bridge: Add MAC Authentication Bypass (MAB) support

Hosts that support 802.1X authentication are able to authenticate
themselves by exchanging EAPOL frames with an authenticator (Ethernet
bridge, in this case) and an authentication server. Access to the
network is only granted by the authenticator to successfully
authenticated hosts.

The above is implemented in the bridge using the "locked" bridge port
option. When enabled, link-local frames (e.g., EAPOL) can be locally
received by the bridge, but all other frames are dropped unless the host
is authenticated. That is, unless the user space control plane installed
an FDB entry according to which the source address of the frame is
located behind the locked ingress port. The entry can be dynamic, in
which case learning needs to be enabled so that the entry will be
refreshed by incoming traffic.

There are deployments in which not all the devices connected to the
authenticator (the bridge) support 802.1X. Such devices can include
printers and cameras. One option to support such deployments is to
unlock the bridge ports connecting these devices, but a slightly more
secure option is to use MAB. When MAB is enabled, the MAC address of the
connected device is used as the user name and password for the
authentication.

For MAB to work, the user space control plane needs to be notified about
MAC addresses that are trying to gain access so that they will be
compared against an allow list. This can be implemented via the regular
learning process with the sole difference that learned FDB entries are
installed with a new "locked" flag indicating that the entry cannot be
used to authenticate the device. The flag cannot be set by user space,
but user space can clear the flag by replacing the entry, thereby
authenticating the device.

Locked FDB entries implement the following semantics with regards to
roaming, aging and forwarding:

1. Roaming: Locked FDB entries can roam to unlocked (authorized) ports,
in which case the "locked" flag is cleared. FDB entries cannot roam
to locked ports regardless of MAB being enabled or not. Therefore,
locked FDB entries are only created if an FDB entry with the given {MAC,
VID} does not already exist. This behavior prevents unauthenticated
devices from disrupting traffic destined to already authenticated
devices.

2. Aging: Locked FDB entries age and refresh by incoming traffic like
regular entries.

3. Forwarding: Locked FDB entries forward traffic like regular entries.
If user space detects an unauthorized MAC behind a locked port and
wishes to prevent traffic with this MAC DA from reaching the host, it
can do so using tc or a different mechanism.

Enable the above behavior using a new bridge port option called "mab".
It can only be enabled on a bridge port that is both locked and has
learning enabled. Locked FDB entries are flushed from the port once MAB
is disabled. A new option is added because there are pure 802.1X
deployments that are not interested in notifications about locked FDB
entries.

Signed-off-by: Hans J. Schultz <[e-mail]>
Signed-off-by: Ido Schimmel <[e-mail]>
Acked-by: Nikolay Aleksandrov <[e-mail]>
Reviewed-by: Vladimir Oltean <[e-mail]>
Signed-off-by: Jakub Kicinski <[e-mail]>

include/linux/if_bridge.h

···
 #define BR_MRP_LOST_IN_CONT	BIT(19)
 #define BR_TX_FWD_OFFLOAD	BIT(20)
 #define BR_PORT_LOCKED		BIT(21)
+#define BR_PORT_MAB		BIT(22)
 
 #define BR_DEFAULT_AGEING_TIME	(300 * HZ)
 

include/uapi/linux/if_link.h

···
 	IFLA_BRPORT_MCAST_EHT_HOSTS_LIMIT,
 	IFLA_BRPORT_MCAST_EHT_HOSTS_CNT,
 	IFLA_BRPORT_LOCKED,
+	IFLA_BRPORT_MAB,
 	__IFLA_BRPORT_MAX
 };
 #define IFLA_BRPORT_MAX (__IFLA_BRPORT_MAX - 1)

include/uapi/linux/neighbour.h

···
 #define NTF_STICKY	(1 << 6)
 #define NTF_ROUTER	(1 << 7)
 /* Extended flags under NDA_FLAGS_EXT: */
-#define NTF_EXT_MANAGED	(1 << 0)
+#define NTF_EXT_MANAGED		(1 << 0)
+#define NTF_EXT_LOCKED		(1 << 1)
 
