mptcp: pm: nl: announce deny-join-id0 flag

commit 2293c57 upstream.

During the connection establishment, a peer can tell the other one that
it cannot establish new subflows to the initial IP address and port by
setting the 'C' flag [1]. Doing so makes sense when the sender is behind
a strict NAT, operating behind a legacy Layer 4 load balancer, or using
anycast IP address for example.

When this 'C' flag is set, the path-managers must then not try to
establish new subflows to the other peer's initial IP address and port.
The in-kernel PM has access to this info, but the userspace PM didn't.

The RFC8684 [1] is strict about that:

  (...) therefore the receiver MUST NOT try to open any additional
  subflows toward this address and port.

So it is important to tell the userspace about that as it is responsible
for the respect of this flag.

When a new connection is created and established, the Netlink events
now contain the existing but not currently used 'flags' attribute. When
MPTCP_PM_EV_FLAG_DENY_JOIN_ID0 is set, it means no other subflows
to the initial IP address and port -- info that are also part of the
event -- can be established.

Link: https://datatracker.ietf.org/doc/html/rfc8684#section-3.1-20.6 [1]
Fixes: 702c2f6 ("mptcp: netlink: allow userspace-driven subflow establishment")
Reported-by: Marek Majkowski <marek@cloudflare.com>
Closes: multipath-tcp/mptcp_net-next#532
Reviewed-by: Mat Martineau <martineau@kernel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Link: https://patch.msgid.link/20250912-net-mptcp-pm-uspace-deny_join_id0-v1-2-40171884ade8@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
[ Conflicts in mptcp_pm.yaml, and mptcp_pm.h, because these files have
  been added later by commit bc8aeb2 ("Documentation: netlink: add
  a YAML spec for mptcp"), and commit 9d1ed17 ("uapi: mptcp: use
  header file generated from YAML spec"), which are not in this version.
  Applying the same modifications, but only in mptcp.h.
  Conflict in pm_netlink.c, because of a difference in the context,
  introduced by commit b9f4554 ("mptcp: annotate lockless access
  for token"), which is not in this version. ]
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Author: matttbe

include/uapi/linux/mptcp.h

@@ -81,6 +81,8 @@ enum {
 
 #define MPTCP_PM_ADDR_ATTR_MAX (__MPTCP_PM_ADDR_ATTR_MAX - 1)
 
+#define MPTCP_PM_EV_FLAG_DENY_JOIN_ID0		_BITUL(0)
+
 #define MPTCP_PM_ADDR_FLAG_SIGNAL			(1 << 0)
 #define MPTCP_PM_ADDR_FLAG_SUBFLOW			(1 << 1)
 #define MPTCP_PM_ADDR_FLAG_BACKUP			(1 << 2)
@@ -127,13 +129,13 @@ struct mptcp_info {
 
 /*
  * MPTCP_EVENT_CREATED: token, family, saddr4 | saddr6, daddr4 | daddr6,
- *                      sport, dport
+ *                      sport, dport, server-side, [flags]
  * A new MPTCP connection has been created. It is the good time to allocate
  * memory and send ADD_ADDR if needed. Depending on the traffic-patterns
  * it can take a long time until the MPTCP_EVENT_ESTABLISHED is sent.
  *
  * MPTCP_EVENT_ESTABLISHED: token, family, saddr4 | saddr6, daddr4 | daddr6,
- *			    sport, dport
+ *			    sport, dport, server-side, [flags]
  * A MPTCP connection is established (can start new subflows).
  *
  * MPTCP_EVENT_CLOSED: token

net/mptcp/pm_netlink.c

@@ -2242,13 +2242,20 @@ static int mptcp_event_created(struct sk_buff *skb,
 			       const struct sock *ssk)
 {
 	int err = nla_put_u32(skb, MPTCP_ATTR_TOKEN, msk->token);
+	u16 flags = 0;
 
 	if (err)
 		return err;
 
 	if (nla_put_u8(skb, MPTCP_ATTR_SERVER_SIDE, READ_ONCE(msk->pm.server_side)))
 		return -EMSGSIZE;
 
+	if (READ_ONCE(msk->pm.remote_deny_join_id0))
+		flags |= MPTCP_PM_EV_FLAG_DENY_JOIN_ID0;
+
+	if (flags && nla_put_u16(skb, MPTCP_ATTR_FLAGS, flags))
+		return -EMSGSIZE;
+
 	return mptcp_event_add_subflow(skb, ssk);
 }