FAILED: patch "[PATCH] tcp: clamp route advmss to TCP_MIN_MSS" failed to apply to 5.15-stable tree

PATCH New [email protected] 2 participants 5 messages Aug 31 – Sep 1

Subsystems (3)

Networking Stable branch TCP

lore

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lore

[email protected]

author maintainer

2026-08-31 12:53 UTC #0 lore raw

The patch below does not apply to the 5.15-stable tree.
If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to <[email protected]>.

To reproduce the conflict and resubmit, you may use the following commands:

git fetch https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/ linux-5.15.y
git checkout FETCH_HEAD
git cherry-pick -x 870a9e42ecc6fe1b8c25d87af043cb0d9c178fe1
# <resolve conflicts, build, test, etc.>
git commit -s
git send-email --to '<[email protected]>' --in-reply-to '2026083106-sterile-daunting-b716@gregkh' --subject-prefix 'PATCH 5.15.y' 'HEAD^..'

Possible dependencies:

thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From 870a9e42ecc6fe1b8c25d87af043cb0d9c178fe1 Mon Sep 17 00:00:00 2001
From: Yong Wang <[email protected]>
Date: Wed, 19 Aug 2026 23:22:04 +0800
Subject: [PATCH] tcp: clamp route advmss to TCP_MIN_MSS

tcp_select_initial_window() assumes that callers never pass an MSS smaller than 1, but route-derived advmss values can violate that assumption.

A too-small explicit RTAX_ADVMSS is one way to get there, but it is not the only one. The same divide-by-zero can also be reached through the "default advmss" path when RTAX_ADVMSS is left at 0 and the effective advmss is later driven down by route MTU and min_adv_mss.

Introduce a tcp_dst_advmss() helper that clamps route advmss to TCP_MIN_MSS before TCP consumes it, and use it in the TCP paths that derive advmss from dst metrics. This keeps the effective MSS from dropping to zero before tcp_select_initial_window() rounds the receive window.

Fixes 1da177e4c3f4 ("Linux-2.6.12-rc2")

Cc [email protected]

Reported-by Vega <[email protected]>

Signed-off-by Yong Wang <[email protected]>

Signed-off-by Ren Wei <[email protected]>

Link https://patch.msgid.link/251eaf8277fa7c66364c9815c5da01662d269181.1787074852.git.edragain@163.com open here

Signed-off-by Jakub Kicinski <[email protected]>

include/net/tcp.h +5 -0

index 2c5b889530b5..670c20876f26 100644
@@ -1782,6 +1782,11 @@ static inline int tcp_full_space(const struct sock *sk)
 	return tcp_win_from_space(sk, READ_ONCE(sk->sk_rcvbuf));
 }
 
+static inline u32 tcp_dst_advmss(const struct dst_entry *dst)
+{
+	return max_t(u32, dst_metric_advmss(dst), TCP_MIN_MSS);
+}
+
 static inline void __tcp_adjust_rcv_ssthresh(struct sock *sk, u32 new_ssthresh)
 {
 	int unused_mem = sk_unused_reserved_mem(sk);

net/ipv4/tcp_ipv4.c +1 -1

index 190c7af4cf92..9f053eb8b46e 100644
@@ -1736,7 +1736,7 @@ struct sock *tcp_v4_syn_recv_sock(const struct sock *sk, struct sk_buff *skb,
 	tcp_ca_openreq_child(newsk, dst);
 
 	tcp_sync_mss(newsk, dst4_mtu(dst));
-	newtp->advmss = tcp_mss_clamp(tcp_sk(sk), dst_metric_advmss(dst));
+	newtp->advmss = tcp_mss_clamp(tcp_sk(sk), tcp_dst_advmss(dst));
 
 	tcp_initialize_rcv_mss(newsk);

net/ipv4/tcp_minisocks.c +1 -1

index 12254e6eb2f3..f3fa0b18eda0 100644
@@ -440,7 +440,7 @@ void tcp_openreq_init_rwin(struct request_sock *req,
 	u32 rcv_wnd;
 	int mss;
 
