net ipv6: Prevent neighbor add if protocol is disabled on device

Disabling IPv6 on an interface removes existing entries but nothing prevents
new entries from being manually added. To that end, add a new neigh_table
operation, allow_add, that is called on RTM_NEWNEIGH to see if neighbor
entries are allowed on a given device. If IPv6 is disabled on the device,
allow_add returns false and passes a message back to the user via extack.

  $ echo 1 > /proc/sys/net/ipv6/conf/eth1/disable_ipv6
  $ ip -6 neigh add fe80::4c88:bff:fe21:2704 dev eth1 lladdr de:ad:be:ef:01:01
  Error: IPv6 is disabled on this device.
Signed-off-by: David Ahern <dsahern@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/neighbour.h

@@ -205,6 +205,8 @@ struct neigh_table {
 	int			(*pconstructor)(struct pneigh_entry *);
 	void			(*pdestructor)(struct pneigh_entry *);
 	void			(*proxy_redo)(struct sk_buff *skb);
+	bool			(*allow_add)(const struct net_device *dev,
+					     struct netlink_ext_ack *extack);
 	char			*id;
 	struct neigh_parms	parms;
 	struct list_head	parms_list;

net/core/neighbour.c

@@ -1920,6 +1920,11 @@ static int neigh_add(struct sk_buff *skb, struct nlmsghdr *nlh,
 		goto out;
 	}
 
+	if (tbl->allow_add && !tbl->allow_add(dev, extack)) {
+		err = -EINVAL;
+		goto out;
+	}
+
 	neigh = neigh_lookup(tbl, dst, dev);
 	if (neigh == NULL) {
 		bool exempt_from_gc;

net/ipv6/ndisc.c

@@ -77,6 +77,8 @@ static u32 ndisc_hash(const void *pkey,
 		      const struct net_device *dev,
 		      __u32 *hash_rnd);
 static bool ndisc_key_eq(const struct neighbour *neigh, const void *pkey);
+static bool ndisc_allow_add(const struct net_device *dev,
+			    struct netlink_ext_ack *extack);
 static int ndisc_constructor(struct neighbour *neigh);
 static void ndisc_solicit(struct neighbour *neigh, struct sk_buff *skb);
 static void ndisc_error_report(struct neighbour *neigh, struct sk_buff *skb);
@@ -117,6 +119,7 @@ struct neigh_table nd_tbl = {
 	.pconstructor =	pndisc_constructor,
 	.pdestructor =	pndisc_destructor,
 	.proxy_redo =	pndisc_redo,
+	.allow_add  =   ndisc_allow_add,
 	.id =		"ndisc_cache",
 	.parms = {
 		.tbl			= &nd_tbl,
@@ -392,6 +395,20 @@ static void pndisc_destructor(struct pneigh_entry *n)
 	ipv6_dev_mc_dec(dev, &maddr);
 }
 
+/* called with rtnl held */
+static bool ndisc_allow_add(const struct net_device *dev,
+			    struct netlink_ext_ack *extack)
+{
+	struct inet6_dev *idev = __in6_dev_get(dev);
+
+	if (!idev || idev->cnf.disable_ipv6) {
+		NL_SET_ERR_MSG(extack, "IPv6 is disabled on this device");
+		return false;
+	}
+
+	return true;
+}
+
 static struct sk_buff *ndisc_alloc_skb(struct net_device *dev,
 				       int len)
 {