commit 2293c57484ae64c9a3c847c8807db8c26a3a4d41 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: 702c2f646d42 ("mptcp: netlink: allow userspace-driven subflow establishment") Reported-by: Marek Majkowski marek@cloudflare.com Closes: https://github.com/multipath-tcp/mptcp_net-next/issues/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-v1-2-401... 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 bc8aeb2045e2 ("Documentation: netlink: add a YAML spec for mptcp"), and commit 9d1ed17f93ce ("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 b9f4554356f6 ("mptcp: annotate lockless access for token"), which is not in this version. ] Signed-off-by: Matthieu Baerts (NGI0) matttbe@kernel.org --- include/uapi/linux/mptcp.h | 6 ++++-- net/mptcp/pm_netlink.c | 7 +++++++ 2 files changed, 11 insertions(+), 2 deletions(-)
diff --git a/include/uapi/linux/mptcp.h b/include/uapi/linux/mptcp.h index dfe19bf13f4c..dacc7af2ea1e 100644 --- a/include/uapi/linux/mptcp.h +++ b/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 diff --git a/net/mptcp/pm_netlink.c b/net/mptcp/pm_netlink.c index cf9244a3644f..7e72862a6b54 100644 --- a/net/mptcp/pm_netlink.c +++ b/net/mptcp/pm_netlink.c @@ -2242,6 +2242,7 @@ 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; @@ -2249,6 +2250,12 @@ static int mptcp_event_created(struct sk_buff *skb, 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); }