net/tcp: Add TCP-AO config and structures

Introduce new kernel config option and common structures as well as
helpers to be used by TCP-AO code.

Co-developed-by: Francesco Ruggeri <[e-mail]>
Signed-off-by: Francesco Ruggeri <[e-mail]>
Co-developed-by: Salam Noureddine <[e-mail]>
Signed-off-by: Salam Noureddine <[e-mail]>
Signed-off-by: Dmitry Safonov <[e-mail]>
Acked-by: David Ahern <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

include/linux/tcp.h

···
 	bool	syn_smc;	/* SYN includes SMC */
 #endif
 
-#ifdef CONFIG_TCP_MD5SIG
-/* TCP AF-Specific parts; only used by MD5 Signature support so far */
+#if defined(CONFIG_TCP_MD5SIG) || defined(CONFIG_TCP_AO)
+/* TCP AF-Specific parts; only used by TCP-AO/MD5 Signature support so far */
 	const struct tcp_sock_af_ops	*af_specific;
 
+#ifdef CONFIG_TCP_MD5SIG
 /* TCP MD5 Signature Option information */
 	struct tcp_md5sig_info	__rcu *md5sig_info;
+#endif
+#ifdef CONFIG_TCP_AO
+	struct tcp_ao_info	__rcu *ao_info;
+#endif
 #endif
 
 /* TCP fastopen related information */

include/net/tcp.h

···
 #include <net/snmp.h>
 #include <net/ip.h>
 #include <net/tcp_states.h>
+#include <net/tcp_ao.h>
 #include <net/inet_ecn.h>
 #include <net/dst.h>
 #include <net/mptcp.h>
···
 	tp->retransmit_skb_hint = NULL;
 }
 
-union tcp_md5_addr {
-	struct in_addr  a4;
-#if IS_ENABLED(CONFIG_IPV6)
-	struct in6_addr	a6;
-#endif
-};
+#define tcp_md5_addr tcp_ao_addr
 
 /* - key database */
 struct tcp_md5sig_key {

include/net/tcp_ao.h

···
+/* SPDX-License-Identifier: GPL-2.0-or-later */
+#ifndef _TCP_AO_H
+#define _TCP_AO_H
+
+#define TCP_AO_KEY_ALIGN	1
+#define __tcp_ao_key_align __aligned(TCP_AO_KEY_ALIGN)
+
+union tcp_ao_addr {
+	struct in_addr  a4;
+#if IS_ENABLED(CONFIG_IPV6)
+	struct in6_addr	a6;
+#endif
+};
+
+struct tcp_ao_hdr {
+	u8	kind;
+	u8	length;
+	u8	keyid;
+	u8	rnext_keyid;
+};
+
+struct tcp_ao_key {
+	struct hlist_node	node;
+	union tcp_ao_addr	addr;
+	u8			key[TCP_AO_MAXKEYLEN] __tcp_ao_key_align;
+	unsigned int		tcp_sigpool_id;
+	unsigned int		digest_size;
+	u8			prefixlen;
+	u8			family;
+	u8			keylen;
+	u8			keyflags;
+	u8			sndid;
+	u8			rcvid;
+	u8			maclen;
+	struct rcu_head		rcu;
+	u8			traffic_keys[];
+};
+
+static inline u8 *rcv_other_key(struct tcp_ao_key *key)
+{
+	return key->traffic_keys;
+}
+
+static inline u8 *snd_other_key(struct tcp_ao_key *key)
+{
+	return key->traffic_keys + key->digest_size;
+}
+
+static inline int tcp_ao_maclen(const struct tcp_ao_key *key)
+{
+	return key->maclen;
+}
+
+static inline int tcp_ao_len(const struct tcp_ao_key *key)
+{
+	return tcp_ao_maclen(key) + sizeof(struct tcp_ao_hdr);
+}
+
+static inline unsigned int tcp_ao_digest_size(struct tcp_ao_key *key)
+{
+	return key->digest_size;
+}
+
+static inline int tcp_ao_sizeof_key(const struct tcp_ao_key *key)
+{
+	return sizeof(struct tcp_ao_key) + (key->digest_size << 1);
+}
+
+struct tcp_ao_info {
+	/* List of tcp_ao_key's */
+	struct hlist_head	head;
+	/* current_key and rnext_key aren't maintained on listen sockets.
+	 * Their purpose is to cache keys on established connections,
+	 * saving needless lookups. Never dereference any of them from
+	 * listen sockets.
+	 * ::current_key may change in RX to the key that was requested by
+	 * the peer, please use READ_ONCE()/WRITE_ONCE() in order to avoid
+	 * load/store tearing.
+	 * Do the same for ::rnext_key, if you don't hold socket lock
+	 * (it's changed only by userspace request in setsockopt()).
+	 */
+	struct tcp_ao_key	*current_key;
+	struct tcp_ao_key	*rnext_key;
+	u32			flags;
+	__be32			lisn;
+	__be32			risn;
+	struct rcu_head		rcu;
+};
+
+#endif /* _TCP_AO_H */

include/uapi/linux/tcp.h

···
 	__u8	tcpm_key[TCP_MD5SIG_MAXKEYLEN];
 };
 
+#define TCP_AO_MAXKEYLEN	80
+
 /* setsockopt(fd, IPPROTO_TCP, TCP_ZEROCOPY_RECEIVE, ...) */
 
 #define TCP_RECEIVE_ZEROCOPY_FLAG_TLB_CLEAN_HINT 0x1

net/ipv4/Kconfig

···
 config TCP_SIGPOOL
 	tristate
 
+config TCP_AO
+	bool "TCP: Authentication Option (RFC5925)"
+	select CRYPTO
+	select TCP_SIGPOOL
+	depends on 64BIT && IPV6 != m # seq-number extension needs WRITE_ONCE(u64)
+	help
+	  TCP-AO specifies the use of stronger Message Authentication Codes (MACs),
+	  protects against replays for long-lived TCP connections, and
+	  provides more details on the association of security with TCP
+	  connections than TCP MD5 (See RFC5925)
+
+	  If unsure, say N.
+
 config TCP_MD5SIG
 	bool "TCP: MD5 Signature Option support (RFC2385)"
 	select CRYPTO