Merge branch 'mptcp-fallback-to-tcp-after-3-mpc-drop-cache'

Matthieu Baerts says:

====================
mptcp: fallback to TCP after 3 MPC drop + cache

The SYN + MPTCP_CAPABLE packets could be explicitly dropped by firewalls
somewhere in the network, e.g. if they decide to drop packets based on
the TCP options, instead of stripping them off.

The idea of this series is to fallback to TCP after 3 SYN+MPC drop
(patch 2). If the connection succeeds after the fallback, it very likely
means a blackhole has been detected. In this case (patch 3), MPTCP can
be disabled for a certain period of time, 1h by default. If after this
period, MPTCP is still blocked, the period is doubled. This technique is
inspired by the one used by TCP FastOpen.

This should help applications which want to use MPTCP by default on the
client side if available.
====================

Link: https://patch.msgid.link/20240909-net-next-mptcp-fallback-x-mpc-v1-0-da7ebb4cd2a3@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

1 files changed, 133 insertions, 0 deletions

diff --git a/net/mptcp/ctrl.c b/net/mptcp/ctrl.c
index 99382c317ebb..38d8121331d4 100644
--- a/net/mptcp/ctrl.c
+++ b/net/mptcp/ctrl.c
@@ -12,6 +12,7 @@
 #include <net/netns/generic.h>
 
 #include "protocol.h"
+#include "mib.h"
 
 #define MPTCP_SYSCTL_PATH "net/mptcp"
 
@@ -27,8 +28,11 @@ struct mptcp_pernet {
 #endif
 
 	unsigned int add_addr_timeout;
+	unsigned int blackhole_timeout;
 	unsigned int close_timeout;
 	unsigned int stale_loss_cnt;
+	atomic_t active_disable_times;
+	unsigned long active_disable_stamp;
 	u8 mptcp_enabled;
 	u8 checksum_enabled;
 	u8 allow_join_initial_addr_port;
@@ -87,6 +91,8 @@ static void mptcp_pernet_set_defaults(struct mptcp_pernet *pernet)
 {
 	pernet->mptcp_enabled = 1;
 	pernet->add_addr_timeout = TCP_RTO_MAX;
+	pernet->blackhole_timeout = 3600;
+	atomic_set(&pernet->active_disable_times, 0);
 	pernet->close_timeout = TCP_TIMEWAIT_LEN;
 	pernet->checksum_enabled = 0;
 	pernet->allow_join_initial_addr_port = 1;
@@ -151,6 +157,20 @@ static int proc_available_schedulers(const struct ctl_table *ctl,
 	return ret;
 }
 
+static int proc_blackhole_detect_timeout(const struct ctl_table *table,
+					 int write, void *buffer, size_t *lenp,
+					 loff_t *ppos)
+{
+	struct mptcp_pernet *pernet = mptcp_get_pernet(current->nsproxy->net_ns);
+	int ret;
+
+	ret = proc_dointvec_minmax(table, write, buffer, lenp, ppos);
+	if (write && ret == 0)
+		atomic_set(&pernet->active_disable_times, 0);
+
+	return ret;
+}
+
 static struct ctl_table mptcp_sysctl_table[] = {
 	{
 		.procname = "enabled",
@@ -217,6 +237,13 @@ static struct ctl_table mptcp_sysctl_table[] = {
 		.mode = 0644,
 		.proc_handler = proc_dointvec_jiffies,
 	},
+	{
+		.procname = "blackhole_timeout",
+		.maxlen = sizeof(unsigned int),
+		.mode = 0644,
+		.proc_handler = proc_blackhole_detect_timeout,
+		.extra1 = SYSCTL_ZERO,
+	},
 };
 
 static int mptcp_pernet_new_table(struct net *net, struct mptcp_pernet *pernet)
@@ -240,6 +267,7 @@ static int mptcp_pernet_new_table(struct net *net, struct mptcp_pernet *pernet)
 	table[6].data = &pernet->scheduler;
 	/* table[7] is for available_schedulers which is read-only info */
 	table[8].data = &pernet->close_timeout;
+	table[9].data = &pernet->blackhole_timeout;
 
 	hdr = register_net_sysctl_sz(net, MPTCP_SYSCTL_PATH, table,
 				     ARRAY_SIZE(mptcp_sysctl_table));
@@ -277,6 +305,111 @@ static void mptcp_pernet_del_table(struct mptcp_pernet *pernet) {}
 
