net: neigh: decrement the family specific qlen

Commit 0ff4eb3d5ebb ("neighbour: make proxy_queue.qlen limit
per-device") introduced the length counter qlen in struct neigh_parms.
There are separate neigh_parms instances for IPv4/ARP and IPv6/ND, and
while the family specific qlen is incremented in pneigh_enqueue(), the
mentioned commit decrements always the IPv4/ARP specific qlen,
regardless of the currently processed family, in pneigh_queue_purge()
and neigh_proxy_process().

As a result, with IPv6/ND, the family specific qlen is only incremented
(and never decremented) until it exceeds PROXY_QLEN, and then, according
to the check in pneigh_enqueue(), neighbor solicitations are not
answered anymore. As an example, this is noted when using the
subnet-router anycast address to access a Linux router. After a certain
amount of time (in the observed case, qlen exceeded PROXY_QLEN after two
days), the Linux router stops answering neighbor solicitations for its
subnet-router anycast address and effectively becomes unreachable.

Another result with IPv6/ND is that the IPv4/ARP specific qlen is
decremented more often than incremented. This leads to negative qlen
values, as a signed integer has been used for the length counter qlen,
and potentially to an integer overflow.

Fix this by introducing the helper function neigh_parms_qlen_dec(),
which decrements the family specific qlen. Thereby, make use of the
existing helper function neigh_get_dev_parms_rcu(), whose definition
therefore needs to be placed earlier in neighbour.c. Take the family
member from struct neigh_table to determine the currently processed
family and appropriately call neigh_parms_qlen_dec() from
pneigh_queue_purge() and neigh_proxy_process().

Additionally, use an unsigned integer for the length counter qlen.

Fixes: 0ff4eb3d5ebb ("neighbour: make proxy_queue.qlen limit per-device")
Signed-off-by: Thomas Zeitlhofer <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

include/net/neighbour.h

···
 	struct rcu_head rcu_head;
 
 	int	reachable_time;
-	int	qlen;
+	u32	qlen;
 	int	data[NEIGH_VAR_DATA_MAX];
 	DECLARE_BITMAP(data_state, NEIGH_VAR_DATA_MAX);
 };

net/core/neighbour.c

···
 	return 0;
 }
 
-static void pneigh_queue_purge(struct sk_buff_head *list, struct net *net)
+static struct neigh_parms *neigh_get_dev_parms_rcu(struct net_device *dev,
+						   int family)
+{
+	switch (family) {
+	case AF_INET:
+		return __in_dev_arp_parms_get_rcu(dev);
+	case AF_INET6:
+		return __in6_dev_nd_parms_get_rcu(dev);
+	}
+	return NULL;
+}
+
+static void neigh_parms_qlen_dec(struct net_device *dev, int family)
+{
+	struct neigh_parms *p;
+
+	rcu_read_lock();
+	p = neigh_get_dev_parms_rcu(dev, family);
+	if (p)
+		p->qlen--;
+	rcu_read_unlock();
+}
+
+static void pneigh_queue_purge(struct sk_buff_head *list, struct net *net,
+			       int family)
 {
 	struct sk_buff_head tmp;
 	unsigned long flags;
···
 		struct net_device *dev = skb->dev;
 
 		if (net == NULL || net_eq(dev_net(dev), net)) {
-			struct in_device *in_dev;
-
-			rcu_read_lock();
-			in_dev = __in_dev_get_rcu(dev);
-			if (in_dev)
-				in_dev->arp_parms->qlen--;
-			rcu_read_unlock();
+			neigh_parms_qlen_dec(dev, family);
 			__skb_unlink(skb, list);
 			__skb_queue_tail(&tmp, skb);
 		}
···
 	write_lock_bh(&tbl->lock);
 	neigh_flush_dev(tbl, dev, skip_perm);
 	pneigh_ifdown_and_unlock(tbl, dev);
-	pneigh_queue_purge(&tbl->proxy_queue, dev ? dev_net(dev) : NULL);
+	pneigh_queue_purge(&tbl->proxy_queue, dev ? dev_net(dev) : NULL,
+			   tbl->family);
 	if (skb_queue_empty_lockless(&tbl->proxy_queue))
 		del_timer_sync(&tbl->proxy_timer);
 	return 0;
···
 
 		if (tdif <= 0) {
 			struct net_device *dev = skb->dev;
-			struct in_device *in_dev;
 
-			rcu_read_lock();
-			in_dev = __in_dev_get_rcu(dev);
-			if (in_dev)
-				in_dev->arp_parms->qlen--;
-			rcu_read_unlock();
+			neigh_parms_qlen_dec(dev, tbl->family);
 			__skb_unlink(skb, &tbl->proxy_queue);
 
 			if (tbl->proxy_redo && netif_running(dev)) {
···
 	cancel_delayed_work_sync(&tbl->managed_work);
 	cancel_delayed_work_sync(&tbl->gc_work);
 	del_timer_sync(&tbl->proxy_timer);
-	pneigh_queue_purge(&tbl->proxy_queue, NULL);
+	pneigh_queue_purge(&tbl->proxy_queue, NULL, tbl->family);
 	neigh_ifdown(tbl, NULL);
 	if (atomic_read(&tbl->entries))
 		pr_crit("neighbour leakage\n");
···
 	if (write && !ret)
 		*(int *)ctl->data = size * SKB_TRUESIZE(ETH_FRAME_LEN);
 	return ret;
-}
-
-static struct neigh_parms *neigh_get_dev_parms_rcu(struct net_device *dev,
-						   int family)
-{
-	switch (family) {
-	case AF_INET:
-		return __in_dev_arp_parms_get_rcu(dev);
-	case AF_INET6:
-		return __in6_dev_nd_parms_get_rcu(dev);
-	}
-	return NULL;
 }
 
 static void neigh_copy_dflt_parms(struct net *net, struct neigh_parms *p,