-	mss = tcp_mss_clamp(tp, dst_metric_advmss(dst));
+	mss = tcp_mss_clamp(tp, tcp_dst_advmss(dst));
 	window_clamp = READ_ONCE(tp->window_clamp);
 	/* Set this up on the first call only */
 	req->rsk_window_clamp = window_clamp ? : dst_metric(dst, RTAX_WINDOW);

net/ipv4/tcp_output.c +3 -3

index fcaa04e65189..f2709d585edb 100644
@@ -143,7 +143,7 @@ static __u16 tcp_advertise_mss(struct sock *sk)
 	int mss = tp->advmss;
 
 	if (dst) {
-		unsigned int metric = dst_metric_advmss(dst);
+		unsigned int metric = tcp_dst_advmss(dst);
 
 		if (metric < mss) {
 			mss = metric;

@@ -3972,7 +3972,7 @@ struct sk_buff *tcp_make_synack(const struct sock *sk, struct dst_entry *dst,
 	}
 	skb_dst_set(skb, dst);
 
-	mss = tcp_mss_clamp(tp, dst_metric_advmss(dst));
+	mss = tcp_mss_clamp(tp, tcp_dst_advmss(dst));
 
 	memset(&opts, 0, sizeof(opts));
 	now = tcp_clock_ns();

@@ -4128,7 +4128,7 @@ static void tcp_connect_init(struct sock *sk)
 
 	if (!tp->window_clamp)
 		WRITE_ONCE(tp->window_clamp, dst_metric(dst, RTAX_WINDOW));
-	tp->advmss = tcp_mss_clamp(tp, dst_metric_advmss(dst));
+	tp->advmss = tcp_mss_clamp(tp, tcp_dst_advmss(dst));
 
 	tcp_initialize_rcv_mss(sk);

net/ipv6/tcp_ipv6.c +1 -1

index 9e9155b1b3aa..df9c29eb5c1f 100644
@@ -1487,7 +1487,7 @@ static struct sock *tcp_v6_syn_recv_sock(const struct sock *sk, struct sk_buff *
 	tcp_ca_openreq_child(newsk, dst);
 
 	tcp_sync_mss(newsk, dst6_mtu(dst));
-	newtp->advmss = tcp_mss_clamp(tcp_sk(sk), dst_metric_advmss(dst));
+	newtp->advmss = tcp_mss_clamp(tcp_sk(sk), tcp_dst_advmss(dst));
 
 	tcp_initialize_rcv_mss(newsk);

Sasha Levin

maintainer

2026-09-01 22:21 UTC #1 lore raw

PATCH 1/4 inet: add dst4_mtu() and dst6_mtu() helpers

From: Eric Dumazet <[email protected]>

[ Upstream commit 8776c4ef3ab22b95f55713f00a7a576e6402507d ]

With CONFIG_MITIGATION_RETPOLINE=y dst_mtu() is a bit fat, because it is generic.

Indeed, clang does not always inline it.

Add dst4_mtu() and dst6_mtu() helpers for callers that expect either ipv4_mtu() or ip6_mtu() to be called.

These helpers are always inlined.

Signed-off-by Eric Dumazet <[email protected]>

Reviewed-by Kuniyuki Iwashima <[email protected]>

Link https://patch.msgid.link/[email protected] open here

Signed-off-by Jakub Kicinski <[email protected]>

Stable-dep-of: 870a9e42ecc6 ("tcp: clamp route advmss to TCP_MIN_MSS")

Signed-off-by Sasha Levin <[email protected]>

---
 include/net/dst.h       | 6 ++++++
 include/net/ip6_route.h | 6 ++++++
 2 files changed, 12 insertions(+)

include/net/dst.h +6 -0

index 9a23986f1071a..9adea166d2d97 100644
@@ -201,6 +201,12 @@ static inline u32 dst_mtu(const struct dst_entry *dst)
 	return INDIRECT_CALL_INET(dst->ops->mtu, ip6_mtu, ipv4_mtu, dst);
 }
 
