Subject: [PATCH net-next] net: Evict neighbor entries on carrier down

From: David Ahern <[email-masked]>
Date: Thu, 11 Oct 2018 20:33:49 -0700

From: David Ahern <[email-masked]>

When a link's carrier goes down it could be a sign of the port changing
networks. If the new network has overlapping addresses with the old one,
then the kernel will continue trying to use neighbor entries established
based on the old network until the entries finally age out - meaning a
potentially long delay with communications not working.

This patch evicts neighbor entries on carrier down with the exception of
those marked permanent. Permanent entries are managed by userspace (either
an admin or a routing daemon such as FRR).

Signed-off-by: David Ahern <[email-masked]>
---
 include/net/neighbour.h |  1 +
 net/core/neighbour.c    | 27 +++++++++++++++++++++++----
 net/ipv4/arp.c          |  2 ++
 net/ipv6/ndisc.c        |  2 ++
 4 files changed, 28 insertions(+), 4 deletions(-)

diff --git a/include/net/neighbour.h b/include/net/neighbour.h
index 0874f7fcd859..f58b384aa6c9 100644
--- a/include/net/neighbour.h
+++ b/include/net/neighbour.h
@@ -323,6 +323,7 @@ void __neigh_set_probe_once(struct neighbour *neigh);
 bool neigh_remove_one(struct neighbour *ndel, struct neigh_table *tbl);
 void neigh_changeaddr(struct neigh_table *tbl, struct net_device *dev);
 int neigh_ifdown(struct neigh_table *tbl, struct net_device *dev);
+int neigh_carrier_down(struct neigh_table *tbl, struct net_device *dev);
 int neigh_resolve_output(struct neighbour *neigh, struct sk_buff *skb);
 int neigh_connected_output(struct neighbour *neigh, struct sk_buff *skb);
 int neigh_direct_output(struct neighbour *neigh, struct sk_buff *skb);
diff --git a/net/core/neighbour.c b/net/core/neighbour.c
index dc1389b8beb1..69c41cb3966d 100644
--- a/net/core/neighbour.c
+++ b/net/core/neighbour.c
@@ -232,7 +232,8 @@ static void pneigh_queue_purge(struct sk_buff_head *list)
 	}
 }
 
-static void neigh_flush_dev(struct neigh_table *tbl, struct net_device *dev)
+static void neigh_flush_dev(struct neigh_table *tbl, struct net_device *dev,
+			    bool skip_perm)
 {
 	int i;
 	struct neigh_hash_table *nht;
@@ -250,6 +251,10 @@ static void neigh_flush_dev(struct neigh_table *tbl, struct net_device *dev)
 				np = &n->next;
 				continue;
 			}
+			if (skip_perm && n->nud_state & NUD_PERMANENT) {
+				np = &n->next;
+				continue;
+			}
 			rcu_assign_pointer(*np,
 				   rcu_dereference_protected(n->next,
 						lockdep_is_held(&tbl->lock)));
@@ -285,21 +290,35 @@ static void neigh_flush_dev(struct neigh_table *tbl, struct net_device *dev)
 void neigh_changeaddr(struct neigh_table *tbl, struct net_device *dev)
 {
 	write_lock_bh(&tbl->lock);
-	neigh_flush_dev(tbl, dev);
+	neigh_flush_dev(tbl, dev, false);
 	write_unlock_bh(&tbl->lock);
 }
 EXPORT_SYMBOL(neigh_changeaddr);
 
-int neigh_ifdown(struct neigh_table *tbl, struct net_device *dev)
+static int __neigh_ifdown(struct neigh_table *tbl, struct net_device *dev,
+			  bool skip_perm)
 {
 	write_lock_bh(&tbl->lock);
-	neigh_flush_dev(tbl, dev);
+	neigh_flush_dev(tbl, dev, skip_perm);
 	pneigh_ifdown_and_unlock(tbl, dev);
 
 	del_timer_sync(&tbl->proxy_timer);
 	pneigh_queue_purge(&tbl->proxy_queue);
 	return 0;
 }
+
+int neigh_carrier_down(struct neigh_table *tbl, struct net_device *dev)
+{
+	__neigh_ifdown(tbl, dev, true);
+	return 0;
+}
+EXPORT_SYMBOL(neigh_carrier_down);
+
+int neigh_ifdown(struct neigh_table *tbl, struct net_device *dev)
+{
+	__neigh_ifdown(tbl, dev, false);
+	return 0;
+}
 EXPORT_SYMBOL(neigh_ifdown);
 
 static struct neighbour *neigh_alloc(struct neigh_table *tbl, struct net_device *dev)
diff --git a/net/ipv4/arp.c b/net/ipv4/arp.c
index e90c89ef8c08..850a6f13a082 100644
--- a/net/ipv4/arp.c
+++ b/net/ipv4/arp.c
@@ -1255,6 +1255,8 @@ static int arp_netdev_event(struct notifier_block *this, unsigned long event,
 		change_info = ptr;
 		if (change_info->flags_changed & IFF_NOARP)
 			neigh_changeaddr(&arp_tbl, dev);
+		if (!netif_carrier_ok(dev))
+			neigh_carrier_down(&arp_tbl, dev);
 		break;
 	default:
 		break;
diff --git a/net/ipv6/ndisc.c b/net/ipv6/ndisc.c
index 51863ada15a4..a25cfdd47c89 100644
--- a/net/ipv6/ndisc.c
+++ b/net/ipv6/ndisc.c
@@ -1784,6 +1784,8 @@ static int ndisc_netdev_event(struct notifier_block *this, unsigned long event,
 		change_info = ptr;
 		if (change_info->flags_changed & IFF_NOARP)
 			neigh_changeaddr(&nd_tbl, dev);
+		if (!netif_carrier_ok(dev))
+			neigh_carrier_down(&nd_tbl, dev);
 		break;
 	case NETDEV_DOWN:
 		neigh_ifdown(&nd_tbl, dev);
-- 
2.11.0