xfrm: support sending NAT keepalives in ESP in UDP states

Add the ability to send out RFC-3948 NAT keepalives from the xfrm stack.

To use, Userspace sets an XFRM_NAT_KEEPALIVE_INTERVAL integer property when
creating XFRM outbound states which denotes the number of seconds between
keepalive messages.

Keepalive messages are sent from a per net delayed work which iterates over
the xfrm states. The logic is guarded by the xfrm state spinlock due to the
xfrm state walk iterator.

Possible future enhancements:

- Adding counters to keep track of sent keepalives.
- deduplicate NAT keepalives between states sharing the same nat keepalive
parameters.
- provisioning hardware offloads for devices capable of implementing this.
- revise xfrm state list to use an rcu list in order to avoid running this
under spinlock.

Suggested-by: Paul Wouters <[e-mail]>
Tested-by: Paul Wouters <[e-mail]>
Tested-by: Antony Antony <[e-mail]>
Signed-off-by: Eyal Birger <[e-mail]>
Signed-off-by: Steffen Klassert <[e-mail]>

include/net/ipv6_stubs.h

···
 #include <net/flow.h>
 #include <net/neighbour.h>
 #include <net/sock.h>
+#include <net/ipv6.h>
 
 /* structs from net/ip6_fib.h */
 struct fib6_info;
···
 			     int (*output)(struct net *, struct sock *, struct sk_buff *));
 	struct net_device *(*ipv6_dev_find)(struct net *net, const struct in6_addr *addr,
 					    struct net_device *dev);
+	int (*ip6_xmit)(const struct sock *sk, struct sk_buff *skb, struct flowi6 *fl6,
+			__u32 mark, struct ipv6_txoptions *opt, int tclass, u32 priority);
 };
 extern const struct ipv6_stub *ipv6_stub __read_mostly;
 

include/net/netns/xfrm.h

···
 
 	spinlock_t xfrm_policy_lock;
 	struct mutex xfrm_cfg_mutex;
+	struct delayed_work	nat_keepalive_work;
 };
 
 #endif

include/net/xfrm.h

···
 	struct xfrm_encap_tmpl	*encap;
 	struct sock __rcu	*encap_sk;
 
+	/* NAT keepalive */
+	u32			nat_keepalive_interval; /* seconds */
+	time64_t		nat_keepalive_expiration;
+
 	/* Data for care-of address */
 	xfrm_address_t	*coaddr;
 
···
 	return 0;
 }
 #endif
+
+int xfrm_nat_keepalive_init(unsigned short family);
+void xfrm_nat_keepalive_fini(unsigned short family);
+int xfrm_nat_keepalive_net_init(struct net *net);
+int xfrm_nat_keepalive_net_fini(struct net *net);
+void xfrm_nat_keepalive_state_updated(struct xfrm_state *x);
 
 #endif	/* _NET_XFRM_H */

include/uapi/linux/xfrm.h

···
 	XFRMA_IF_ID,		/* __u32 */
 	XFRMA_MTIMER_THRESH,	/* __u32 in seconds for input SA */
 	XFRMA_SA_DIR,		/* __u8 */
+	XFRMA_NAT_KEEPALIVE_INTERVAL,	/* __u32 in seconds for NAT keepalive */
 	__XFRMA_MAX
 
 #define XFRMA_OUTPUT_MARK XFRMA_SET_MARK	/* Compatibility */

net/ipv6/af_inet6.c

···
 	.nd_tbl	= &nd_tbl,
 	.ipv6_fragment = ip6_fragment,
 	.ipv6_dev_find = ipv6_dev_find,
+	.ip6_xmit = ip6_xmit,
 };
 
 static const struct ipv6_bpf_stub ipv6_bpf_stub_impl = {

net/ipv6/xfrm6_policy.c

···
 	ret = register_pernet_subsys(&xfrm6_net_ops);
 	if (ret)
 		goto out_protocol;
+
+	ret = xfrm_nat_keepalive_init(AF_INET6);
+	if (ret)
+		goto out_nat_keepalive;
 out:
 	return ret;
+out_nat_keepalive:
+	unregister_pernet_subsys(&xfrm6_net_ops);
 out_protocol:
 	xfrm6_protocol_fini();
 out_state:
···
 