+/* Variant of dst_mtu() for IPv4 users. */
+static inline u32 dst4_mtu(const struct dst_entry *dst)
+{
+	return INDIRECT_CALL_1(dst->ops->mtu, ipv4_mtu, dst);
+}
+
 /* RTT metrics are stored in milliseconds for user ABI, but used as jiffies */
 static inline unsigned long dst_metric_rtt(const struct dst_entry *dst, int metric)
 {

include/net/ip6_route.h +6 -0

index 5efd0b71dc673..299d964850fd3 100644
@@ -263,6 +263,12 @@ static inline bool ipv6_anycast_destination(const struct dst_entry *dst,
 int ip6_fragment(struct net *net, struct sock *sk, struct sk_buff *skb,
 		 int (*output)(struct net *, struct sock *, struct sk_buff *));
 
+/* Variant of dst_mtu() for IPv6 users */
+static inline u32 dst6_mtu(const struct dst_entry *dst)
+{
+	return INDIRECT_CALL_1(dst->ops->mtu, ip6_mtu, dst);
+}
+
 static inline unsigned int ip6_skb_dst_mtu(struct sk_buff *skb)
 {
 	unsigned int mtu;

Signature · 1 line

2.53.0

Sasha Levin

maintainer

2026-09-01 22:21 UTC #2 lore raw

PATCH 2/4 ipv6: use dst6_mtu() instead of dst_mtu()

From: Eric Dumazet <[email protected]>

[ Upstream commit b40f0130a23b6bab8b359e158a40af81f11f14be ]

When we expect an IPv6 dst, use dst6_mtu() instead of dst_mtu() to save some code space.

Due to current dst6_mtu() implementation, only convert users in IPv6 stack.

Signed-off-by Eric Dumazet <[email protected]>

Reviewed-by Kuniyuki Iwashima <[email protected]>

Link https://patch.msgid.link/[email protected] open here

Signed-off-by Jakub Kicinski <[email protected]>

Stable-dep-of: 870a9e42ecc6 ("tcp: clamp route advmss to TCP_MIN_MSS")

Signed-off-by Sasha Levin <[email protected]>

---
 net/ipv6/ip6_gre.c       |  3 ++-
 net/ipv6/ip6_output.c    |  6 +++---
 net/ipv6/ip6_tunnel.c    |  4 ++--
 net/ipv6/ipv6_sockglue.c |  4 ++--
 net/ipv6/route.c         | 10 ++++++----
 net/ipv6/tcp_ipv6.c      |  9 +++++----
 6 files changed, 20 insertions(+), 16 deletions(-)

net/ipv6/ip6_gre.c +2 -1

index f9c088cc94395..3ad4edba5f32b 100644
@@ -1071,9 +1071,10 @@ static netdev_tx_t ip6erspan_tunnel_xmit(struct sk_buff *skb,
 	gre_build_header(skb, 8, TUNNEL_SEQ, proto, 0, htonl(atomic_fetch_inc(&t->o_seqno)));
 
 	/* TooBig packet may have updated dst->dev's mtu */
-	if (!t->parms.collect_md && dst && dst_mtu(dst) > dst->dev->mtu)
+	if (!t->parms.collect_md && dst && dst6_mtu(dst) > dst->dev->mtu)
 		dst->ops->update_pmtu(dst, NULL, skb, dst->dev->mtu, false);
 
+
 	err = ip6_tnl_xmit(skb, dev, dsfield, &fl6, encap_limit, &mtu,
 			   NEXTHDR_GRE);
 	if (err != 0) {

net/ipv6/ip6_output.c +3 -3

index ab745cda1179b..1dbcd79dd9bb0 100644
@@ -323,7 +323,7 @@ int ip6_xmit(const struct sock *sk, struct sk_buff *skb, struct flowi6 *fl6,
 	skb->priority = priority;
 	skb->mark = mark;
 
-	mtu = dst_mtu(dst);
+	mtu = dst6_mtu(dst);
 	if ((skb->len <= mtu) || skb->ignore_df || skb_is_gso(skb)) {
 		IP6_UPD_PO_STATS(net, idev, IPSTATS_MIB_OUT, skb->len);