 #endif /* CONFIG_SYSCTL */
 
+/* The following code block is to deal with middle box issues with MPTCP,
+ * similar to what is done with TFO.
+ * The proposed solution is to disable active MPTCP globally when SYN+MPC are
+ * dropped, while SYN without MPC aren't. In this case, active side MPTCP is
+ * disabled globally for 1hr at first. Then if it happens again, it is disabled
+ * for 2h, then 4h, 8h, ...
+ * The timeout is reset back to 1hr when a successful active MPTCP connection is
+ * fully established.
+ */
+
+/* Disable active MPTCP and record current jiffies and active_disable_times */
+void mptcp_active_disable(struct sock *sk)
+{
+	struct net *net = sock_net(sk);
+	struct mptcp_pernet *pernet;
+
+	pernet = mptcp_get_pernet(net);
+
+	if (!READ_ONCE(pernet->blackhole_timeout))
+		return;
+
+	/* Paired with READ_ONCE() in mptcp_active_should_disable() */
+	WRITE_ONCE(pernet->active_disable_stamp, jiffies);
+
+	/* Paired with smp_rmb() in mptcp_active_should_disable().
+	 * We want pernet->active_disable_stamp to be updated first.
+	 */
+	smp_mb__before_atomic();
+	atomic_inc(&pernet->active_disable_times);
+
+	MPTCP_INC_STATS(net, MPTCP_MIB_BLACKHOLE);
+}
+
+/* Calculate timeout for MPTCP active disable
+ * Return true if we are still in the active MPTCP disable period
+ * Return false if timeout already expired and we should use active MPTCP
+ */
+bool mptcp_active_should_disable(struct sock *ssk)
+{
+	struct net *net = sock_net(ssk);
+	unsigned int blackhole_timeout;
+	struct mptcp_pernet *pernet;
+	unsigned long timeout;
+	int disable_times;
+	int multiplier;
+
+	pernet = mptcp_get_pernet(net);
+	blackhole_timeout = READ_ONCE(pernet->blackhole_timeout);
+
+	if (!blackhole_timeout)
+		return false;
+
+	disable_times = atomic_read(&pernet->active_disable_times);
+	if (!disable_times)
+		return false;
+
+	/* Paired with smp_mb__before_atomic() in mptcp_active_disable() */
+	smp_rmb();
+
+	/* Limit timeout to max: 2^6 * initial timeout */
+	multiplier = 1 << min(disable_times - 1, 6);
+
+	/* Paired with the WRITE_ONCE() in mptcp_active_disable(). */
+	timeout = READ_ONCE(pernet->active_disable_stamp) +
+		  multiplier * blackhole_timeout * HZ;
+
+	return time_before(jiffies, timeout);
+}
+
+/* Enable active MPTCP and reset active_disable_times if needed */
+void mptcp_active_enable(struct sock *sk)
+{
+	struct mptcp_pernet *pernet = mptcp_get_pernet(sock_net(sk));
+
+	if (atomic_read(&pernet->active_disable_times)) {
+		struct dst_entry *dst = sk_dst_get(sk);
+
+		if (dst && dst->dev && (dst->dev->flags & IFF_LOOPBACK))
+			atomic_set(&pernet->active_disable_times, 0);
+	}
+}
+
+/* Check the number of retransmissions, and fallback to TCP if needed */
+void mptcp_active_detect_blackhole(struct sock *ssk, bool expired)
+{
+	struct mptcp_subflow_context *subflow;
+	u32 timeouts;
+
+	if (!sk_is_mptcp(ssk))
+		return;
+
+	timeouts = inet_csk(ssk)->icsk_retransmits;
+	subflow = mptcp_subflow_ctx(ssk);
+
+	if (subflow->request_mptcp && ssk->sk_state == TCP_SYN_SENT) {
+		if (timeouts == 2 || (timeouts < 2 && expired)) {
+			MPTCP_INC_STATS(sock_net(ssk), MPTCP_MIB_MPCAPABLEACTIVEDROP);
+			subflow->mpc_drop = 1;
+			mptcp_subflow_early_fallback(mptcp_sk(subflow->conn), subflow);
+		} else {
+			subflow->mpc_drop = 0;
+		}
+	}
+}
+
 static int __net_init mptcp_net_init(struct net *net)
 {
 	struct mptcp_pernet *pernet = mptcp_get_pernet(net);