net: support marking accepting TCP sockets

When using mark-based routing, sockets returned from accept()
may need to be marked differently depending on the incoming
connection request.

This is the case, for example, if different socket marks identify
different networks: a listening socket may want to accept
connections from all networks, but each connection should be
marked with the network that the request came in on, so that
subsequent packets are sent on the correct network.

This patch adds a sysctl to mark TCP sockets based on the fwmark
of the incoming SYN packet. If enabled, and an unmarked socket
receives a SYN, then the SYN packet's fwmark is written to the
connection's inet_request_sock, and later written back to the
accepted socket when the connection is established.  If the
socket already has a nonzero mark, then the behaviour is the same
as it is today, i.e., the listening socket's fwmark is used.

Black-box tested using user-mode linux:

- IPv4/IPv6 SYN+ACK, FIN, etc. packets are routed based on the
  mark of the incoming SYN packet.
- The socket returned by accept() is marked with the mark of the
  incoming SYN packet.
- Tested with syncookies=1 and syncookies=2.

Change-Id: I5e8c9b989762a93f3eb5a0c1b4df44f62d57f3cb
Signed-off-by: Lorenzo Colitti <lorenzo@google.com>

include/net/inet_sock.h

@@ -89,6 +89,7 @@ struct inet_request_sock {
 				no_srccheck: 1;
 	kmemcheck_bitfield_end(flags);
 	struct ip_options_rcu	*opt;
+	u32                     ir_mark;
 };
 
 static inline struct inet_request_sock *inet_rsk(const struct request_sock *sk)
@@ -96,6 +97,15 @@ static inline struct inet_request_sock *inet_rsk(const struct request_sock *sk)
 	return (struct inet_request_sock *)sk;
 }
 
+static inline u32 inet_request_mark(struct sock *sk, struct sk_buff *skb)
+{
+	if (!sk->sk_mark && sock_net(sk)->ipv4.sysctl_tcp_fwmark_accept) {
+		return skb->mark;
+	} else {
+		return sk->sk_mark;
+	}
+}
+
 struct inet_cork {
 	unsigned int		flags;
 	__be32			addr;

include/net/netns/ipv4.h

@@ -57,6 +57,7 @@ struct netns_ipv4 {
 	unsigned int sysctl_ping_group_range[2];
 	long sysctl_tcp_mem[3];
 	int sysctl_fwmark_reflect;
+	int sysctl_tcp_fwmark_accept;
 
 	atomic_t rt_genid;
 	atomic_t dev_addr_genid;

net/ipv4/inet_connection_sock.c

@@ -361,7 +361,7 @@ struct dst_entry *inet_csk_route_req(struct sock *sk,
 	struct ip_options_rcu *opt = inet_rsk(req)->opt;
 	struct net *net = sock_net(sk);
 
-	flowi4_init_output(fl4, sk->sk_bound_dev_if, sk->sk_mark,
+	flowi4_init_output(fl4, sk->sk_bound_dev_if, ireq->ir_mark,
 			   RT_CONN_FLAGS(sk), RT_SCOPE_UNIVERSE,
 			   sk->sk_protocol, inet_sk_flowi_flags(sk),
 			   (opt && opt->opt.srr) ? opt->opt.faddr : ireq->rmt_addr,
@@ -394,7 +394,7 @@ struct dst_entry *inet_csk_route_child_sock(struct sock *sk,
 	struct rtable *rt;
 
 	fl4 = &newinet->cork.fl.u.ip4;
-	flowi4_init_output(fl4, sk->sk_bound_dev_if, sk->sk_mark,
+	flowi4_init_output(fl4, sk->sk_bound_dev_if, ireq->ir_mark,
 			   RT_CONN_FLAGS(sk), RT_SCOPE_UNIVERSE,
 			   sk->sk_protocol, inet_sk_flowi_flags(sk),
 			   (opt && opt->opt.srr) ? opt->opt.faddr : ireq->rmt_addr,
@@ -616,6 +616,8 @@ struct sock *inet_csk_clone_lock(const struct sock *sk,
 		inet_sk(newsk)->inet_sport = inet_rsk(req)->loc_port;
 		newsk->sk_write_space = sk_stream_write_space;
 