@@ -1364,10 +1364,10 @@ static int ip6_setup_cork(struct sock *sk, struct inet_cork_full *cork,
 	v6_cork->tclass = ipc6->tclass;
 	if (rt->dst.flags & DST_XFRM_TUNNEL)
 		mtu = np->pmtudisc >= IPV6_PMTUDISC_PROBE ?
-		      READ_ONCE(rt->dst.dev->mtu) : dst_mtu(&rt->dst);
+		      READ_ONCE(rt->dst.dev->mtu) : dst6_mtu(&rt->dst);
 	else
 		mtu = np->pmtudisc >= IPV6_PMTUDISC_PROBE ?
-			READ_ONCE(rt->dst.dev->mtu) : dst_mtu(xfrm_dst_path(&rt->dst));
+			READ_ONCE(rt->dst.dev->mtu) : dst6_mtu(xfrm_dst_path(&rt->dst));
 	if (np->frag_size < mtu) {
 		if (np->frag_size)
 			mtu = np->frag_size;

net/ipv6/ip6_tunnel.c +2 -2

index 45e0cec505c26..9f4c71889d911 100644
@@ -643,7 +643,7 @@ ip4ip6_err(struct sk_buff *skb, struct inet6_skb_parm *opt,
 
 	/* change mtu on this route */
 	if (rel_type == ICMP_DEST_UNREACH && rel_code == ICMP_FRAG_NEEDED) {
-		if (rel_info > dst_mtu(skb_dst(skb2)))
+		if (rel_info > dst6_mtu(skb_dst(skb2)))
 			goto out;
 
 		skb_dst_update_pmtu_no_confirm(skb2, rel_info);

@@ -1185,7 +1185,7 @@ int ip6_tnl_xmit(struct sk_buff *skb, struct net_device *dev, __u8 dsfield,
 				     t->parms.name);
 		goto tx_err_dst_release;
 	}
-	mtu = dst_mtu(dst) - eth_hlen - psh_hlen - t->tun_hlen;
+	mtu = dst6_mtu(dst) - eth_hlen - psh_hlen - t->tun_hlen;
 	if (encap_limit >= 0) {
 		max_headroom += 8;
 		mtu -= 8;

net/ipv6/ipv6_sockglue.c +2 -2

index 41e8b02c12c83..3d598d7fde1f8 100644
@@ -1210,7 +1210,7 @@ static int do_ipv6_getsockopt(struct sock *sk, int level, int optname,
 		rcu_read_lock();
 		dst = __sk_dst_get(sk);
 		if (dst)
-			val = dst_mtu(dst);
+			val = dst6_mtu(dst);
 		rcu_read_unlock();
 		if (!val)
 			return -ENOTCONN;

@@ -1309,7 +1309,7 @@ static int do_ipv6_getsockopt(struct sock *sk, int level, int optname,
 		rcu_read_lock();
 		dst = __sk_dst_get(sk);
 		if (dst)
-			mtuinfo.ip6m_mtu = dst_mtu(dst);
+			mtuinfo.ip6m_mtu = dst6_mtu(dst);
 		rcu_read_unlock();
 		if (!mtuinfo.ip6m_mtu)
 			return -ENOTCONN;

net/ipv6/route.c +6 -4

index 43b8995142e2e..63e8435c4b82f 100644
@@ -1990,6 +1990,8 @@ static void rt6_update_exception_stamp_rt(struct rt6_info *rt)
 static bool rt6_mtu_change_route_allowed(struct inet6_dev *idev,
 					 struct rt6_info *rt, int mtu)
 {
+	u32 dmtu = dst6_mtu(&rt->dst);
+
 	/* If the new MTU is lower than the route PMTU, this new MTU will be the
 	 * lowest MTU in the path: always allow updating the route PMTU to
 	 * reflect PMTU decreases.

