From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-alma10-1.taild15c8.ts.net [100.103.45.18]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 63C10220698 for ; Wed, 2 Sep 2026 00:33:32 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=100.103.45.18 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1788309213; cv=none; b=DXe9FfOPFNrl8s+6BQvDZS5gorqfXmL8fwZQrzCMFH7PXED7RdW7p0zABeHEbddkxpeP4oOLY6iDxlf1iCClAj6s/jITRWRizVuwFp95Ib6PhT1L/I4pbD8XN14q4iyZvLt5V8bsfW+vM4fVP38T/SzDXB4tCJ+pWe5vPFc0iv0= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1788309213; c=relaxed/simple; bh=a1pBh9ATPqbTnk3vinGnd6HktT8MFEGMJ0zkT5blEME=; h=From:To:Cc:Subject:Date:Message-ID:In-Reply-To:References: MIME-Version; b=ZvGjAU4Ro9m/QvbBVdqRHeeGqGakrpEsKDwJCidlmqgLr1sZpmbXH57R2XoQ5xGkARcl42yeMdt/pgCdmqFSvzxGMdnmNwEiQBXVLGrNkv5APgZ6h5FOtll86YWwkyiHQr5OqgSufjV+koMzPdAyAlmuTaxMqVShqTWFdEwMH8A= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=RmpEfnUB; arc=none smtp.client-ip=100.103.45.18 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="RmpEfnUB" Received: by smtp.kernel.org (Postfix) with ESMTPSA id AC3F91F00A3D; Wed, 2 Sep 2026 00:33:31 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1788309212; bh=VXr3OLsXKhnbCdRvoTz3NvVVVCB/eUVzL05VU98PXL8=; h=From:To:Cc:Subject:Date:In-Reply-To:References; b=RmpEfnUBO6rxhCBrgJQLFBYP9oJyDq4M/COjFh34FE7d/NYzwtrOsMHBvMmil+gGE TTxE5U4VBQp9HxBEod6AdM5x5KOIFnT/G8+PxbdkyTX8V4KMkr8cpBa4HJuUWHPY8l Y0iXSIO4w1R1fxVk9j4heKc79zphR1jaHYcrLXXr2HgrAd2nY7wVWNTH3o2WQt0oCT uRw6EwGkZVpMdNxelS4cBcXOqQyNGJV+hTuhfV181847NdUd/1fLYfVNt8eOtdJrIK pN4j9e8uve6RhmBQxFpOrNj5AE5sg6ZPiOiO9xXXusCxrZ2TDG4gVuNE08egmoHIZY FLe3zBTxOwpRA== From: Sasha Levin To: stable@vger.kernel.org Cc: Jiayuan Chen , Eric Dumazet , Jakub Kicinski , Sasha Levin Subject: [PATCH 5.15.y 3/3] net: advertise TCP MSS from the configured MTU, not the learned PMTU Date: Tue, 1 Sep 2026 20:33:28 -0400 Message-ID: <20260902003328.1792382-3-sashal@kernel.org> X-Mailer: git-send-email 2.53.0 In-Reply-To: <20260902003328.1792382-1-sashal@kernel.org> References: <2026083119-uncoated-animal-60e3@gregkh> <20260902003328.1792382-1-sashal@kernel.org> Precedence: bulk X-Mailing-List: stable@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit From: Jiayuan Chen [ Upstream commit 2640e64195948a601430d230c9864f5426574cde ] The MSS a host puts in its SYN tells the peer how big a segment it may send us. Right now we can shrink it with a PMTU we learned on our own send path, which is the wrong direction entirely. On asymmetric paths this bites - think DSR load balancers, where the request side goes through a smaller-MTU overlay. We learn a small PMTU going out, then advertise a small MSS, and the peer stays capped for the whole connection even though its path back to us is wide. MSS only shows up in the SYN and never grows back. On symmetric paths we lose nothing by dropping it either: the peer runs its own PMTU discovery and usually already knows the real path MTU. So work out the advertised MSS from the configured route or device MTU and ignore the learned PMTU. Our send side is unchanged, still clamped by tcp_current_mss(). Add ip_dst_mtu_configured()/ip6_dst_mtu_configured() and use them from the two default_advmss() paths. Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2") Fixes: 164a5e7ad531 ("ipv4: ipv4_default_advmss() should use route mtu") Cc: stable@vger.kernel.org Signed-off-by: Jiayuan Chen Reviewed-by: Eric Dumazet Link: https://patch.msgid.link/20260815070413.294559-1-jiayuan.chen@linux.dev Signed-off-by: Jakub Kicinski Signed-off-by: Sasha Levin --- include/net/ip.h | 25 +++++++++++++++++++++++++ include/net/ip6_route.h | 37 +++++++++++++++++++++++++++++++++++++ net/ipv4/route.c | 4 ++-- net/ipv6/route.c | 2 +- 4 files changed, 65 insertions(+), 3 deletions(-) diff --git a/include/net/ip.h b/include/net/ip.h index 8c9f7b40d6fef..28740cc6f3167 100644 --- a/include/net/ip.h +++ b/include/net/ip.h @@ -477,6 +477,31 @@ 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 ebab85b70641a..7b5006fc86297 100644 --- a/include/net/ip6_route.h +++ b/include/net/ip6_route.h @@ -341,6 +341,43 @@ 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 65b539c94125f..654ff3ecb6869 100644 --- a/net/ipv4/route.c +++ b/net/ipv4/route.c @@ -1377,8 +1377,8 @@ 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 8d3dc03238682..c925ba10eb96a 100644 --- a/net/ipv6/route.c +++ b/net/ipv6/route.c @@ -3183,7 +3183,7 @@ EXPORT_SYMBOL_GPL(ip6_sk_redirect); 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); -- 2.53.0