ipv6: add option to drop unsolicited neighbor advertisements

In certain 802.11 wireless deployments, there will be NA proxies
that use knowledge of the network to correctly answer requests.
To prevent unsolicitd advertisements on the shared medium from
being a problem, on such deployments wireless needs to drop them.

Enable this by providing an option called "drop_unsolicited_na".
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

Documentation/networking/ip-sysctl.txt

@@ -1680,6 +1680,13 @@ drop_unicast_in_l2_multicast - BOOLEAN
 
 	By default this is turned off.
 
+drop_unsolicited_na - BOOLEAN
+	Drop all unsolicited neighbor advertisements, for example if there's
+	a known good NA proxy on the network and such frames need not be used
+	(or in the case of 802.11, must not be used to prevent attacks.)
+
+	By default this is turned off.
+
 icmp/*:
 ratelimit - INTEGER
 	Limit the maximal rates for sending ICMPv6 packets.

include/linux/ipv6.h

@@ -56,6 +56,7 @@ struct ipv6_devconf {
 	__s32           ndisc_notify;
 	__s32		suppress_frag_ndisc;
 	__s32		accept_ra_mtu;
+	__s32		drop_unsolicited_na;
 	struct ipv6_stable_secret {
 		bool initialized;
 		struct in6_addr secret;

include/uapi/linux/ipv6.h

@@ -175,6 +175,7 @@ enum {
 	DEVCONF_ACCEPT_RA_MIN_HOP_LIMIT,
 	DEVCONF_IGNORE_ROUTES_WITH_LINKDOWN,
 	DEVCONF_DROP_UNICAST_IN_L2_MULTICAST,
+	DEVCONF_DROP_UNSOLICITED_NA,
 	DEVCONF_MAX
 };
 

net/ipv6/addrconf.c

@@ -4712,6 +4712,7 @@ static inline void ipv6_store_devconf(struct ipv6_devconf *cnf,
 	/* we omit DEVCONF_STABLE_SECRET for now */
 	array[DEVCONF_USE_OIF_ADDRS_ONLY] = cnf->use_oif_addrs_only;
 	array[DEVCONF_DROP_UNICAST_IN_L2_MULTICAST] = cnf->drop_unicast_in_l2_multicast;
+	array[DEVCONF_DROP_UNSOLICITED_NA] = cnf->drop_unsolicited_na;
 }
 
 static inline size_t inet6_ifla6_size(void)
@@ -5792,6 +5793,13 @@ static struct addrconf_sysctl_table
 			.mode		= 0644,
 			.proc_handler	= proc_dointvec,
 		},
+		{
+			.procname	= "drop_unsolicited_na",
+			.data		= &ipv6_devconf.drop_unsolicited_na,
+			.maxlen		= sizeof(int),
+			.mode		= 0644,
+			.proc_handler	= proc_dointvec,
+		},
 		{
 			/* sentinel */
 		}

net/ipv6/ndisc.c

@@ -883,6 +883,7 @@ static void ndisc_recv_na(struct sk_buff *skb)
 				    offsetof(struct nd_msg, opt));
 	struct ndisc_options ndopts;
 	struct net_device *dev = skb->dev;
+	struct inet6_dev *idev = __in6_dev_get(dev);
 	struct inet6_ifaddr *ifp;
 	struct neighbour *neigh;
 
@@ -902,6 +903,14 @@ static void ndisc_recv_na(struct sk_buff *skb)
 		return;
 	}
 
+	/* For some 802.11 wireless deployments (and possibly other networks),
+	 * there will be a NA proxy and unsolicitd packets are attacks
+	 * and thus should not be accepted.
+	 */
+	if (!msg->icmph.icmp6_solicited && idev &&
+	    idev->cnf.drop_unsolicited_na)
+		return;
+
 	if (!ndisc_parse_options(msg->opt, ndoptlen, &ndopts)) {
 		ND_PRINTK(2, warn, "NS: invalid ND option\n");
 		return;