[PATCH v2 21/35] net/tcp: Ignore specific ICMPs for TCP-AO connections

Dmitry Safonov
2022-09-23T20:15:58Z

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>
---
include/net/tcp_ao.h | 9 +++++
include/uapi/linux/snmp.h | 1 +
include/uapi/linux/tcp.h | 1 +
net/ipv4/proc.c | 1 +
net/ipv4/tcp_ao.c | 70 ++++++++++++++++++++++++++++++++++++++-
net/ipv4/tcp_ipv4.c | 5 +++
net/ipv6/tcp_ipv6.c | 4 +++
7 files changed, 90 insertions(+), 1 deletion(-)

diff --git a/include/net/tcp_ao.h b/include/net/tcp_ao.h
index e99c8f300a5a..743a910ba508 100644
--- a/include/net/tcp_ao.h
+++ b/include/net/tcp_ao.h
@@ -25,6 +25,7 @@ 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 {
@@ -77,6 +78,9 @@ static inline unsigned int tcp_ao_digest_size(struct tcp_ao_key *key)
return key->digest_size;
}

+/* bits in 'ao_flags' */
+#define AO_ACCEPT_ICMPS BIT(0)
+
struct tcp_ao_info {
struct hlist_head head;
struct rcu_head rcu;
@@ -169,6 +173,7 @@ u32 tcp_ao_compute_sne(u32 sne, u32 seq, u32 new_seq);
void tcp_ao_time_wait(struct tcp_timewait_sock *tcptw, struct tcp_sock *tp);
int tcp_ao_cache_traffic_keys(const struct sock *sk, struct tcp_ao_info *ao,
struct tcp_ao_key *ao_key);
+bool tcp_ao_ignore_icmp(struct sock *sk, 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,
@@ -239,6 +244,10 @@ void tcp_ao_connect_init(struct sock *sk);

#else /* CONFIG_TCP_AO */

+static inline bool tcp_ao_ignore_icmp(struct sock *sk, 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)
diff --git a/include/uapi/linux/snmp.h b/include/uapi/linux/snmp.h
index f09119db8b40..bc7655394e9a 100644
--- a/include/uapi/linux/snmp.h
+++ b/include/uapi/linux/snmp.h
@@ -296,6 +296,7 @@ enum
LINUX_MIB_TCPAOBAD, /* TCPAOBad */
LINUX_MIB_TCPAOKEYNOTFOUND, /* TCPAOKeyNotFound */
LINUX_MIB_TCPAOGOOD, /* TCPAOGood */
+ LINUX_MIB_TCPAODROPPEDICMPS, /* TCPAODroppedIcmps */
__LINUX_MIB_MAX
};

diff --git a/include/uapi/linux/tcp.h b/include/uapi/linux/tcp.h
index 5369458ae89f..508bedbc6ad8 100644
--- a/include/uapi/linux/tcp.h
+++ b/include/uapi/linux/tcp.h
@@ -349,6 +349,7 @@ struct tcp_diag_md5sig {

#define TCP_AO_CMDF_CURR (1 << 0) /* Only checks field sndid */
#define TCP_AO_CMDF_NEXT (1 << 1) /* Only checks field rcvid */
+#define TCP_AO_CMDF_ACCEPT_ICMP (1 << 2) /* Accept incoming ICMPs */

struct tcp_ao { /* setsockopt(TCP_AO) */
struct __kernel_sockaddr_storage tcpa_addr;
diff --git a/net/ipv4/proc.c b/net/ipv4/proc.c
index 1b5a078adcf1..ccfb7f51e82f 100644
--- a/net/ipv4/proc.c
+++ b/net/ipv4/proc.c
@@ -301,6 +301,7 @@ 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 700e9a8bc983..5fb36863810d 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)
@@ -52,6 +53,63 @@ int tcp_ao_calc_traffic_key(struct tcp_ao_key *mkt, u8 *key, void *ctx,
return 1;
}

