ipv4: add option to drop unicast encapsulated in L2 multicast

In order to solve a problem with 802.11, the so-called hole-196 attack,
add an option (sysctl) called "drop_unicast_in_l2_multicast" which, if
enabled, causes the stack to drop IPv4 unicast packets encapsulated in
link-layer multi- or broadcast frames. Such frames can (as an attack)
be created by any member of the same wireless network and transmitted
as valid encrypted frames since the symmetric key for broadcast frames
is shared between all stations.

Additionally, enabling this option provides compliance with a SHOULD
clause of RFC 1122.

Reviewed-by: Julian Anastasov <[e-mail]>
Signed-off-by: Johannes Berg <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

Documentation/networking/ip-sysctl.txt

···
 	promote a corresponding secondary IP address instead of
 	removing all the corresponding secondary IP addresses.
 
+drop_unicast_in_l2_multicast - BOOLEAN
+	Drop any unicast IP packets that are received in link-layer
+	multicast (or broadcast) frames.
+	This behavior (for multicast) is actually a SHOULD in RFC
+	1122, but is disabled by default for compatibility reasons.
+	Default: off (0)
+
 
 tag - INTEGER
 	Allows you to write a number, which can be used as required.

include/uapi/linux/ip.h

···
 	IPV4_DEVCONF_IGMPV2_UNSOLICITED_REPORT_INTERVAL,
 	IPV4_DEVCONF_IGMPV3_UNSOLICITED_REPORT_INTERVAL,
 	IPV4_DEVCONF_IGNORE_ROUTES_WITH_LINKDOWN,
+	IPV4_DEVCONF_DROP_UNICAST_IN_L2_MULTICAST,
 	__IPV4_DEVCONF_MAX
 };
 

net/ipv4/devinet.c

···
 					      "promote_secondaries"),
 		DEVINET_SYSCTL_FLUSHING_ENTRY(ROUTE_LOCALNET,
 					      "route_localnet"),
+		DEVINET_SYSCTL_FLUSHING_ENTRY(DROP_UNICAST_IN_L2_MULTICAST,
+					      "drop_unicast_in_l2_multicast"),
 	},
 };
 

net/ipv4/ip_input.c

···
 	rt = skb_rtable(skb);
 	if (rt->rt_type == RTN_MULTICAST) {
 		IP_UPD_PO_STATS_BH(net, IPSTATS_MIB_INMCAST, skb->len);
-	} else if (rt->rt_type == RTN_BROADCAST)
+	} else if (rt->rt_type == RTN_BROADCAST) {
 		IP_UPD_PO_STATS_BH(net, IPSTATS_MIB_INBCAST, skb->len);
+	} else if (skb->pkt_type == PACKET_BROADCAST ||
+		   skb->pkt_type == PACKET_MULTICAST) {
+		struct in_device *in_dev = __in_dev_get_rcu(skb->dev);
+
+		/* RFC 1122 3.3.6:
+		 *
+		 *   When a host sends a datagram to a link-layer broadcast
+		 *   address, the IP destination address MUST be a legal IP
+		 *   broadcast or IP multicast address.
+		 *
+		 *   A host SHOULD silently discard a datagram that is received
+		 *   via a link-layer broadcast (see Section 2.4) but does not
+		 *   specify an IP multicast or broadcast destination address.
+		 *
+		 * This doesn't explicitly say L2 *broadcast*, but broadcast is
+		 * in a way a form of multicast and the most common use case for
+		 * this is 802.11 protecting against cross-station spoofing (the
+		 * so-called "hole-196" attack) so do it for both.
+		 */
+		if (in_dev &&
+		    IN_DEV_ORCONF(in_dev, DROP_UNICAST_IN_L2_MULTICAST))
+			goto drop;
+	}
 
 	return dst_input(skb);