@@ -2000,10 +2002,10 @@ static bool rt6_mtu_change_route_allowed(struct inet6_dev *idev,
 	 * handle this.
 	 */
 
-	if (dst_mtu(&rt->dst) >= mtu)
+	if (dmtu >= mtu)
 		return true;
 
-	if (dst_mtu(&rt->dst) == idev->cnf.mtu6)
+	if (dmtu == idev->cnf.mtu6)
 		return true;
 
 	return false;

@@ -2867,7 +2869,7 @@ static void __ip6_rt_update_pmtu(struct dst_entry *dst, const struct sock *sk,
 
 	if (mtu < IPV6_MIN_MTU)
 		return;
-	if (mtu >= dst_mtu(dst))
+	if (mtu >= dst6_mtu(dst))
 		return;
 
 	if (!rt6_cache_allowed_for_pmtu(rt6)) {

@@ -3184,7 +3186,7 @@ EXPORT_SYMBOL_GPL(ip6_sk_redirect);
 static unsigned int ip6_default_advmss(const struct dst_entry *dst)
 {
 	struct net_device *dev = dst->dev;
-	unsigned int mtu = dst_mtu(dst);
+	unsigned int mtu = dst6_mtu(dst);
 	struct net *net;
 
 	mtu -= sizeof(struct ipv6hdr) + sizeof(struct tcphdr);

net/ipv6/tcp_ipv6.c +5 -4

index dea59b123eb75..5c6b0d98730d9 100644
@@ -354,7 +354,7 @@ static int tcp_v6_connect(struct sock *sk, struct sockaddr *uaddr,
 static void tcp_v6_mtu_reduced(struct sock *sk)
 {
 	struct dst_entry *dst;
-	u32 mtu;
+	u32 mtu, dmtu;
 
 	if ((1 << sk->sk_state) & (TCPF_LISTEN | TCPF_CLOSE))
 		return;

@@ -371,8 +371,9 @@ static void tcp_v6_mtu_reduced(struct sock *sk)
 	if (!dst)
 		return;
 
-	if (inet_csk(sk)->icsk_pmtu_cookie > dst_mtu(dst)) {
-		tcp_sync_mss(sk, dst_mtu(dst));
+	dmtu = dst6_mtu(dst);
+	if (inet_csk(sk)->icsk_pmtu_cookie > dmtu) {
+		tcp_sync_mss(sk, dmtu);
 		tcp_simple_retransmit(sk);
 	}
 }

@@ -1394,7 +1395,7 @@ static struct sock *tcp_v6_syn_recv_sock(const struct sock *sk, struct sk_buff *
 
 	tcp_ca_openreq_child(newsk, dst);
 
-	tcp_sync_mss(newsk, dst_mtu(dst));
+	tcp_sync_mss(newsk, dst6_mtu(dst));
 	newtp->advmss = tcp_mss_clamp(tcp_sk(sk), dst_metric_advmss(dst));
 
 	tcp_initialize_rcv_mss(newsk);

Signature · 1 line

2.53.0

Sasha Levin

maintainer

2026-09-01 22:21 UTC #3 lore raw

PATCH 3/4 ipv4: use dst4_mtu() instead of dst_mtu()

In reply to Sasha Levin

From: Eric Dumazet <[email protected]>

[ Upstream commit fe8570186f100b6cc499b2f7705946baf1388cde ]

When we expect an IPv4 dst, use dst4_mtu() instead of dst_mtu() to save some code space.

Signed-off-by Eric Dumazet <[email protected]>

Reviewed-by Kuniyuki Iwashima <[email protected]>

Link https://patch.msgid.link/[email protected] open here

Signed-off-by Jakub Kicinski <[email protected]>

Stable-dep-of: 870a9e42ecc6 ("tcp: clamp route advmss to TCP_MIN_MSS")