 void xfrm6_fini(void)
 {
+	xfrm_nat_keepalive_fini(AF_INET6);
 	unregister_pernet_subsys(&xfrm6_net_ops);
 	xfrm6_protocol_fini();
 	xfrm6_policy_fini();

net/xfrm/Makefile

···
 
 obj-$(CONFIG_XFRM) := xfrm_policy.o xfrm_state.o xfrm_hash.o \
 		      xfrm_input.o xfrm_output.o \
-		      xfrm_sysctl.o xfrm_replay.o xfrm_device.o
+		      xfrm_sysctl.o xfrm_replay.o xfrm_device.o \
+		      xfrm_nat_keepalive.o
 obj-$(CONFIG_XFRM_STATISTICS) += xfrm_proc.o
 obj-$(CONFIG_XFRM_ALGO) += xfrm_algo.o
 obj-$(CONFIG_XFRM_USER) += xfrm_user.o

net/xfrm/xfrm_compat.c

···
 	[XFRMA_IF_ID]		= { .type = NLA_U32 },
 	[XFRMA_MTIMER_THRESH]	= { .type = NLA_U32 },
 	[XFRMA_SA_DIR]          = NLA_POLICY_RANGE(NLA_U8, XFRM_SA_DIR_IN, XFRM_SA_DIR_OUT),
+	[XFRMA_NAT_KEEPALIVE_INTERVAL]	= { .type = NLA_U32 },
 };
 
 static struct nlmsghdr *xfrm_nlmsg_put_compat(struct sk_buff *skb,
···
 	case XFRMA_IF_ID:
 	case XFRMA_MTIMER_THRESH:
 	case XFRMA_SA_DIR:
+	case XFRMA_NAT_KEEPALIVE_INTERVAL:
 		return xfrm_nla_cpy(dst, src, nla_len(src));
 	default:
-		BUILD_BUG_ON(XFRMA_MAX != XFRMA_SA_DIR);
+		BUILD_BUG_ON(XFRMA_MAX != XFRMA_NAT_KEEPALIVE_INTERVAL);
 		pr_warn_once("unsupported nla_type %d\n", src->nla_type);
 		return -EOPNOTSUPP;
 	}
···
 	int err;
 
 	if (type > XFRMA_MAX) {
-		BUILD_BUG_ON(XFRMA_MAX != XFRMA_SA_DIR);
+		BUILD_BUG_ON(XFRMA_MAX != XFRMA_NAT_KEEPALIVE_INTERVAL);
 		NL_SET_ERR_MSG(extack, "Bad attribute");
 		return -EOPNOTSUPP;
 	}

