[PATCH net 03/14] netfilter: nft_fwd_netdev: use recursion counter in neigh egress path

Pablo Neira Ayuso
2026-05-01T12:22:47Z

From: Weiming Shi <bestswngs@gmail.com>

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().

[ Updated to cache the nf_get_nf_dup_skb_recursion pointer. --pablo ]

Fixes: f87b9464d152 ("netfilter: nft_fwd_netdev: Support egress hook")
Reported-by: Xiang Mei <xmei5@asu.edu>
Signed-off-by: Weiming Shi <bestswngs@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
---
include/net/netfilter/nf_dup_netdev.h | 13 +++++++++++++
net/netfilter/nf_dup_netdev.c | 16 ----------------
net/netfilter/nft_fwd_netdev.c | 8 ++++++++
3 files changed, 21 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 e348fb90b8dc..3b0a70e154cd 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 605b1d42abce..b9e88d7cf308 100644
--- a/net/netfilter/nft_fwd_netdev.c
+++ b/net/netfilter/nft_fwd_netdev.c
@@ -95,6 +95,7 @@ static void nft_fwd_neigh_eval(const struct nft_expr *expr,
struct nft_regs *regs,
const struct nft_pktinfo *pkt)
{
+ u8 *nf_dup_skb_recursion = nf_get_nf_dup_skb_recursion();
struct nft_fwd_neigh *priv = nft_expr_priv(expr);
void *addr = &regs->data[priv->sreg_addr];
int oif = regs->data[priv->sreg_dev];
@@ -153,6 +154,11 @@ static void nft_fwd_neigh_eval(const struct nft_expr *expr,
goto out;
}

+ if (*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) {
verdict = NF_DROP;
@@ -170,7 +176,9 @@ static void nft_fwd_neigh_eval(const struct nft_expr *expr,

skb->dev = dev;
skb_clear_tstamp(skb);
+ (*nf_dup_skb_recursion)++;
neigh_xmit(neigh_table, dev, addr, skb);
+ (*nf_dup_skb_recursion)--;
out:
regs->verdict.code = verdict;
}
--
2.47.3