Babel: Add MAC authentication support

This implements support for MAC authentication in the Babel protocol, as
specified by RFC 8967. The implementation seeks to follow the RFC as close
as possible, with the only deliberate deviation being the addition of
support for all the HMAC algorithms already supported by Bird, as well as
the Blake2b variant of the Blake algorithm.

For description of applicability, assumptions and security properties,
see RFC 8967 sections 1.1 and 1.2.

1 files changed, 147 insertions, 9 deletions

diff --git a/proto/babel/babel.c b/proto/babel/babel.c
index fdebc352..82ba7da1 100644
--- a/proto/babel/babel.c
+++ b/proto/babel/babel.c
@@ -38,6 +38,8 @@
 #include <stdlib.h>
 #include "babel.h"
 
+#define LOG_PKT_AUTH(msg, args...) \
+  log_rl(&p->log_pkt_tbf, L_AUTH "%s: " msg, p->p.name, args)
 
 /*
  * Is one number greater or equal than another mod 2^16? This is based on the
@@ -55,6 +57,7 @@ static void babel_send_seqno_request(struct babel_proto *p, struct babel_entry *
 static void babel_update_cost(struct babel_neighbor *n);
 static inline void babel_kick_timer(struct babel_proto *p);
 static inline void babel_iface_kick_timer(struct babel_iface *ifa);
+static void babel_auth_init_neighbor(struct babel_neighbor *n);
 
 /*
  *	Functions to maintain data structures
@@ -428,6 +431,7 @@ babel_get_neighbor(struct babel_iface *ifa, ip_addr addr)
   init_list(&nbr->routes);
   init_list(&nbr->requests);
   add_tail(&ifa->neigh_list, NODE nbr);
+  babel_auth_init_neighbor(nbr);
 
   return nbr;
 }
@@ -509,11 +513,13 @@ babel_expire_neighbors(struct babel_proto *p)
 
       if (nbr->hello_expiry && nbr->hello_expiry <= now_)
         babel_expire_hello(p, nbr, now_);
+
+      if (nbr->auth_expiry && nbr->auth_expiry <= now_)
+        babel_flush_neighbor(p, nbr);
     }
   }
 }
 
-
 /*
  *	Best route selection
  */
@@ -1388,6 +1394,127 @@ babel_handle_seqno_request(union babel_msg *m, struct babel_iface *ifa)
   }
 }
 
