diff --git a/include/net/ip.h b/include/net/ip.h
index 7f2fe1a8401b..a8f57b4f4aa2 100644
--- a/include/net/ip.h
+++ b/include/net/ip.h
@@ -506,6 +506,31 @@ static inline unsigned int ip_dst_mtu_maybe_forward(const struct dst_entry *dst,
 	return res;
 }
 
+/* Configured/administrative MTU of a route, for advertising the TCP MSS.
+ *
+ * Unlike ip_dst_mtu_maybe_forward(), this deliberately ignores the
+ * ICMP-learned path MTU (rt->rt_pmtu).  The advertised MSS bounds what the
+ * peer may send to us and must reflect our receive capability (the device or
+ * route-configured MTU), not a path MTU learned on the reverse (send)
+ * direction, which may not apply to the peer->us path and outlives the fnhe
+ * for the whole connection.  See RFC 2923 section 2.3 and the comment above
+ * tcp_advertise_mss().
+ */
+static inline unsigned int ip_dst_mtu_configured(const struct dst_entry *dst)
+{
+	unsigned int mtu, res;
+
+	rcu_read_lock();
+	mtu = dst_metric_raw(dst, RTAX_MTU);
+	if (!mtu)
+		mtu = READ_ONCE(dst_dev_rcu(dst)->mtu);
+	mtu = min_t(unsigned int, mtu, IP_MAX_MTU);
+	res = mtu - lwtunnel_headroom(dst->lwtstate, mtu);
+	rcu_read_unlock();
+
+	return res;
+}
+
 static inline unsigned int ip_skb_dst_mtu(struct sock *sk,
 					  const struct sk_buff *skb)
 {
diff --git a/include/net/ip6_route.h b/include/net/ip6_route.h
index cc045705862d..c69f1c871922 100644
--- a/include/net/ip6_route.h
+++ b/include/net/ip6_route.h
@@ -387,6 +387,43 @@ static inline unsigned int ip6_dst_mtu_maybe_forward(const struct dst_entry *dst
 	return mtu - lwtunnel_headroom(dst->lwtstate, mtu);
 }
 
+/* Configured/administrative MTU of a route, for advertising the TCP MSS.
+ *
+ * Unlike ip6_dst_mtu_maybe_forward(), this ignores any ICMPv6-learned path
+ * MTU (which is kept on the RTF_CACHE exception route) and returns the MTU of
+ * the underlying route (fib6_pmtu) or the egress device.  The advertised MSS
+ * bounds what the peer may send to us and must reflect our receive
+ * capability, not a path MTU learned on the reverse (send) direction.  See
+ * RFC 2923 section 2.3 and the comment above tcp_advertise_mss().
+ */
+static inline unsigned int ip6_dst_mtu_configured(const struct dst_entry *dst)
+{
+	const struct rt6_info *rt = dst_rt6_info(dst);
+	const struct fib6_info *from;
+	struct inet6_dev *idev;
+	unsigned int mtu = 0;
+
+	rcu_read_lock();
+	/* IPv6 keeps the learned PMTU and the configured MTU in the same
+	 * RTAX_MTU slot: the learned value sits on this (possibly RTF_CACHE)
+	 * dst, the configured one on the underlying route.  Reach the latter
+	 * via ->from (fib6_pmtu), populated by ip6_route_info_create().
+	 */
+	from = rcu_dereference(rt->from);
+	if (from)
+		mtu = from->fib6_pmtu;
+	if (!mtu) {
+		mtu = IPV6_MIN_MTU;
+		idev = __in6_dev_get(dst_dev_rcu(dst));
+		if (idev)
+			mtu = max_t(unsigned int, mtu, READ_ONCE(idev->cnf.mtu6));
+	}
+	rcu_read_unlock();
+
+	mtu = min_t(unsigned int, mtu, IP6_MAX_MTU);
+	return mtu - lwtunnel_headroom(dst->lwtstate, mtu);
+}
+
 u32 ip6_mtu_from_fib6(const struct fib6_result *res,
 		      const struct in6_addr *daddr,
 		      const struct in6_addr *saddr);
diff --git a/net/ipv4/route.c b/net/ipv4/route.c
index 604cc51dfd9b..37674d76f90f 100644
--- a/net/ipv4/route.c
+++ b/net/ipv4/route.c
@@ -1363,8 +1363,8 @@ static unsigned int ipv4_default_advmss(const struct dst_entry *dst)
 
 	rcu_read_lock();
 	net = dst_dev_net_rcu(dst);
-	advmss = max_t(unsigned int, ipv4_mtu(dst) - header_size,
-				   net->ipv4.ip_rt_min_advmss);
+	advmss = max_t(unsigned int, ip_dst_mtu_configured(dst) - header_size,
+		       net->ipv4.ip_rt_min_advmss);
 	rcu_read_unlock();
 
 	return min(advmss, IPV4_MAX_PMTU - header_size);
diff --git a/net/ipv6/route.c b/net/ipv6/route.c
index 526535387512..93b4168044d9 100644
--- a/net/ipv6/route.c
+++ b/net/ipv6/route.c
@@ -3261,7 +3261,7 @@ void ip6_sk_redirect(struct sk_buff *skb, struct sock *sk)
 
 static unsigned int ip6_default_advmss(const struct dst_entry *dst)
 {
-	unsigned int mtu = dst6_mtu(dst);
+	unsigned int mtu = ip6_dst_mtu_configured(dst);
 	struct net *net;
 
 	mtu -= sizeof(struct ipv6hdr) + sizeof(struct tcphdr);