 /*
  *	Neighbor Cache Entry States.
···
  * NTF_EXT_MANAGED flagged neigbor entries are managed by the kernel on behalf
  * of a user space control plane, and automatically refreshed so that (if
  * possible) they remain in NUD_REACHABLE state.
+ *
+ * NTF_EXT_LOCKED flagged bridge FDB entries are entries generated by the
+ * bridge in response to a host trying to communicate via a locked bridge port
+ * with MAB enabled. Their purpose is to notify user space that a host requires
+ * authentication.
  */
 
 struct nda_cacheinfo {

net/bridge/br_fdb.c

···
 	struct nda_cacheinfo ci;
 	struct nlmsghdr *nlh;
 	struct ndmsg *ndm;
+	u32 ext_flags = 0;
 
 	nlh = nlmsg_put(skb, portid, seq, type, sizeof(*ndm), flags);
 	if (nlh == NULL)
···
 		ndm->ndm_flags |= NTF_EXT_LEARNED;
 	if (test_bit(BR_FDB_STICKY, &fdb->flags))
 		ndm->ndm_flags |= NTF_STICKY;
+	if (test_bit(BR_FDB_LOCKED, &fdb->flags))
+		ext_flags |= NTF_EXT_LOCKED;
 
 	if (nla_put(skb, NDA_LLADDR, ETH_ALEN, &fdb->key.addr))
 		goto nla_put_failure;
 	if (nla_put_u32(skb, NDA_MASTER, br->dev->ifindex))
 		goto nla_put_failure;
+	if (nla_put_u32(skb, NDA_FLAGS_EXT, ext_flags))
+		goto nla_put_failure;
+
 	ci.ndm_used	 = jiffies_to_clock_t(now - fdb->used);
 	ci.ndm_confirmed = 0;
 	ci.ndm_updated	 = jiffies_to_clock_t(now - fdb->updated);
···
 	return NLMSG_ALIGN(sizeof(struct ndmsg))
 		+ nla_total_size(ETH_ALEN) /* NDA_LLADDR */
 		+ nla_total_size(sizeof(u32)) /* NDA_MASTER */
+		+ nla_total_size(sizeof(u32)) /* NDA_FLAGS_EXT */
 		+ nla_total_size(sizeof(u16)) /* NDA_VLAN */
 		+ nla_total_size(sizeof(struct nda_cacheinfo))
 		+ nla_total_size(0) /* NDA_FDB_EXT_ATTRS */
···
 						      &fdb->flags)))
 					clear_bit(BR_FDB_ADDED_BY_EXT_LEARN,
 						  &fdb->flags);
+				/* Clear locked flag when roaming to an
+				 * unlocked port.
+				 */
+				if (unlikely(test_bit(BR_FDB_LOCKED, &fdb->flags)))
+					clear_bit(BR_FDB_LOCKED, &fdb->flags);
 			}
 
 			if (unlikely(test_bit(BR_FDB_ADDED_BY_USER, &flags)))
···
 		modified = true;
 	}
 
+	if (test_and_clear_bit(BR_FDB_LOCKED, &fdb->flags))
+		modified = true;
+
 	if (fdb_handle_notify(fdb, notify))
 		modified = true;
 
···
 	struct net_bridge_port *p = NULL;
 	struct net_bridge_vlan *v;
 	struct net_bridge *br = NULL;
+	u32 ext_flags = 0;
 	int err = 0;
 
 	trace_br_fdb_add(ndm, dev, addr, vid, nlh_flags);
···
 		}
 		br = p->br;
 		vg = nbp_vlan_group(p);
+	}
+
+	if (tb[NDA_FLAGS_EXT])
+		ext_flags = nla_get_u32(tb[NDA_FLAGS_EXT]);
+
+	if (ext_flags & NTF_EXT_LOCKED) {
+		NL_SET_ERR_MSG_MOD(extack, "Cannot add FDB entry with \"locked\" flag set");
+		return -EINVAL;
 	}
 
 	if (tb[NDA_FDB_EXT_ATTRS]) {

net/bridge/br_input.c

···
 		struct net_bridge_fdb_entry *fdb_src =
 			br_fdb_find_rcu(br, eth_hdr(skb)->h_source, vid);
 
-		if (!fdb_src || READ_ONCE(fdb_src->dst) != p ||
-		    test_bit(BR_FDB_LOCAL, &fdb_src->flags))
+		if (!fdb_src) {
+			/* FDB miss. Create locked FDB entry if MAB is enabled
+			 * and drop the packet.
+			 */
+			if (p->flags & BR_PORT_MAB)
+				br_fdb_update(br, p, eth_hdr(skb)->h_source,
+					      vid, BIT(BR_FDB_LOCKED));
 			goto drop;
+		} else if (READ_ONCE(fdb_src->dst) != p ||
+			   test_bit(BR_FDB_LOCAL, &fdb_src->flags)) {
+			/* FDB mismatch. Drop the packet without roaming. */
+			goto drop;
+		} else if test_bit(BR_FDB_LOCKED, &fdb_src->flags) {
+			/* FDB match, but entry is locked. Refresh it and drop
+			 * the packet.
+			 */
+			br_fdb_update(br, p, eth_hdr(skb)->h_source, vid,
+				      BIT(BR_FDB_LOCKED));
+			goto drop;
+		}
 	}
 