+/*
+ *      Authentication functions
+ */
+
+/**
+ * babel_auth_reset_index - Reset authentication index on interface
+ * @ifa: Interface to reset
+ *
+ * This function resets the authentication index and packet counter for an
+ * interface, and should be called on interface configuration, or when the
+ * packet counter overflows.
+ */
+void
+babel_auth_reset_index(struct babel_iface *ifa)
+{
+  random_bytes(ifa->auth_index, BABEL_AUTH_INDEX_LEN);
+  ifa->auth_pc = 1;
+}
+
+/**
+ * babel_auth_init_neighbor - Initialise authentication data for neighbor
+ * @n: Neighbor to initialise
+ *
+ * This function initialises the authentication-related state for a new neighbor
+ * that has just been created.
+ */
+void
+babel_auth_init_neighbor(struct babel_neighbor *n)
+{
+  if (n->ifa->cf->auth_type != BABEL_AUTH_NONE)
+    n->auth_expiry = current_time() + BABEL_AUTH_NEIGHBOR_TIMEOUT;
+}
+
+static void
+babel_auth_send_challenge(struct babel_iface *ifa, struct babel_neighbor *n)
+{
+  struct babel_proto *p = ifa->proto;
+  union babel_msg msg = {};
+
+  TRACE(D_PACKETS, "Sending AUTH challenge to %I on %s",
+	n->addr, ifa->ifname);
+
+  random_bytes(n->auth_nonce, BABEL_AUTH_NONCE_LEN);
+  n->auth_nonce_expiry = current_time() + BABEL_AUTH_CHALLENGE_TIMEOUT;
+  n->auth_next_challenge = current_time() + BABEL_AUTH_CHALLENGE_INTERVAL;
+
+  msg.type = BABEL_TLV_CHALLENGE_REQ;
+  msg.challenge.nonce_len = BABEL_AUTH_NONCE_LEN;
+  msg.challenge.nonce = n->auth_nonce;
+
+  babel_send_unicast(&msg, ifa, n->addr);
+}
+
+int
+babel_auth_check_pc(struct babel_iface *ifa, struct babel_msg_auth *msg)
+{
+  struct babel_proto *p = ifa->proto;
+  struct babel_neighbor *n;
+
+  TRACE(D_PACKETS, "Handling MAC check from %I on %s",
+        msg->sender,  ifa->ifname);
+
+  /* We create the neighbour entry at this point because it makes it easier to
+   * rate limit challenge replies; this is explicitly allowed by the spec (see
+   *  Section 4.3).
+   */
+  n = babel_get_neighbor(ifa, msg->sender);
+
+  if (msg->challenge_seen && n->auth_next_challenge_reply <= current_time())
+  {
+    union babel_msg resp = {};
+    TRACE(D_PACKETS, "Sending MAC challenge response to %I", msg->sender);
+    resp.type = BABEL_TLV_CHALLENGE_REPLY;
+    resp.challenge.nonce_len = msg->challenge_len;
+    resp.challenge.nonce = msg->challenge;
+    n->auth_next_challenge_reply = current_time() + BABEL_AUTH_CHALLENGE_INTERVAL;
+    babel_send_unicast(&resp, ifa, msg->sender);
+  }
+
+  if (msg->index_len > BABEL_AUTH_INDEX_LEN || !msg->pc_seen)
+  {
+    LOG_PKT_AUTH("Invalid index or no PC from %I on %s",
+                 msg->sender, ifa->ifname);
+    return 1;
+  }
+
+  /* On successful challenge, update PC and index to current values */
+  if (msg->challenge_reply_seen &&
+      n->auth_nonce_expiry &&
+      n->auth_nonce_expiry >= current_time() &&
+      !memcmp(msg->challenge_reply, n->auth_nonce, BABEL_AUTH_NONCE_LEN))
+  {
+    n->auth_index_len = msg->index_len;
+    memcpy(n->auth_index, msg->index, msg->index_len);
+    n->auth_pc = msg->pc;
+  }
+
+  /* If index differs, send challenge */
+  if ((n->auth_index_len != msg->index_len ||
+      memcmp(n->auth_index, msg->index, msg->index_len)) &&
+      n->auth_next_challenge <= current_time())
+  {
+    LOG_PKT_AUTH("Index mismatch from %I on %s; sending challenge",
+                 msg->sender, ifa->ifname);
+    babel_auth_send_challenge(ifa, n);
+    return 1;
+  }
+
+  /* Index matches; only accept if PC is greater than last */
+  if (n->auth_pc >= msg->pc)
+  {
+    LOG_PKT_AUTH("Packet counter too low from %I on %s",
+                 msg->sender, ifa->ifname);
+    return 1;
+  }
+
+  n->auth_pc = msg->pc;
+  n->auth_expiry = current_time() + BABEL_AUTH_NEIGHBOR_TIMEOUT;
+  n->auth_passed = 1;
+  return 0;
+}
 
 /*
  *	Babel interfaces
@@ -1556,6 +1683,8 @@ babel_iface_update_buffers(struct babel_iface *ifa)
   sk_set_tbsize(ifa->sk, tbsize);
 
   ifa->tx_length = tbsize - BABEL_OVERHEAD;
+
+  babel_auth_set_tx_overhead(ifa);
 }
 
 static struct babel_iface*
@@ -1615,6 +1744,9 @@ babel_add_iface(struct babel_proto *p, struct iface *new, struct babel_iface_con
   init_list(&ifa->neigh_list);
   ifa->hello_seqno = 1;
 