Signed-off-by Sasha Levin <[email protected]>

---
 net/ipv4/icmp.c                     |  2 +-
 net/ipv4/ip_output.c                |  4 ++--
 net/ipv4/ip_sockglue.c              |  2 +-
 net/ipv4/ipmr.c                     |  2 +-
 net/ipv4/netfilter/nf_reject_ipv4.c |  2 +-
 net/ipv4/tcp_ipv4.c                 | 13 ++++++-------
 net/ipv6/sit.c                      |  2 +-
 7 files changed, 13 insertions(+), 14 deletions(-)

net/ipv4/icmp.c +1 -1

index 49c94aebac32d..b1812362f0965 100644
@@ -768,7 +768,7 @@ void __icmp_send(struct sk_buff *skb_in, int type, int code, __be32 info,
 
 	/* RFC says return as much as we can without exceeding 576 bytes. */
 
-	room = dst_mtu(&rt->dst);
+	room = dst4_mtu(&rt->dst);
 	if (room > 576)
 		room = 576;
 	room -= sizeof(struct iphdr) + icmp_param.replyopts.opt.opt.optlen;

net/ipv4/ip_output.c +2 -2

index fb64f5fcc422f..ffa8d6363a820 100644
@@ -1262,7 +1262,7 @@ static int ip_setup_cork(struct sock *sk, struct inet_cork *cork,
 		return -EFAULT;
 
 	cork->fragsize = ip_sk_use_pmtu(sk) ?
-			 dst_mtu(&rt->dst) : READ_ONCE(rt->dst.dev->mtu);
+			 dst4_mtu(&rt->dst) : READ_ONCE(rt->dst.dev->mtu);
 
 	if (!inetdev_valid_mtu(cork->fragsize))
 		return -ENETUNREACH;

@@ -1528,7 +1528,7 @@ struct sk_buff *__ip_make_skb(struct sock *sk,
 	 * locally. */
 	if (inet->pmtudisc == IP_PMTUDISC_DO ||
 	    inet->pmtudisc == IP_PMTUDISC_PROBE ||
-	    (skb->len <= dst_mtu(&rt->dst) &&
+	    (skb->len <= dst4_mtu(&rt->dst) &&
 	     ip_dont_fragment(sk, &rt->dst)))
 		df = htons(IP_DF);

net/ipv4/ip_sockglue.c +1 -1

index 3620dc739a5bf..f985d9a0dcdac 100644
@@ -1631,7 +1631,7 @@ static int do_ip_getsockopt(struct sock *sk, int level, int optname,
 		val = 0;
 		dst = sk_dst_get(sk);
 		if (dst) {
-			val = dst_mtu(dst);
+			val = dst4_mtu(dst);
 			dst_release(dst);
 		}
 		if (!val) {

net/ipv4/ipmr.c +1 -1

index 5e582eab1ea19..1a9d499d914bf 100644
@@ -1860,7 +1860,7 @@ static void ipmr_queue_xmit(struct net *net, struct mr_table *mrt,
 
 	dev = rt->dst.dev;
 
-	if (skb->len+encap > dst_mtu(&rt->dst) && (ntohs(iph->frag_off) & IP_DF)) {
+	if (skb->len+encap > dst4_mtu(&rt->dst) && (ntohs(iph->frag_off) & IP_DF)) {
 		/* Do not fragment multicasts. Alas, IPv4 does not
 		 * allow to send ICMP, so that packets will disappear
 		 * to blackhole.

net/ipv4/netfilter/nf_reject_ipv4.c +1 -1

index fbd264aa0a7ec..f64552b2cb832 100644
@@ -298,7 +298,7 @@ void nf_send_reset(struct net *net, struct sock *sk, struct sk_buff *oldskb,
 	niph = ip_hdr(nskb);
 
 	/* "Never happens" */
-	if (nskb->len > dst_mtu(skb_dst(nskb)))
+	if (nskb->len > dst4_mtu(skb_dst(nskb)))
 		goto free_nskb;
 
 	nf_ct_attach(nskb, oldskb);

net/ipv4/tcp_ipv4.c +6 -7

index f7693166c5161..6f97dfbc5343d 100644
@@ -352,7 +352,7 @@ void tcp_v4_mtu_reduced(struct sock *sk)
 {
 	struct inet_sock *inet = inet_sk(sk);
 	struct dst_entry *dst;
-	u32 mtu;
+	u32 mtu, dmtu;
 
 	if ((1 << sk->sk_state) & (TCPF_LISTEN | TCPF_CLOSE))
 		return;