+		newsk->sk_mark = inet_rsk(req)->ir_mark;
+
 		newicsk->icsk_retransmits = 0;
 		newicsk->icsk_backoff	  = 0;
 		newicsk->icsk_probes_out  = 0;

net/ipv4/syncookies.c

@@ -312,6 +312,7 @@ struct sock *cookie_v4_check(struct sock *sk, struct sk_buff *skb,
 	ireq->rmt_port		= th->source;
 	ireq->loc_addr		= ip_hdr(skb)->daddr;
 	ireq->rmt_addr		= ip_hdr(skb)->saddr;
+	ireq->ir_mark		= inet_request_mark(sk, skb);
 	ireq->ecn_ok		= ecn_ok;
 	ireq->snd_wscale	= tcp_opt.snd_wscale;
 	ireq->sack_ok		= tcp_opt.sack_ok;
@@ -347,7 +348,7 @@ struct sock *cookie_v4_check(struct sock *sk, struct sk_buff *skb,
 	 * hasn't changed since we received the original syn, but I see
 	 * no easy way to do this.
 	 */
-	flowi4_init_output(&fl4, 0, sk->sk_mark, RT_CONN_FLAGS(sk),
+	flowi4_init_output(&fl4, 0, ireq->ir_mark, RT_CONN_FLAGS(sk),
 			   RT_SCOPE_UNIVERSE, IPPROTO_TCP,
 			   inet_sk_flowi_flags(sk),
 			   (opt && opt->srr) ? opt->faddr : ireq->rmt_addr,

net/ipv4/sysctl_net_ipv4.c

@@ -794,6 +794,13 @@ static struct ctl_table ipv4_net_table[] = {
 		.mode		= 0644,
 		.proc_handler	= proc_dointvec,
 	},
+	{
+		.procname	= "tcp_fwmark_accept",
+		.data		= &init_net.ipv4.sysctl_tcp_fwmark_accept,
+		.maxlen		= sizeof(int),
+		.mode		= 0644,
+		.proc_handler	= proc_dointvec,
+	},
 	{ }
 };
 

net/ipv4/tcp_ipv4.c

@@ -1350,6 +1350,7 @@ int tcp_v4_conn_request(struct sock *sk, struct sk_buff *skb)
 	ireq->rmt_addr = saddr;
 	ireq->no_srccheck = inet_sk(sk)->transparent;
 	ireq->opt = tcp_v4_save_options(sk, skb);
+	ireq->ir_mark = inet_request_mark(sk, skb);
 
 	if (security_inet_conn_request(sk, skb, req))
 		goto drop_and_free;

net/ipv6/inet6_connection_sock.c

@@ -69,7 +69,7 @@ struct dst_entry *inet6_csk_route_req(struct sock *sk,
 	final_p = fl6_update_dst(&fl6, np->opt, &final);
 	fl6.saddr = treq->loc_addr;
 	fl6.flowi6_oif = sk->sk_bound_dev_if;
-	fl6.flowi6_mark = sk->sk_mark;
+	fl6.flowi6_mark = inet_rsk(req)->ir_mark;
 	fl6.fl6_dport = inet_rsk(req)->rmt_port;
 	fl6.fl6_sport = inet_rsk(req)->loc_port;
 	security_req_classify_flow(req, flowi6_to_flowi(&fl6));

net/ipv6/syncookies.c

@@ -215,6 +215,8 @@ struct sock *cookie_v6_check(struct sock *sk, struct sk_buff *skb)
 	    ipv6_addr_type(&ireq6->rmt_addr) & IPV6_ADDR_LINKLOCAL)
 		ireq6->iif = inet6_iif(skb);
 
+	ireq->ir_mark = inet_request_mark(sk, skb);
+
 	req->expires = 0UL;
 	req->retrans = 0;
 	ireq->ecn_ok		= ecn_ok;
@@ -241,7 +243,7 @@ struct sock *cookie_v6_check(struct sock *sk, struct sk_buff *skb)
 		final_p = fl6_update_dst(&fl6, np->opt, &final);
 		fl6.saddr = ireq6->loc_addr;
 		fl6.flowi6_oif = sk->sk_bound_dev_if;
-		fl6.flowi6_mark = sk->sk_mark;
+		fl6.flowi6_mark = inet_rsk(req)->ir_mark;
 		fl6.fl6_dport = inet_rsk(req)->rmt_port;
 		fl6.fl6_sport = inet_sk(sk)->inet_sport;
 		security_req_classify_flow(req, flowi6_to_flowi(&fl6));

net/ipv6/tcp_ipv6.c

@@ -493,7 +493,7 @@ static int tcp_v6_send_synack(struct sock *sk, struct request_sock *req,
 	fl6.saddr = treq->loc_addr;
 	fl6.flowlabel = 0;
 	fl6.flowi6_oif = treq->iif;
-	fl6.flowi6_mark = sk->sk_mark;
+	fl6.flowi6_mark = inet_rsk(req)->ir_mark;
 	fl6.fl6_dport = inet_rsk(req)->rmt_port;
 	fl6.fl6_sport = inet_rsk(req)->loc_port;
 	security_req_classify_flow(req, flowi6_to_flowi(&fl6));
@@ -1144,6 +1144,7 @@ static int tcp_v6_conn_request(struct sock *sk, struct sk_buff *skb)
 		TCP_ECN_create_request(req, tcp_hdr(skb));
 
 	treq->iif = sk->sk_bound_dev_if;
+	inet_rsk(req)->ir_mark = inet_request_mark(sk, skb);
 
 	/* So that link locals have meaning */
 	if (!sk->sk_bound_dev_if &&