net/xfrm/xfrm_nat_keepalive.c

···
+// SPDX-License-Identifier: GPL-2.0-only
+/*
+ * xfrm_nat_keepalive.c
+ *
+ * (c) 2024 Eyal Birger <[e-mail]>
+ */
+
+#include <net/inet_common.h>
+#include <net/ip6_checksum.h>
+#include <net/xfrm.h>
+
+static DEFINE_PER_CPU(struct sock *, nat_keepalive_sk_ipv4);
+#if IS_ENABLED(CONFIG_IPV6)
+static DEFINE_PER_CPU(struct sock *, nat_keepalive_sk_ipv6);
+#endif
+
+struct nat_keepalive {
+	struct net *net;
+	u16 family;
+	xfrm_address_t saddr;
+	xfrm_address_t daddr;
+	__be16 encap_sport;
+	__be16 encap_dport;
+	__u32 smark;
+};
+
+static void nat_keepalive_init(struct nat_keepalive *ka, struct xfrm_state *x)
+{
+	ka->net = xs_net(x);
+	ka->family = x->props.family;
+	ka->saddr = x->props.saddr;
+	ka->daddr = x->id.daddr;
+	ka->encap_sport = x->encap->encap_sport;
+	ka->encap_dport = x->encap->encap_dport;
+	ka->smark = xfrm_smark_get(0, x);
+}
+
+static int nat_keepalive_send_ipv4(struct sk_buff *skb,
+				   struct nat_keepalive *ka)
+{
+	struct net *net = ka->net;
+	struct flowi4 fl4;
+	struct rtable *rt;
+	struct sock *sk;
+	__u8 tos = 0;
+	int err;
+
+	flowi4_init_output(&fl4, 0 /* oif */, skb->mark, tos,
+			   RT_SCOPE_UNIVERSE, IPPROTO_UDP, 0,
+			   ka->daddr.a4, ka->saddr.a4, ka->encap_dport,
+			   ka->encap_sport, sock_net_uid(net, NULL));
+
+	rt = ip_route_output_key(net, &fl4);
+	if (IS_ERR(rt))
+		return PTR_ERR(rt);
+
+	skb_dst_set(skb, &rt->dst);
+
+	sk = *this_cpu_ptr(&nat_keepalive_sk_ipv4);
+	sock_net_set(sk, net);
+	err = ip_build_and_send_pkt(skb, sk, fl4.saddr, fl4.daddr, NULL, tos);
+	sock_net_set(sk, &init_net);
+	return err;
+}
+
+#if IS_ENABLED(CONFIG_IPV6)
+static int nat_keepalive_send_ipv6(struct sk_buff *skb,
+				   struct nat_keepalive *ka,
+				   struct udphdr *uh)
+{
+	struct net *net = ka->net;
+	struct dst_entry *dst;
+	struct flowi6 fl6;
+	struct sock *sk;
+	__wsum csum;
+	int err;
+
+	csum = skb_checksum(skb, 0, skb->len, 0);
+	uh->check = csum_ipv6_magic(&ka->saddr.in6, &ka->daddr.in6,
+				    skb->len, IPPROTO_UDP, csum);
+	if (uh->check == 0)
+		uh->check = CSUM_MANGLED_0;
+
+	memset(&fl6, 0, sizeof(fl6));
+	fl6.flowi6_mark = skb->mark;
+	fl6.saddr = ka->saddr.in6;
+	fl6.daddr = ka->daddr.in6;
+	fl6.flowi6_proto = IPPROTO_UDP;
+	fl6.fl6_sport = ka->encap_sport;
+	fl6.fl6_dport = ka->encap_dport;
+
+	sk = *this_cpu_ptr(&nat_keepalive_sk_ipv6);
+	sock_net_set(sk, net);
+	dst = ipv6_stub->ipv6_dst_lookup_flow(net, sk, &fl6, NULL);
+	if (IS_ERR(dst))
+		return PTR_ERR(dst);
+
+	skb_dst_set(skb, dst);
+	err = ipv6_stub->ip6_xmit(sk, skb, &fl6, skb->mark, NULL, 0, 0);
+	sock_net_set(sk, &init_net);
+	return err;
+}
+#endif
+
+static void nat_keepalive_send(struct nat_keepalive *ka)
+{
+	const int nat_ka_hdrs_len = max(sizeof(struct iphdr),
+					sizeof(struct ipv6hdr)) +
+				    sizeof(struct udphdr);
+	const u8 nat_ka_payload = 0xFF;
+	int err = -EAFNOSUPPORT;
+	struct sk_buff *skb;
+	struct udphdr *uh;
+
+	skb = alloc_skb(nat_ka_hdrs_len + sizeof(nat_ka_payload), GFP_ATOMIC);
+	if (unlikely(!skb))
+		return;
+
+	skb_reserve(skb, nat_ka_hdrs_len);
+
+	skb_put_u8(skb, nat_ka_payload);
+
+	uh = skb_push(skb, sizeof(*uh));
+	uh->source = ka->encap_sport;
+	uh->dest = ka->encap_dport;
+	uh->len = htons(skb->len);
+	uh->check = 0;
+
+	skb->mark = ka->smark;
+
+	switch (ka->family) {
+	case AF_INET:
+		err = nat_keepalive_send_ipv4(skb, ka);
+		break;
+#if IS_ENABLED(CONFIG_IPV6)
+	case AF_INET6:
+		err = nat_keepalive_send_ipv6(skb, ka, uh);