@@ -364,15 +364,14 @@ void tcp_v4_mtu_reduced(struct sock *sk)
 	/* Something is about to be wrong... Remember soft error
 	 * for the case, if this connection will not able to recover.
 	 */
-	if (mtu < dst_mtu(dst) && ip_dont_fragment(sk, dst))
+	dmtu = dst4_mtu(dst);
+	if (mtu < dmtu && ip_dont_fragment(sk, dst))
 		WRITE_ONCE(sk->sk_err_soft, EMSGSIZE);
 
-	mtu = dst_mtu(dst);
-
 	if (inet->pmtudisc != IP_PMTUDISC_DONT &&
 	    ip_sk_accept_pmtu(sk) &&
-	    inet_csk(sk)->icsk_pmtu_cookie > mtu) {
-		tcp_sync_mss(sk, mtu);
+	    inet_csk(sk)->icsk_pmtu_cookie > dmtu) {
+		tcp_sync_mss(sk, dmtu);
 
 		/* Resend the TCP packet because it's
 		 * clear that the old packet has been

@@ -1614,7 +1613,7 @@ struct sock *tcp_v4_syn_recv_sock(const struct sock *sk, struct sk_buff *skb,
 
 	tcp_ca_openreq_child(newsk, dst);
 
-	tcp_sync_mss(newsk, dst_mtu(dst));
+	tcp_sync_mss(newsk, dst4_mtu(dst));
 	newtp->advmss = tcp_mss_clamp(tcp_sk(sk), dst_metric_advmss(dst));
 
 	tcp_initialize_rcv_mss(newsk);

net/ipv6/sit.c +1 -1

index 4b16ddc3d6c7f..3f8e03cded539 100644
@@ -974,7 +974,7 @@ static netdev_tx_t ipip6_tunnel_xmit(struct sk_buff *skb,
 	iph6 = ipv6_hdr(skb);
 
 	if (df) {
-		mtu = dst_mtu(&rt->dst) - t_hlen;
+		mtu = dst4_mtu(&rt->dst) - t_hlen;
 
 		if (mtu < IPV4_MIN_MTU) {
 			DEV_STATS_INC(dev, collisions);

Signature · 1 line

2.53.0

Sasha Levin

maintainer

2026-09-01 22:21 UTC #4 lore raw

PATCH 4/4 tcp: clamp route advmss to TCP_MIN_MSS

In reply to Sasha Levin

From: Yong Wang <[email protected]>

[ Upstream commit 870a9e42ecc6fe1b8c25d87af043cb0d9c178fe1 ]

tcp_select_initial_window() assumes that callers never pass an MSS smaller than 1, but route-derived advmss values can violate that assumption.

Fixes 1da177e4c3f4 ("Linux-2.6.12-rc2")

Cc [email protected]

Reported-by Vega <[email protected]>

Signed-off-by Yong Wang <[email protected]>

Signed-off-by Ren Wei <[email protected]>

Link https://patch.msgid.link/251eaf8277fa7c66364c9815c5da01662d269181.1787074852.git.edragain@163.com open here

Signed-off-by Jakub Kicinski <[email protected]>

[ discarded upstream ssthresh refactor context and kept existing WRITE_ONCE-free window_clamp assignment style, replacing dst_metric_advmss() with tcp_dst_advmss() ]

Signed-off-by Sasha Levin <[email protected]>

---
 include/net/tcp.h        | 5 +++++
 net/ipv4/tcp_ipv4.c      | 2 +-
 net/ipv4/tcp_minisocks.c | 2 +-
 net/ipv4/tcp_output.c    | 6 +++---
 net/ipv6/tcp_ipv6.c      | 2 +-
 5 files changed, 11 insertions(+), 6 deletions(-)

include/net/tcp.h +5 -0

index be91d81d66ab3..48e0a1d363e84 100644
@@ -1436,6 +1436,11 @@ static inline int tcp_full_space(const struct sock *sk)
 	return tcp_win_from_space(sk, READ_ONCE(sk->sk_rcvbuf));
 }
 
