mptcp: pm: drop TCP TS with ADD_ADDRv6 + port

With TCP-timestamps (padded) taking 12 bytes and ADD_ADDR IPv6 + port
taking 30 bytes, the 40-byte limit for the TCP options is reached. In
this case, it is then not possible to send the signal.

To be able to send this ADD_ADDR, the TCP timestamps option can now be
dropped. This is done, when needed by setting the *drop_ts parameter
from mptcp_established_options. This feature is controlled by a new
net.mptcp.add_addr_v6_port_drop_ts sysctl knob, enabled by default.

It is important to keep in mind that dropping the TCP timestamps option
for one packet of the connection could eventually disrupt some
middleboxes: even if it should be unlikely, they could drop the packet
or even block the connection. That's why this new feature can be
controlled by a sysctl knob.

Closes: https://github.com/multipath-tcp/mptcp_net-next/issues/448
Reviewed-by: Mat Martineau <martineau@kernel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Link: https://patch.msgid.link/20260605-net-next-mptcp-add-addr6-port-ts-v2-6-758e7ca73f4d@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

3 files changed, 21 insertions, 4 deletions

diff --git a/net/mptcp/options.c b/net/mptcp/options.c
index 95f16f9f0ce2..8d0680a588dd 100644
--- a/net/mptcp/options.c
+++ b/net/mptcp/options.c
@@ -659,11 +659,13 @@ static u64 add_addr_generate_hmac(u64 key1, u64 key2,
 static bool mptcp_established_options_add_addr(struct sock *sk,
 					       struct sk_buff *skb, int *size,
 					       unsigned int remaining,
+					       bool has_ts,
 					       struct mptcp_out_options *opts)
 {
 	struct mptcp_subflow_context *subflow = mptcp_subflow_ctx(sk);
 	struct mptcp_sock *msk = mptcp_sk(subflow->conn);
 	struct mptcp_addr_info addr;
+	bool drop_ts = has_ts;
 	bool echo;
 
 	/* add addr will strip the existing options, be sure to avoid breaking
@@ -672,11 +674,13 @@ static bool mptcp_established_options_add_addr(struct sock *sk,
 	if (!mptcp_pm_should_add_signal(msk) ||
 	    (opts->suboptions & (OPTION_MPTCP_MPJ_ACK | OPTION_MPTCP_MPC_ACK)) ||
 	    !skb || !skb_is_tcp_pure_ack(skb) ||
-	    !mptcp_pm_add_addr_signal(msk, size, remaining, &addr, &echo))
+	    !mptcp_pm_add_addr_signal(msk, size, remaining, &addr, &echo,
+				      &drop_ts))
 		return false;
 
 	pr_debug("drop other suboptions\n");
 	opts->suboptions = OPTION_MPTCP_ADD_ADDR;
+	opts->drop_ts = drop_ts;
 	opts->addr = addr;
 	if (!echo) {
 		MPTCP_INC_STATS(sock_net(sk), MPTCP_MIB_ADDADDRTX);
@@ -859,7 +863,8 @@ int mptcp_established_options(struct sock *sk, struct sk_buff *skb,
 
 	total_size += opt_size;
 	remaining -= opt_size;
-	if (mptcp_established_options_add_addr(sk, skb, &opt_size, remaining, opts)) {
+	if (mptcp_established_options_add_addr(sk, skb, &opt_size, remaining,
+					       has_ts, opts)) {
 		total_size += opt_size;
 		remaining -= opt_size;
 		ret = true;
diff --git a/net/mptcp/pm.c b/net/mptcp/pm.c
index 59dc598d343d..ac7de4141738 100644
--- a/net/mptcp/pm.c
+++ b/net/mptcp/pm.c
@@ -903,7 +903,8 @@ static int mptcp_add_addr_len(int family, bool echo, bool port)
 }
 
 bool mptcp_pm_add_addr_signal(struct mptcp_sock *msk, int *size, int remaining,
-			      struct mptcp_addr_info *addr, bool *echo)
+			      struct mptcp_addr_info *addr, bool *echo,
+			      bool *drop_ts)
 {
 	bool skip_add_addr = false;
 	bool ret = false;
@@ -941,6 +942,13 @@ bool mptcp_pm_add_addr_signal(struct mptcp_sock *msk, int *size, int remaining,
 	if (len > remaining) {
 		struct net *net = sock_net((struct sock *)msk);
 
+		if (*drop_ts && mptcp_add_addr_v6_port_drop_ts(net)) {
+			/* OK without TCP Timestamps? */
+			len -= TCPOLEN_TSTAMP_ALIGNED;
+			if (len <= remaining)
+				goto enough_space;
+		}
+
 		if (*echo) {
 			MPTCP_INC_STATS(net, MPTCP_MIB_ECHOADDTXDROP);
 		} else {
@@ -950,6 +958,9 @@ bool mptcp_pm_add_addr_signal(struct mptcp_sock *msk, int *size, int remaining,
 		goto drop_signal_mark;
 	}
 
+	*drop_ts = false;
+
+enough_space:
 	ret = true;
 	*size = len;
 
diff --git a/net/mptcp/protocol.h b/net/mptcp/protocol.h
index b43dae72e7de..e69fcb4d48af 100644
--- a/net/mptcp/protocol.h
+++ b/net/mptcp/protocol.h
@@ -1208,7 +1208,8 @@ static inline bool mptcp_pm_is_kernel(const struct mptcp_sock *msk)
 }
 
 bool mptcp_pm_add_addr_signal(struct mptcp_sock *msk, int *size, int remaining,
-			      struct mptcp_addr_info *addr, bool *echo);
+			      struct mptcp_addr_info *addr, bool *echo,
+			      bool *drop_ts);
 bool mptcp_pm_rm_addr_signal(struct mptcp_sock *msk, unsigned int remaining,
 			     struct mptcp_rm_list *rm_list, int *len);
 int mptcp_pm_get_local_id(struct mptcp_sock *msk, struct sock_common *skc);