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

[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().

Fixes: f87b9464d152 ("netfilter: nft_fwd_netdev: Support egress hook")
Reported-by: Xiang Mei <xmei5@asu.edu>
Signed-off-by: Weiming Shi <bestswngs@gmail.com>
---
 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