+		break;
+#endif
+	}
+	if (err)
+		kfree_skb(skb);
+}
+
+struct nat_keepalive_work_ctx {
+	time64_t next_run;
+	time64_t now;
+};
+
+static int nat_keepalive_work_single(struct xfrm_state *x, int count, void *ptr)
+{
+	struct nat_keepalive_work_ctx *ctx = ptr;
+	bool send_keepalive = false;
+	struct nat_keepalive ka;
+	time64_t next_run;
+	u32 interval;
+	int delta;
+
+	interval = x->nat_keepalive_interval;
+	if (!interval)
+		return 0;
+
+	spin_lock(&x->lock);
+
+	delta = (int)(ctx->now - x->lastused);
+	if (delta < interval) {
+		x->nat_keepalive_expiration = ctx->now + interval - delta;
+		next_run = x->nat_keepalive_expiration;
+	} else if (x->nat_keepalive_expiration > ctx->now) {
+		next_run = x->nat_keepalive_expiration;
+	} else {
+		next_run = ctx->now + interval;
+		nat_keepalive_init(&ka, x);
+		send_keepalive = true;
+	}
+
+	spin_unlock(&x->lock);
+
+	if (send_keepalive)
+		nat_keepalive_send(&ka);
+
+	if (!ctx->next_run || next_run < ctx->next_run)
+		ctx->next_run = next_run;
+	return 0;
+}
+
+static void nat_keepalive_work(struct work_struct *work)
+{
+	struct nat_keepalive_work_ctx ctx;
+	struct xfrm_state_walk walk;
+	struct net *net;
+
+	ctx.next_run = 0;
+	ctx.now = ktime_get_real_seconds();
+
+	net = container_of(work, struct net, xfrm.nat_keepalive_work.work);
+	xfrm_state_walk_init(&walk, IPPROTO_ESP, NULL);
+	xfrm_state_walk(net, &walk, nat_keepalive_work_single, &ctx);
+	xfrm_state_walk_done(&walk, net);
+	if (ctx.next_run)
+		schedule_delayed_work(&net->xfrm.nat_keepalive_work,
+				      (ctx.next_run - ctx.now) * HZ);
+}
+
+static int nat_keepalive_sk_init(struct sock * __percpu *socks,
+				 unsigned short family)
+{
+	struct sock *sk;
+	int err, i;
+
+	for_each_possible_cpu(i) {
+		err = inet_ctl_sock_create(&sk, family, SOCK_RAW, IPPROTO_UDP,
+					   &init_net);
+		if (err < 0)
+			goto err;
+
+		*per_cpu_ptr(socks, i) = sk;
+	}
+
+	return 0;
+err:
+	for_each_possible_cpu(i)
+		inet_ctl_sock_destroy(*per_cpu_ptr(socks, i));
+	return err;
+}
+
+static void nat_keepalive_sk_fini(struct sock * __percpu *socks)
+{
+	int i;
+
+	for_each_possible_cpu(i)
+		inet_ctl_sock_destroy(*per_cpu_ptr(socks, i));
+}
+
+void xfrm_nat_keepalive_state_updated(struct xfrm_state *x)
+{
+	struct net *net;
+
+	if (!x->nat_keepalive_interval)
+		return;
+
+	net = xs_net(x);
+	schedule_delayed_work(&net->xfrm.nat_keepalive_work, 0);
+}
+
+int __net_init xfrm_nat_keepalive_net_init(struct net *net)
+{
+	INIT_DELAYED_WORK(&net->xfrm.nat_keepalive_work, nat_keepalive_work);
+	return 0;
+}
+
+int xfrm_nat_keepalive_net_fini(struct net *net)
+{
+	cancel_delayed_work_sync(&net->xfrm.nat_keepalive_work);
+	return 0;
+}
+
+int xfrm_nat_keepalive_init(unsigned short family)
+{
+	int err = -EAFNOSUPPORT;
+
+	switch (family) {
+	case AF_INET:
+		err = nat_keepalive_sk_init(&nat_keepalive_sk_ipv4, PF_INET);
+		break;
+#if IS_ENABLED(CONFIG_IPV6)
+	case AF_INET6:
+		err = nat_keepalive_sk_init(&nat_keepalive_sk_ipv6, PF_INET6);
+		break;
+#endif
+	}
+
+	if (err)
+		pr_err("xfrm nat keepalive init: failed to init err:%d\n", err);
+	return err;
+}
+EXPORT_SYMBOL_GPL(xfrm_nat_keepalive_init);
+
+void xfrm_nat_keepalive_fini(unsigned short family)
+{
+	switch (family) {
+	case AF_INET:
+		nat_keepalive_sk_fini(&nat_keepalive_sk_ipv4);
+		break;
+#if IS_ENABLED(CONFIG_IPV6)
+	case AF_INET6:
+		nat_keepalive_sk_fini(&nat_keepalive_sk_ipv6);
+		break;
+#endif
+	}
+}
+EXPORT_SYMBOL_GPL(xfrm_nat_keepalive_fini);