 	nbp_switchdev_frame_mark(p, skb);

net/bridge/br_netlink.c

···
 		+ nla_total_size(1)	/* IFLA_BRPORT_NEIGH_SUPPRESS */
 		+ nla_total_size(1)	/* IFLA_BRPORT_ISOLATED */
 		+ nla_total_size(1)	/* IFLA_BRPORT_LOCKED */
+		+ nla_total_size(1)	/* IFLA_BRPORT_MAB */
 		+ nla_total_size(sizeof(struct ifla_bridge_id))	/* IFLA_BRPORT_ROOT_ID */
 		+ nla_total_size(sizeof(struct ifla_bridge_id))	/* IFLA_BRPORT_BRIDGE_ID */
 		+ nla_total_size(sizeof(u16))	/* IFLA_BRPORT_DESIGNATED_PORT */
···
 	    nla_put_u8(skb, IFLA_BRPORT_MRP_IN_OPEN,
 		       !!(p->flags & BR_MRP_LOST_IN_CONT)) ||
 	    nla_put_u8(skb, IFLA_BRPORT_ISOLATED, !!(p->flags & BR_ISOLATED)) ||
-	    nla_put_u8(skb, IFLA_BRPORT_LOCKED, !!(p->flags & BR_PORT_LOCKED)))
+	    nla_put_u8(skb, IFLA_BRPORT_LOCKED, !!(p->flags & BR_PORT_LOCKED)) ||
+	    nla_put_u8(skb, IFLA_BRPORT_MAB, !!(p->flags & BR_PORT_MAB)))
 		return -EMSGSIZE;
 
 	timerval = br_timer_value(&p->message_age_timer);
···
 	[IFLA_BRPORT_NEIGH_SUPPRESS] = { .type = NLA_U8 },
 	[IFLA_BRPORT_ISOLATED]	= { .type = NLA_U8 },
 	[IFLA_BRPORT_LOCKED] = { .type = NLA_U8 },
+	[IFLA_BRPORT_MAB] = { .type = NLA_U8 },
 	[IFLA_BRPORT_BACKUP_PORT] = { .type = NLA_U32 },
 	[IFLA_BRPORT_MCAST_EHT_HOSTS_LIMIT] = { .type = NLA_U32 },
 };
···
 	br_set_port_flag(p, tb, IFLA_BRPORT_NEIGH_SUPPRESS, BR_NEIGH_SUPPRESS);
 	br_set_port_flag(p, tb, IFLA_BRPORT_ISOLATED, BR_ISOLATED);
 	br_set_port_flag(p, tb, IFLA_BRPORT_LOCKED, BR_PORT_LOCKED);
+	br_set_port_flag(p, tb, IFLA_BRPORT_MAB, BR_PORT_MAB);
+
+	if ((p->flags & BR_PORT_MAB) &&
+	    (!(p->flags & BR_PORT_LOCKED) || !(p->flags & BR_LEARNING))) {
+		NL_SET_ERR_MSG(extack, "Bridge port must be locked and have learning enabled when MAB is enabled");
+		p->flags = old_flags;
+		return -EINVAL;
+	} else if (!(p->flags & BR_PORT_MAB) && (old_flags & BR_PORT_MAB)) {
+		struct net_bridge_fdb_flush_desc desc = {
+			.flags = BIT(BR_FDB_LOCKED),
+			.flags_mask = BIT(BR_FDB_LOCKED),
+			.port_ifindex = p->dev->ifindex,
+		};
+
+		br_fdb_flush(p->br, &desc);
+	}
 
 	changed_mask = old_flags ^ p->flags;
 

net/bridge/br_private.h

···
 	BR_FDB_ADDED_BY_EXT_LEARN,
 	BR_FDB_OFFLOADED,
 	BR_FDB_NOTIFY,
-	BR_FDB_NOTIFY_INACTIVE
+	BR_FDB_NOTIFY_INACTIVE,
+	BR_FDB_LOCKED,
 };
 
 struct net_bridge_fdb_key {

net/core/rtnetlink.c

···
 		return err;
 	}
 
+	if (tb[NDA_FLAGS_EXT]) {
+		netdev_info(dev, "invalid flags given to default FDB implementation\n");
+		return err;
+	}
+
 	if (vid) {
 		netdev_info(dev, "vlans aren't supported yet for dev_uc|mc_add()\n");
 		return err;