net/tcp: Ignore specific ICMPs for TCP-AO connections

Similarly to IPsec, RFC5925 prescribes:
  ">> A TCP-AO implementation MUST default to ignore incoming ICMPv4
  messages of Type 3 (destination unreachable), Codes 2-4 (protocol
  unreachable, port unreachable, and fragmentation needed -- ’hard
  errors’), and ICMPv6 Type 1 (destination unreachable), Code 1
  (administratively prohibited) and Code 4 (port unreachable) intended
  for connections in synchronized states (ESTABLISHED, FIN-WAIT-1, FIN-
  WAIT-2, CLOSE-WAIT, CLOSING, LAST-ACK, TIME-WAIT) that match MKTs."

A selftest (later in patch series) verifies that this attack is not
possible in this TCP-AO implementation.

Co-developed-by: Francesco Ruggeri <fruggeri@arista.com>
Signed-off-by: Francesco Ruggeri <fruggeri@arista.com>
Co-developed-by: Salam Noureddine <noureddine@arista.com>
Signed-off-by: Salam Noureddine <noureddine@arista.com>
Signed-off-by: Dmitry Safonov <dima@arista.com>
Acked-by: David Ahern <dsahern@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

3 files changed, 66 insertions, 0 deletions

diff --git a/net/ipv4/proc.c b/net/ipv4/proc.c
index f5b37ebc18c0..5f4654ebff48 100644
--- a/net/ipv4/proc.c
+++ b/net/ipv4/proc.c
@@ -303,6 +303,7 @@ static const struct snmp_mib snmp4_net_list[] = {
 	SNMP_MIB_ITEM("TCPAOBad", LINUX_MIB_TCPAOBAD),
 	SNMP_MIB_ITEM("TCPAOKeyNotFound", LINUX_MIB_TCPAOKEYNOTFOUND),
 	SNMP_MIB_ITEM("TCPAOGood", LINUX_MIB_TCPAOGOOD),
+	SNMP_MIB_ITEM("TCPAODroppedIcmps", LINUX_MIB_TCPAODROPPEDICMPS),
 	SNMP_MIB_SENTINEL
 };
 
diff --git a/net/ipv4/tcp_ao.c b/net/ipv4/tcp_ao.c
index f76fcb93499d..223af5c9eaf3 100644
--- a/net/ipv4/tcp_ao.c
+++ b/net/ipv4/tcp_ao.c
@@ -15,6 +15,7 @@
 
 #include <net/tcp.h>
 #include <net/ipv6.h>
+#include <net/icmp.h>
 
 int tcp_ao_calc_traffic_key(struct tcp_ao_key *mkt, u8 *key, void *ctx,
 			    unsigned int len, struct tcp_sigpool *hp)
@@ -44,6 +45,60 @@ clear_hash:
 	return 1;
 }
 
+bool tcp_ao_ignore_icmp(const struct sock *sk, int family, int type, int code)
+{
+	bool ignore_icmp = false;
+	struct tcp_ao_info *ao;
+
+	/* RFC5925, 7.8:
+	 * >> A TCP-AO implementation MUST default to ignore incoming ICMPv4
+	 * messages of Type 3 (destination unreachable), Codes 2-4 (protocol
+	 * unreachable, port unreachable, and fragmentation needed -- ’hard
+	 * errors’), and ICMPv6 Type 1 (destination unreachable), Code 1
+	 * (administratively prohibited) and Code 4 (port unreachable) intended
+	 * for connections in synchronized states (ESTABLISHED, FIN-WAIT-1, FIN-
+	 * WAIT-2, CLOSE-WAIT, CLOSING, LAST-ACK, TIME-WAIT) that match MKTs.
+	 */
+	if (family == AF_INET) {
+		if (type != ICMP_DEST_UNREACH)
+			return false;
+		if (code < ICMP_PROT_UNREACH || code > ICMP_FRAG_NEEDED)
+			return false;
+	} else {
+		if (type != ICMPV6_DEST_UNREACH)
+			return false;
+		if (code != ICMPV6_ADM_PROHIBITED && code != ICMPV6_PORT_UNREACH)
+			return false;
+	}
+
+	rcu_read_lock();
+	switch (sk->sk_state) {
+	case TCP_TIME_WAIT:
+		ao = rcu_dereference(tcp_twsk(sk)->ao_info);
+		break;
+	case TCP_SYN_SENT:
+	case TCP_SYN_RECV:
+	case TCP_LISTEN:
+	case TCP_NEW_SYN_RECV:
+		/* RFC5925 specifies to ignore ICMPs *only* on connections
+		 * in synchronized states.
+		 */
+		rcu_read_unlock();
+		return false;
+	default:
+		ao = rcu_dereference(tcp_sk(sk)->ao_info);
+	}
+
+	if (ao && !ao->accept_icmps) {
+		ignore_icmp = true;
+		__NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPAODROPPEDICMPS);
+		atomic64_inc(&ao->counters.dropped_icmp);
+	}
+	rcu_read_unlock();
+
+	return ignore_icmp;
+}
+
 /* Optimized version of tcp_ao_do_lookup(): only for sockets for which
  * it's known that the keys in ao_info are matching peer's
  * family/address/VRF/etc.
@@ -1086,6 +1141,7 @@ int tcp_ao_copy_all_matching(const struct sock *sk, struct sock *newsk,
 	new_ao->lisn = htonl(tcp_rsk(req)->snt_isn);
 	new_ao->risn = htonl(tcp_rsk(req)->rcv_isn);
 	new_ao->ao_required = ao->ao_required;
+	new_ao->accept_icmps = ao->accept_icmps;
 
 	if (family == AF_INET) {
 		addr = (union tcp_ao_addr *)&newsk->sk_daddr;
@@ -1792,9 +1848,11 @@ static int tcp_ao_info_cmd(struct sock *sk, unsigned short int family,
 		atomic64_set(&ao_info->counters.pkt_bad, cmd.pkt_bad);
 		atomic64_set(&ao_info->counters.key_not_found, cmd.pkt_key_not_found);
 		atomic64_set(&ao_info->counters.ao_required, cmd.pkt_ao_required);
+		atomic64_set(&ao_info->counters.dropped_icmp, cmd.pkt_dropped_icmp);
 	}
 
 	ao_info->ao_required = cmd.ao_required;
+	ao_info->accept_icmps = cmd.accept_icmps;
 	if (new_current)
 		WRITE_ONCE(ao_info->current_key, new_current);
 	if (new_rnext)
diff --git a/net/ipv4/tcp_ipv4.c b/net/ipv4/tcp_ipv4.c
index bdec99707028..8f98c58e2689 100644
--- a/net/ipv4/tcp_ipv4.c
+++ b/net/ipv4/tcp_ipv4.c
@@ -494,6 +494,8 @@ int tcp_v4_err(struct sk_buff *skb, u32 info)
 		return -ENOENT;
 	}
 	if (sk->sk_state == TCP_TIME_WAIT) {
+		/* To increase the counter of ignored icmps for TCP-AO */
+		tcp_ao_ignore_icmp(sk, AF_INET, type, code);
 		inet_twsk_put(inet_twsk(sk));
 		return 0;
 	}
@@ -507,6 +509,11 @@ int tcp_v4_err(struct sk_buff *skb, u32 info)
 		return 0;
 	}
 
+	if (tcp_ao_ignore_icmp(sk, AF_INET, type, code)) {
+		sock_put(sk);
+		return 0;
+	}
+
 	bh_lock_sock(sk);
 	/* If too many ICMPs get dropped on busy
 	 * servers this needs to be solved differently.