+  if (ic->auth_type != BABEL_AUTH_NONE)
+    babel_auth_reset_index(ifa);
+
   ifa->timer = tm_new_init(ifa->pool, babel_iface_timer, ifa, 0, 0);
 
   init_list(&ifa->msg_queue);
@@ -1722,6 +1854,9 @@ babel_reconfigure_iface(struct babel_proto *p, struct babel_iface *ifa, struct b
   ifa->next_hop_ip4 = ipa_nonzero(new->next_hop_ip4) ? new->next_hop_ip4 : addr4;
   ifa->next_hop_ip6 = ipa_nonzero(new->next_hop_ip6) ? new->next_hop_ip6 : ifa->addr;
 
+  if (new->auth_type != BABEL_AUTH_NONE && old->auth_type != new->auth_type)
+    babel_auth_reset_index(ifa);
+
   if (ipa_zero(ifa->next_hop_ip4) && p->ip4_channel)
     log(L_WARN "%s: Missing IPv4 next hop address for %s", p->p.name, ifa->ifname);
 
@@ -1910,8 +2045,8 @@ babel_show_interfaces(struct proto *P, const char *iff)
   }
 
   cli_msg(-1023, "%s:", p->p.name);
-  cli_msg(-1023, "%-10s %-6s %7s %6s %7s %-15s %s",
-	  "Interface", "State", "RX cost", "Nbrs", "Timer",
+  cli_msg(-1023, "%-10s %-6s %-5s %7s %6s %7s %-15s %s",
+	  "Interface", "State", "Auth", "RX cost", "Nbrs", "Timer",
 	  "Next hop (v4)", "Next hop (v6)");
 
   WALK_LIST(ifa, p->interfaces)
@@ -1924,8 +2059,10 @@ babel_show_interfaces(struct proto *P, const char *iff)
 	nbrs++;
 
     btime timer = MIN(ifa->next_regular, ifa->next_hello) - current_time();
-    cli_msg(-1023, "%-10s %-6s %7u %6u %7t %-15I %I",
+    cli_msg(-1023, "%-10s %-6s %-5s %7u %6u %7t %-15I %I",
 	    ifa->iface->name, (ifa->up ? "Up" : "Down"),
+            (ifa->cf->auth_type == BABEL_AUTH_MAC ?
+             (ifa->cf->auth_permissive ? "Perm" : "Yes") : "No"),
 	    ifa->cf->rxcost, nbrs, MAX(timer, 0),
 	    ifa->next_hop_ip4, ifa->next_hop_ip6);
   }
@@ -1946,8 +2083,8 @@ babel_show_neighbors(struct proto *P, const char *iff)
   }
 
   cli_msg(-1024, "%s:", p->p.name);
-  cli_msg(-1024, "%-25s %-10s %6s %6s %6s %7s",
-	  "IP address", "Interface", "Metric", "Routes", "Hellos", "Expires");
+  cli_msg(-1024, "%-25s %-10s %6s %6s %6s %7s %4s",
+	  "IP address", "Interface", "Metric", "Routes", "Hellos", "Expires", "Auth");
 
   WALK_LIST(ifa, p->interfaces)
   {
@@ -1961,9 +2098,10 @@ babel_show_neighbors(struct proto *P, const char *iff)
         rts++;
 
       uint hellos = u32_popcount(n->hello_map);
-      btime timer = n->hello_expiry - current_time();
-      cli_msg(-1024, "%-25I %-10s %6u %6u %6u %7t",
-	      n->addr, ifa->iface->name, n->cost, rts, hellos, MAX(timer, 0));
+      btime timer = (n->hello_expiry ?: n->auth_expiry) - current_time();
+      cli_msg(-1024, "%-25I %-10s %6u %6u %6u %7t %-4s",
+	      n->addr, ifa->iface->name, n->cost, rts, hellos, MAX(timer, 0),
+              n->auth_passed ? "Yes" : "No");
     }
   }
 }