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>

include/net/tcp_ao.h

@@ -24,6 +24,7 @@ struct tcp_ao_counters {
 	atomic64_t	pkt_bad;
 	atomic64_t	key_not_found;
 	atomic64_t	ao_required;
+	atomic64_t	dropped_icmp;
 };
 
 struct tcp_ao_key {
@@ -92,7 +93,8 @@ struct tcp_ao_info {
 	struct tcp_ao_key	*rnext_key;
 	struct tcp_ao_counters	counters;
 	u32			ao_required	:1,
-				__unused	:31;
+				accept_icmps	:1,
+				__unused	:30;
 	__be32			lisn;
 	__be32			risn;
 	/* Sequence Number Extension (SNE) are upper 4 bytes for SEQ,
@@ -191,6 +193,7 @@ int tcp_ao_calc_traffic_key(struct tcp_ao_key *mkt, u8 *key, void *ctx,
 			    unsigned int len, struct tcp_sigpool *hp);
 void tcp_ao_destroy_sock(struct sock *sk, bool twsk);
 void tcp_ao_time_wait(struct tcp_timewait_sock *tcptw, struct tcp_sock *tp);
+bool tcp_ao_ignore_icmp(const struct sock *sk, int family, int type, int code);
 enum skb_drop_reason tcp_inbound_ao_hash(struct sock *sk,
 			const struct sk_buff *skb, unsigned short int family,
 			const struct request_sock *req,
@@ -274,6 +277,12 @@ static inline void tcp_ao_syncookie(struct sock *sk, const struct sk_buff *skb,
 {
 }
 
+static inline bool tcp_ao_ignore_icmp(const struct sock *sk, int family,
+				      int type, int code)
+{
+	return false;
+}
+
 static inline enum skb_drop_reason tcp_inbound_ao_hash(struct sock *sk,
 		const struct sk_buff *skb, unsigned short int family,
 		const struct request_sock *req, const struct tcp_ao_hdr *aoh)

include/uapi/linux/snmp.h

@@ -301,6 +301,7 @@ enum
 	LINUX_MIB_TCPAOBAD,			/* TCPAOBad */
 	LINUX_MIB_TCPAOKEYNOTFOUND,		/* TCPAOKeyNotFound */
 	LINUX_MIB_TCPAOGOOD,			/* TCPAOGood */
+	LINUX_MIB_TCPAODROPPEDICMPS,		/* TCPAODroppedIcmps */
 	__LINUX_MIB_MAX
 };
 

include/uapi/linux/tcp.h

@@ -405,7 +405,8 @@ struct tcp_ao_info_opt { /* setsockopt(TCP_AO_INFO) */
 		set_rnext	:1,	/* corresponding ::rnext */
 		ao_required	:1,	/* don't accept non-AO connects */
 		set_counters	:1,	/* set/clear ::pkt_* counters */
-		reserved	:28;	/* must be 0 */
+		accept_icmps	:1,	/* accept incoming ICMPs */
+		reserved	:27;	/* must be 0 */
 	__u16	reserved2;		/* padding, must be 0 */
 	__u8	current_key;		/* KeyID to set as Current_key */
 	__u8	rnext;			/* KeyID to set as Rnext_key */
@@ -413,6 +414,7 @@ struct tcp_ao_info_opt { /* setsockopt(TCP_AO_INFO) */
 	__u64	pkt_bad;		/* failed verification */
 	__u64	pkt_key_not_found;	/* could not find a key to verify */
 	__u64	pkt_ao_required;	/* segments missing TCP-AO sign */
+	__u64	pkt_dropped_icmp;	/* ICMPs that were ignored */
 } __attribute__((aligned(8)));
 
 /* setsockopt(fd, IPPROTO_TCP, TCP_ZEROCOPY_RECEIVE, ...) */

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
 };
 

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 @@ int tcp_ao_calc_traffic_key(struct tcp_ao_key *mkt, u8 *key, void *ctx,
 	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)

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.

net/ipv6/tcp_ipv6.c

@@ -396,6 +396,8 @@ static int tcp_v6_err(struct sk_buff *skb, struct inet6_skb_parm *opt,
 	}
 
 	if (sk->sk_state == TCP_TIME_WAIT) {
+		/* To increase the counter of ignored icmps for TCP-AO */
+		tcp_ao_ignore_icmp(sk, AF_INET6, type, code);
 		inet_twsk_put(inet_twsk(sk));
 		return 0;
 	}
@@ -406,6 +408,11 @@ static int tcp_v6_err(struct sk_buff *skb, struct inet6_skb_parm *opt,
 		return 0;
 	}
 
+	if (tcp_ao_ignore_icmp(sk, AF_INET6, type, code)) {
+		sock_put(sk);
+		return 0;
+	}
+
 	bh_lock_sock(sk);
 	if (sock_owned_by_user(sk) && type != ICMPV6_PKT_TOOBIG)
 		__NET_INC_STATS(net, LINUX_MIB_LOCKDROPPEDICMPS);