+bool tcp_ao_ignore_icmp(struct sock *sk, int type, int code)
+{
+ struct tcp_ao_info *ao;
+ bool ignore_icmp = false;
+
+ /* 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 (sk->sk_family == AF_INET) {
+ if (type != ICMP_DEST_UNREACH)
+ return false;
+ if (code < ICMP_PROT_UNREACH || code > ICMP_FRAG_NEEDED)
+ return false;
+ } else if (sk->sk_family == AF_INET6) {
+ if (type != ICMPV6_DEST_UNREACH)
+ return false;
+ if (code != ICMPV6_ADM_PROHIBITED && code != ICMPV6_PORT_UNREACH)
+ return false;
+ } else {
+ WARN_ON_ONCE(1);
+ 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->ao_flags & 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;
+}
+
struct tcp_ao_key *tcp_ao_do_lookup_keyid(struct tcp_ao_info *ao,
int sndid, int rcvid)
{
@@ -1399,7 +1457,7 @@ static inline bool tcp_ao_mkt_overlap_v6(struct tcp_ao *cmd,

#define TCP_AO_KEYF_ALL (0)
#define TCP_AO_CMDF_ADDMOD_VALID \
- (TCP_AO_CMDF_CURR | TCP_AO_CMDF_NEXT)
+ (TCP_AO_CMDF_CURR | TCP_AO_CMDF_NEXT | TCP_AO_CMDF_ACCEPT_ICMP)
#define TCP_AO_CMDF_DEL_VALID \
(TCP_AO_CMDF_CURR | TCP_AO_CMDF_NEXT)

@@ -1482,6 +1540,11 @@ static int tcp_ao_add_cmd(struct sock *sk, unsigned short int family,
atomic64_set(&key->pkt_good, 0);
atomic64_set(&key->pkt_bad, 0);

+ if (cmd.tcpa_flags & TCP_AO_CMDF_ACCEPT_ICMP)
+ ao_info->ao_flags |= AO_ACCEPT_ICMPS;
+ else
+ ao_info->ao_flags &= ~AO_ACCEPT_ICMPS;
+
ret = tcp_ao_parse_crypto(&cmd, key);
if (ret < 0)
goto err_free_sock;
@@ -1640,6 +1703,11 @@ static int tcp_ao_mod_cmd(struct sock *sk, unsigned short int family,
if (!ao_info)
return -ENOENT;
/* TODO: make tcp_ao_current_rnext() and flags set atomic */
+ if (cmd.tcpa_flags & TCP_AO_CMDF_ACCEPT_ICMP)
+ ao_info->ao_flags |= AO_ACCEPT_ICMPS;
+ else
+ ao_info->ao_flags &= ~AO_ACCEPT_ICMPS;
+
return tcp_ao_current_rnext(sk, cmd.tcpa_flags,
cmd.tcpa_current, cmd.tcpa_rnext);
}
diff --git a/net/ipv4/tcp_ipv4.c b/net/ipv4/tcp_ipv4.c
index ca4c6be886b7..372fbca343af 100644
--- a/net/ipv4/tcp_ipv4.c
+++ b/net/ipv4/tcp_ipv4.c
@@ -484,6 +484,8 @@ 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, type, code);
inet_twsk_put(inet_twsk(sk));
return 0;
}
@@ -498,6 +500,9 @@ int tcp_v4_err(struct sk_buff *skb, u32 info)
}

bh_lock_sock(sk);
+ if (tcp_ao_ignore_icmp(sk, type, code))
+ goto out;
+
/* If too many ICMPs get dropped on busy
* servers this needs to be solved differently.
* We do take care of PMTU discovery (RFC1191) special case :
diff --git a/net/ipv6/tcp_ipv6.c b/net/ipv6/tcp_ipv6.c
index 8a27408549cd..78994d1cbc45 100644
--- a/net/ipv6/tcp_ipv6.c
+++ b/net/ipv6/tcp_ipv6.c
@@ -399,6 +399,8 @@ 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, type, code);
inet_twsk_put(inet_twsk(sk));
return 0;
}
@@ -410,6 +412,8 @@ static int tcp_v6_err(struct sk_buff *skb, struct inet6_skb_parm *opt,
}

bh_lock_sock(sk);
+ if (tcp_ao_ignore_icmp(sk, type, code))
+ goto out;
if (sock_owned_by_user(sk) && type != ICMPV6_PKT_TOOBIG)
__NET_INC_STATS(net, LINUX_MIB_LOCKDROPPEDICMPS);

--
2.37.2