+static inline u32 tcp_dst_advmss(const struct dst_entry *dst)
+{
+	return max_t(u32, dst_metric_advmss(dst), TCP_MIN_MSS);
+}
+
 void tcp_cleanup_rbuf(struct sock *sk, int copied);
 
 /* We provision sk_rcvbuf around 200% of sk_rcvlowat.

net/ipv4/tcp_ipv4.c +1 -1

index 6f97dfbc5343d..dac9c151f671b 100644
@@ -1614,7 +1614,7 @@ struct sock *tcp_v4_syn_recv_sock(const struct sock *sk, struct sk_buff *skb,
 	tcp_ca_openreq_child(newsk, dst);
 
 	tcp_sync_mss(newsk, dst4_mtu(dst));
-	newtp->advmss = tcp_mss_clamp(tcp_sk(sk), dst_metric_advmss(dst));
+	newtp->advmss = tcp_mss_clamp(tcp_sk(sk), tcp_dst_advmss(dst));
 
 	tcp_initialize_rcv_mss(newsk);

net/ipv4/tcp_minisocks.c +1 -1

index 061225d645e6b..e36c466b4b65a 100644
@@ -369,7 +369,7 @@ void tcp_openreq_init_rwin(struct request_sock *req,
 	u32 rcv_wnd;
 	int mss;
 
-	mss = tcp_mss_clamp(tp, dst_metric_advmss(dst));
+	mss = tcp_mss_clamp(tp, tcp_dst_advmss(dst));
 	window_clamp = READ_ONCE(tp->window_clamp);
 	/* Set this up on the first call only */
 	req->rsk_window_clamp = window_clamp ? : dst_metric(dst, RTAX_WINDOW);

net/ipv4/tcp_output.c +3 -3

index a2d5e9f1fe241..462c704cc18a9 100644
@@ -125,7 +125,7 @@ static __u16 tcp_advertise_mss(struct sock *sk)
 	int mss = tp->advmss;
 
 	if (dst) {
-		unsigned int metric = dst_metric_advmss(dst);
+		unsigned int metric = tcp_dst_advmss(dst);
 
 		if (metric < mss) {
 			mss = metric;

@@ -3597,7 +3597,7 @@ struct sk_buff *tcp_make_synack(const struct sock *sk, struct dst_entry *dst,
 	}
 	skb_dst_set(skb, dst);
 
-	mss = tcp_mss_clamp(tp, dst_metric_advmss(dst));
+	mss = tcp_mss_clamp(tp, tcp_dst_advmss(dst));
 
 	memset(&opts, 0, sizeof(opts));
 	now = tcp_clock_ns();

@@ -3713,7 +3713,7 @@ static void tcp_connect_init(struct sock *sk)
 
 	if (!tp->window_clamp)
 		tp->window_clamp = dst_metric(dst, RTAX_WINDOW);
-	tp->advmss = tcp_mss_clamp(tp, dst_metric_advmss(dst));
+	tp->advmss = tcp_mss_clamp(tp, tcp_dst_advmss(dst));
 
 	tcp_initialize_rcv_mss(sk);

net/ipv6/tcp_ipv6.c +1 -1

index 5c6b0d98730d9..7dc0d53c3c249 100644
@@ -1396,7 +1396,7 @@ static struct sock *tcp_v6_syn_recv_sock(const struct sock *sk, struct sk_buff *
 	tcp_ca_openreq_child(newsk, dst);
 
 	tcp_sync_mss(newsk, dst6_mtu(dst));
-	newtp->advmss = tcp_mss_clamp(tcp_sk(sk), dst_metric_advmss(dst));
+	newtp->advmss = tcp_mss_clamp(tcp_sk(sk), tcp_dst_advmss(dst));
 
 	tcp_initialize_rcv_mss(newsk);

Signature · 1 line

2.53.0