net/xfrm/xfrm_policy.c

···
 	if (rv < 0)
 		goto out_sysctl;
 
+	rv = xfrm_nat_keepalive_net_init(net);
+	if (rv < 0)
+		goto out_nat_keepalive;
+
 	return 0;
 
+out_nat_keepalive:
+	xfrm_sysctl_fini(net);
 out_sysctl:
 	xfrm_policy_fini(net);
 out_policy:
···
 
 static void __net_exit xfrm_net_exit(struct net *net)
 {
+	xfrm_nat_keepalive_net_fini(net);
 	xfrm_sysctl_fini(net);
 	xfrm_policy_fini(net);
 	xfrm_state_fini(net);
···
 #endif
 
 	register_xfrm_state_bpf();
+	xfrm_nat_keepalive_init(AF_INET);
 }
 
 #ifdef CONFIG_AUDITSYSCALL

net/xfrm/xfrm_state.c

···
 		if (x->id.spi)
 			hlist_del_rcu(&x->byspi);
 		net->xfrm.state_num--;
+		xfrm_nat_keepalive_state_updated(x);
 		spin_unlock(&net->xfrm.xfrm_state_lock);
 
 		if (x->encap_sk)
···
 	net->xfrm.state_num++;
 
 	xfrm_hash_grow_check(net, x->bydst.next != NULL);
+	xfrm_nat_keepalive_state_updated(x);
 }
 
 /* net->xfrm.xfrm_state_lock is held */
···
 		err = xfrm_init_replay(x, extack);
 		if (err)
 			goto error;
+	}
+
+	if (x->nat_keepalive_interval) {
+		if (x->dir != XFRM_SA_DIR_OUT) {
+			NL_SET_ERR_MSG(extack, "NAT keepalive is only supported for outbound SAs");
+			err = -EINVAL;
+			goto error;
+		}
+
+		if (!x->encap || x->encap->encap_type != UDP_ENCAP_ESPINUDP) {
+			NL_SET_ERR_MSG(extack,
+				       "NAT keepalive is only supported for UDP encapsulation");
+			err = -EINVAL;
+			goto error;
+		}
 	}
 
 error:

net/xfrm/xfrm_user.c

···
 	if (attrs[XFRMA_SA_DIR])
 		x->dir = nla_get_u8(attrs[XFRMA_SA_DIR]);
 
+	if (attrs[XFRMA_NAT_KEEPALIVE_INTERVAL])
+		x->nat_keepalive_interval =
+			nla_get_u32(attrs[XFRMA_NAT_KEEPALIVE_INTERVAL]);
+
 	err = __xfrm_init_state(x, false, attrs[XFRMA_OFFLOAD_DEV], extack);
 	if (err)
 		goto error;
···
 	}
 	if (x->dir)
 		ret = nla_put_u8(skb, XFRMA_SA_DIR, x->dir);
+
+	if (x->nat_keepalive_interval) {
+		ret = nla_put_u32(skb, XFRMA_NAT_KEEPALIVE_INTERVAL,
+				  x->nat_keepalive_interval);
+		if (ret)
+			goto out;
+	}
 out:
 	return ret;
 }
···
 	[XFRMA_IF_ID]		= { .type = NLA_U32 },
 	[XFRMA_MTIMER_THRESH]   = { .type = NLA_U32 },
 	[XFRMA_SA_DIR]          = NLA_POLICY_RANGE(NLA_U8, XFRM_SA_DIR_IN, XFRM_SA_DIR_OUT),
+	[XFRMA_NAT_KEEPALIVE_INTERVAL] = { .type = NLA_U32 },
 };
 EXPORT_SYMBOL_GPL(xfrma_policy);
 
···
 
 	if (x->dir)
 		l += nla_total_size(sizeof(x->dir));
+
+	if (x->nat_keepalive_interval)
+		l += nla_total_size(sizeof(x->nat_keepalive_interval));
 
 	return l;
 }