM +0800, Weiming Shi wrote:
> nft_fwd_neigh can be used in egress chains (NF_NETDEV_EGRESS). When the
> forwarding rule targets the same device or two devices forward to each
> other, neigh_xmit() triggers dev_queue_xmit() which re-enters
> nf_hook_egress(), causing infinite recursion and stack overflow.
> 
> Move the nf_get_nf_dup_skb_recursion() accessor and NF_RECURSION_LIMIT
> to the shared header nf_dup_netdev.h as a static inline, so that
> nft_fwd_netdev can use the recursion counter directly without exported
> function call overhead. Guard neigh_xmit() with the same recursion
> limit already used in nf_do_netdev_egress().
> 
> Fixes: f87b9464d152 ("netfilter: nft_fwd_netdev: Support egress hook")

I would just restrict this "feature", I don't see a point in allowing
this from egress?

> Reported-by: Xiang Mei <[email-masked]>
> Signed-off-by: Weiming Shi <[email-masked]>
> ---
>  include/net/netfilter/nf_dup_netdev.h | 13 +++++++++++++
>  net/netfilter/nf_dup_netdev.c         | 16 ----------------
>  net/netfilter/nft_fwd_netdev.c        |  7 +++++++
>  3 files changed, 20 insertions(+), 16 deletions(-)
> 
> diff --git a/include/net/netfilter/nf_dup_netdev.h b/include/net/netfilter/nf_dup_netdev.h
> index b175d271aec9..609bcf422a9b 100644
> --- a/include/net/netfilter/nf_dup_netdev.h
> +++ b/include/net/netfilter/nf_dup_netdev.h
> @@ -3,10 +3,23 @@
>  #define _NF_DUP_NETDEV_H_
>  
>  #include <net/netfilter/nf_tables.h>
> +#include <linux/netdevice.h>
> +#include <linux/sched.h>
>  
>  void nf_dup_netdev_egress(const struct nft_pktinfo *pkt, int oif);
>  void nf_fwd_netdev_egress(const struct nft_pktinfo *pkt, int oif);
>  
> +#define NF_RECURSION_LIMIT	2
> +
> +static inline u8 *nf_get_nf_dup_skb_recursion(void)
> +{
> +#ifndef CONFIG_PREEMPT_RT
> +	return this_cpu_ptr(&softnet_data.xmit.nf_dup_skb_recursion);
> +#else
> +	return &current->net_xmit.nf_dup_skb_recursion;
> +#endif
> +}
> +
>  struct nft_offload_ctx;
>  struct nft_flow_rule;
>  
> diff --git a/net/netfilter/nf_dup_netdev.c b/net/netfilter/nf_dup_netdev.c
> index fab8b9011098..a958a1b0c5be 100644
> --- a/net/netfilter/nf_dup_netdev.c
> +++ b/net/netfilter/nf_dup_netdev.c
> @@ -13,22 +13,6 @@
>  #include <net/netfilter/nf_tables_offload.h>
>  #include <net/netfilter/nf_dup_netdev.h>
>  
> -#define NF_RECURSION_LIMIT	2
> -
> -#ifndef CONFIG_PREEMPT_RT
> -static u8 *nf_get_nf_dup_skb_recursion(void)
> -{
> -	return this_cpu_ptr(&softnet_data.xmit.nf_dup_skb_recursion);
> -}
> -#else
> -
> -static u8 *nf_get_nf_dup_skb_recursion(void)
> -{
> -	return &current->net_xmit.nf_dup_skb_recursion;
> -}
> -
> -#endif
> -
>  static void nf_do_netdev_egress(struct sk_buff *skb, struct net_device *dev,
>  				enum nf_dev_hooks hook)
>  {
> diff --git a/net/netfilter/nft_fwd_netdev.c b/net/netfilter/nft_fwd_netdev.c
> index 152a9fb4d23a..492bb599a499 100644
> --- a/net/netfilter/nft_fwd_netdev.c
> +++ b/net/netfilter/nft_fwd_netdev.c
> @@ -141,13 +141,20 @@ static void nft_fwd_neigh_eval(const struct nft_expr *expr,
>  		goto out;
>  	}
>  
> +	if (*nf_get_nf_dup_skb_recursion() > NF_RECURSION_LIMIT) {
> +		verdict = NF_DROP;
> +		goto out;
> +	}
> +
>  	dev = dev_get_by_index_rcu(nft_net(pkt), oif);
>  	if (dev == NULL)
>  		return;
>  
>  	skb->dev = dev;
>  	skb_clear_tstamp(skb);
> +	(*nf_get_nf_dup_skb_recursion())++;
>  	neigh_xmit(neigh_table, dev, addr, skb);
> +	(*nf_get_nf_dup_skb_recursion())--;
>  out:
>  	regs->verdict.code = verdict;
>  }
> -- 
> 2.43.0
> 
>