ipv6: tcp: enable flowlabel reflection in some RST packets

When RST packets are sent because no socket could be found,
it makes sense to use flowlabel_reflect sysctl to decide
if a reflection of the flowlabel is requested.

This extends commit 22b6722b ("ipv6: Add sysctl for per
namespace flow label reflection"), for some TCP RST packets.

In order to provide full control of this new feature,
flowlabel_reflect becomes a bitmask.
Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

Documentation/networking/ip-sysctl.txt

@@ -1429,14 +1429,24 @@ flowlabel_state_ranges - BOOLEAN
 	FALSE: disabled
 	Default: true
 
-flowlabel_reflect - BOOLEAN
-	Automatically reflect the flow label. Needed for Path MTU
+flowlabel_reflect - INTEGER
+	Control flow label reflection. Needed for Path MTU
 	Discovery to work with Equal Cost Multipath Routing in anycast
 	environments. See RFC 7690 and:
 	https://tools.ietf.org/html/draft-wang-6man-flow-label-reflection-01
-	TRUE: enabled
-	FALSE: disabled
-	Default: FALSE
+
+	This is a mask of two bits.
+	1: enabled for established flows
+
+	Note that this prevents automatic flowlabel changes, as done
+	in "tcp: change IPv6 flow-label upon receiving spurious retransmission"
+	and "tcp: Change txhash on every SYN and RTO retransmit"
+
+	2: enabled for TCP RESET packets (no active listener)
+	If set, a RST packet sent in response to a SYN packet on a closed
+	port will reflect the incoming flow label.
+
+	Default: 0
 
 fib_multipath_hash_policy - INTEGER
 	Controls which hash policy to use for multipath routes.

net/ipv6/af_inet6.c

@@ -212,7 +212,7 @@ static int inet6_create(struct net *net, struct socket *sock, int protocol,
 	np->mc_loop	= 1;
 	np->mc_all	= 1;
 	np->pmtudisc	= IPV6_PMTUDISC_WANT;
-	np->repflow	= net->ipv6.sysctl.flowlabel_reflect;
+	np->repflow	= net->ipv6.sysctl.flowlabel_reflect & 1;
 	sk->sk_ipv6only	= net->ipv6.sysctl.bindv6only;
 
 	/* Init the ipv4 part of the socket since we can have sockets

net/ipv6/sysctl_net_ipv6.c

@@ -23,6 +23,7 @@
 
 static int zero;
 static int one = 1;
+static int three = 3;
 static int auto_flowlabels_min;
 static int auto_flowlabels_max = IP6_AUTO_FLOW_LABEL_MAX;
 
@@ -114,6 +115,8 @@ static struct ctl_table ipv6_table_template[] = {
 		.maxlen		= sizeof(int),
 		.mode		= 0644,
 		.proc_handler	= proc_dointvec,
+		.extra1		= &zero,
+		.extra2		= &three,
 	},
 	{
 		.procname	= "max_dst_opts_number",

net/ipv6/tcp_ipv6.c

@@ -916,15 +916,17 @@ static void tcp_v6_send_response(const struct sock *sk, struct sk_buff *skb, u32
 static void tcp_v6_send_reset(const struct sock *sk, struct sk_buff *skb)
 {
 	const struct tcphdr *th = tcp_hdr(skb);
+	struct ipv6hdr *ipv6h = ipv6_hdr(skb);
 	u32 seq = 0, ack_seq = 0;
 	struct tcp_md5sig_key *key = NULL;
 #ifdef CONFIG_TCP_MD5SIG
 	const __u8 *hash_location = NULL;
-	struct ipv6hdr *ipv6h = ipv6_hdr(skb);
 	unsigned char newhash[16];
 	int genhash;
 	struct sock *sk1 = NULL;
 #endif
+	__be32 label = 0;
+	struct net *net;
 	int oif = 0;
 
 	if (th->rst)
@@ -936,6 +938,7 @@ static void tcp_v6_send_reset(const struct sock *sk, struct sk_buff *skb)
 	if (!sk && !ipv6_unicast_destination(skb))
 		return;
 
+	net = dev_net(skb_dst(skb)->dev);
 #ifdef CONFIG_TCP_MD5SIG
 	rcu_read_lock();
 	hash_location = tcp_parse_md5sig_option(th);
@@ -949,7 +952,7 @@ static void tcp_v6_send_reset(const struct sock *sk, struct sk_buff *skb)
 		 * Incoming packet is checked with md5 hash with finding key,
 		 * no RST generated if md5 hash doesn't match.
 		 */
-		sk1 = inet6_lookup_listener(dev_net(skb_dst(skb)->dev),
+		sk1 = inet6_lookup_listener(net,
 					   &tcp_hashinfo, NULL, 0,
 					   &ipv6h->saddr,
 					   th->source, &ipv6h->daddr,
@@ -979,9 +982,13 @@ static void tcp_v6_send_reset(const struct sock *sk, struct sk_buff *skb)
 		oif = sk->sk_bound_dev_if;
 		if (sk_fullsock(sk))
 			trace_tcp_send_reset(sk, skb);
+	} else {
+		if (net->ipv6.sysctl.flowlabel_reflect & 2)
+			label = ip6_flowlabel(ipv6h);
 	}
 
-	tcp_v6_send_response(sk, skb, seq, ack_seq, 0, 0, 0, oif, key, 1, 0, 0);
+	tcp_v6_send_response(sk, skb, seq, ack_seq, 0, 0, 0, oif, key, 1, 0,
+			     label);
 
 #ifdef CONFIG_TCP_MD5SIG
 out: