[PATCH net 0/2] Revert advertising the TCP MSS from the configured MTU

[PATCH net 0/2] Revert advertising the TCP MSS from the configured MTU

[Esko Mobius via B4 Relay]

Commit 2640e6419594 ("net: advertise TCP MSS from the configured MTU,
not the learned PMTU") breaks TCP for clients behind networks that
filter inbound ICMP, when the server reaches them through a hop with a
smaller MTU: the handshake completes, then the connection stalls as
soon as the client sends full-size segments. This was reported in [1]
and confirmed on 7.3-rc7; reverting the commit on top of 7.3-rc7 fixes
it. Details and numbers are in patch 2.

The asymmetric (DSR-style) case that commit targets can still be served
by the existing per-route advmss metric, which overrides the default
after the revert; patch 2 includes a test of that as well. If you would
rather keep the new behaviour as an explicit opt-in some other way, I
am happy to test or rework this.

Patch 1 removes the packetdrill tests that assert the reverted
behaviour, patch 2 is the revert itself.

[1] https://lore.kernel.org/regressions/4F0FAEB6-A13B-47F3-BFF1-C706686C6F64@gmail.com/

Signed-off-by: Esko Mobius <echoowall@gmail.com>
---
Esko Mobius (2):
      Revert "selftests: net: packetdrill: add tests for advertised MSS with PMTU exceptions"
      Revert "net: advertise TCP MSS from the configured MTU, not the learned PMTU"

 include/net/ip.h                                   | 25 --------
 include/net/ip6_route.h                            | 37 ------------
 net/ipv4/route.c                                   |  4 +-
 net/ipv6/route.c                                   |  2 +-
 .../net/packetdrill/tcp_advmss_pmtu_ipv4.pkt       | 67 ----------------------
 .../net/packetdrill/tcp_advmss_pmtu_ipv6.pkt       | 67 ----------------------
 6 files changed, 3 insertions(+), 199 deletions(-)
---
base-commit: 7076f684d5132e6a6145e1da2fa74771ecd052e5
change-id: 20261026-advmss-revert-5805de17369f

Best regards,
--  
Esko Mobius <echoowall@gmail.com>

[PATCH net 1/2] Revert "selftests: net: packetdrill: add tests for advertised MSS with PMTU exceptions"

[Esko Mobius via B4 Relay]

From: Esko Mobius <echoowall@gmail.com>

This reverts commit e5c8e301b497 ("selftests: net: packetdrill: add
tests for advertised MSS with PMTU exceptions").

These tests assert the behaviour introduced by commit 2640e6419594
("net: advertise TCP MSS from the configured MTU, not the learned
PMTU"), which is reverted in the next patch because it causes a
regression. Remove them first so the series stays bisectable.

Signed-off-by: Esko Mobius <echoowall@gmail.com>
---
 .../net/packetdrill/tcp_advmss_pmtu_ipv4.pkt       | 67 ----------------------
 .../net/packetdrill/tcp_advmss_pmtu_ipv6.pkt       | 67 ----------------------
 2 files changed, 134 deletions(-)

diff --git a/tools/testing/selftests/net/packetdrill/tcp_advmss_pmtu_ipv4.pkt b/tools/testing/selftests/net/packetdrill/tcp_advmss_pmtu_ipv4.pkt
deleted file mode 100644
index f2ef931b7..000000000
--- a/tools/testing/selftests/net/packetdrill/tcp_advmss_pmtu_ipv4.pkt
+++ /dev/null
@@ -1,67 +0,0 @@
-// SPDX-License-Identifier: GPL-2.0
-//
-// Test that IPv4 advertised MSS in SYN-ACK is derived from the configured
-// interface MTU (1500 -> MSS 1460), not the ICMP-learned Path MTU.
-
---ip_version=ipv4
-
-`./defaults.sh
-ethtool -K tun0 tso off
-`
-
-//
-// Connection 1: Learn PMTU exception (MTU 1200 -> MSS 1160)
-//
-    0 socket(..., SOCK_STREAM, IPPROTO_TCP) = 3
-   +0 setsockopt(3, SOL_SOCKET, SO_REUSEADDR, [1], 4) = 0
-   +0 bind(3, ..., ...) = 0
-   +0 listen(3, 1) = 0
-
-   +0 < S 0:0(0) win 65535 <mss 1460,sackOK,nop,nop,nop,wscale 8>
-   +0 > S. 0:0(0) ack 1 <mss 1460,nop,nop,sackOK,nop,wscale 8>
-  +.1 < . 1:1(0) ack 1 win 257
-   +0 accept(3, ..., ...) = 4
-
-// Send a full 1460-byte segment
-   +0 write(4, ..., 1460) = 1460
-   +0 > P. 1:1461(1460) ack 1
-
-// ICMP Fragmentation Needed arrives indicating next-hop MTU 1200
-   +0 < icmp unreachable frag_needed mtu 1200 [1:1461(1460)]
-
-// Local host retransmits using the learned MTU 1200 (MSS = 1200 - 40 = 1160)
-   +0 > . 1:1161(1160) ack 1
-   +0 > P. 1161:1461(300) ack 1
-   +0 < R 1:1(0) ack 1461 win 0
-
-// Close connection 1 and listener
-   +0 close(4) = 0
-   +0 close(3) = 0
-
-//
-// Connection 2: New connection from the same peer
-//
-   +0 socket(..., SOCK_STREAM, IPPROTO_TCP) = 3
-   +0 setsockopt(3, SOL_SOCKET, SO_REUSEADDR, [1], 4) = 0
-   +0 bind(3, ..., ...) = 0
-   +0 listen(3, 1) = 0
-
-   +0 < S 0:0(0) win 65535 <mss 1460,sackOK,nop,nop,nop,wscale 8>
-
-// Verify: SYN-ACK MUST advertise configured MSS 1460, NOT the learned PMTU MSS 1160
-   +0 > S. 0:0(0) ack 1 <mss 1460,nop,nop,sackOK,nop,wscale 8>
-   +0 < . 1:1(0) ack 1 win 257
-   +0 accept(3, ..., ...) = 4
-
-// Verify: Outgoing transmit MSS is still constrained by the learned PMTU 1200
-   +0 write(4, ..., 1460) = 1460
-   +0 > . 1:1161(1160) ack 1
-   +0 > P. 1161:1461(300) ack 1
-   +0 < . 1:1(0) ack 1461 win 257
-
-// Clean up
-   +0 close(4) = 0
-   +0 > F. 1461:1461(0) ack 1
-   +0 < F. 1:1(0) ack 1462 win 257
-   +0 > . 1462:1462(0) ack 2
-   +0 close(3) = 0
diff --git a/tools/testing/selftests/net/packetdrill/tcp_advmss_pmtu_ipv6.pkt b/tools/testing/selftests/net/packetdrill/tcp_advmss_pmtu_ipv6.pkt
deleted file mode 100644
index c7638b11a..000000000
--- a/tools/testing/selftests/net/packetdrill/tcp_advmss_pmtu_ipv6.pkt
+++ /dev/null
@@ -1,67 +0,0 @@
-// SPDX-License-Identifier: GPL-2.0
-//
-// Test that IPv6 advertised MSS in SYN-ACK is derived from the configured
-// interface MTU (1520 -> MSS 1460), not the ICMPv6-learned Path MTU.
-
---ip_version=ipv6
-
-`./defaults.sh
-ethtool -K tun0 tso off
-`
-
-//
-// Connection 1: Learn PMTU exception (MTU 1280 -> MSS 1220)
-//
-    0 socket(..., SOCK_STREAM, IPPROTO_TCP) = 3
-   +0 setsockopt(3, SOL_SOCKET, SO_REUSEADDR, [1], 4) = 0
-   +0 bind(3, ..., ...) = 0
-   +0 listen(3, 1) = 0
-
-   +0 < S 0:0(0) win 65535 <mss 1460,sackOK,nop,nop,nop,wscale 8>
-   +0 > S. 0:0(0) ack 1 <mss 1460,nop,nop,sackOK,nop,wscale 8>
-  +.1 < . 1:1(0) ack 1 win 257
-   +0 accept(3, ..., ...) = 4
-
-// Send a full 1460-byte segment
-   +0 write(4, ..., 1460) = 1460
-   +0 > P. 1:1461(1460) ack 1
-
-// ICMPv6 Packet Too Big arrives indicating next-hop MTU 1280
-   +0 < icmp packet_too_big mtu 1280 [1:1461(1460)]
-
-// Local host retransmits using the learned MTU 1280 (MSS = 1280 - 40 - 20 = 1220)
-   +0 > . 1:1221(1220) ack 1
-   +0 > P. 1221:1461(240) ack 1
-   +0 < R 1:1(0) ack 1461 win 0
-
-// Close connection 1 and listener
-   +0 close(4) = 0
-   +0 close(3) = 0
-
-//
-// Connection 2: New connection from the same peer
-//
-   +0 socket(..., SOCK_STREAM, IPPROTO_TCP) = 3
-   +0 setsockopt(3, SOL_SOCKET, SO_REUSEADDR, [1], 4) = 0
-   +0 bind(3, ..., ...) = 0
-   +0 listen(3, 1) = 0
-
-   +0 < S 0:0(0) win 65535 <mss 1460,sackOK,nop,nop,nop,wscale 8>
-
-// Verify: SYN-ACK MUST advertise configured MSS 1460, NOT the learned PMTU MSS 1220
-   +0 > S. 0:0(0) ack 1 <mss 1460,nop,nop,sackOK,nop,wscale 8>
-   +0 < . 1:1(0) ack 1 win 257
-   +0 accept(3, ..., ...) = 4
-
-// Verify: Outgoing transmit MSS is still constrained by the learned PMTU 1280
-   +0 write(4, ..., 1460) = 1460
-   +0 > . 1:1221(1220) ack 1
-   +0 > P. 1221:1461(240) ack 1
-   +0 < . 1:1(0) ack 1461 win 257
-
-// Clean up
-   +0 close(4) = 0
-   +0 > F. 1461:1461(0) ack 1
-   +0 < F. 1:1(0) ack 1462 win 257
-   +0 > . 1462:1462(0) ack 2
-   +0 close(3) = 0

-- 
Git-157)

[PATCH net 2/2] Revert "net: advertise TCP MSS from the configured MTU, not the learned PMTU"

[Esko Mobius via B4 Relay]

From: Esko Mobius <echoowall@gmail.com>

This reverts commit 2640e6419594 ("net: advertise TCP MSS from the
configured MTU, not the learned PMTU").

That commit stopped the learned PMTU from lowering the MSS we advertise
in SYN and SYN-ACK. For symmetric paths it argued that "the peer runs
its own PMTU discovery and usually already knows the real path MTU".
That only holds if ICMP "fragmentation needed" can reach the peer. When
the peer sits behind a firewall that drops inbound ICMP, its PMTU
discovery is a black hole (RFC 2923, section 2.1), and the smaller MSS
we advertised was the only thing keeping its full-size segments off the
narrow hop.

Inbound ICMP filtering is very common in enterprise and carrier
networks, and it is pervasive in mainland China. We hit this in
production after the commit was backported to 6.12.111: TCP connections
that reach a server through a relay with a smaller-MTU hop complete the
handshake, then stall as soon as the client sends full-size segments.
Clients on networks that let ICMP through, and connections that bypass
the relay, are unaffected, which made it hard to trace back to the
kernel.

Reproducer, network namespaces on a single host:

  cli --(1500)-- r1 --(1456)-- r2 --(1500)-- srv

r1 drops ICMP frag-needed towards cli. srv has already learned PMTU
1456 towards cli, then cli uploads 1 MB to srv:

  kernel                     SYN-ACK MSS   1 MB upload
  6.12.107                   1416          OK
  6.12.111                   1460          stalls, times out
  7.3-rc7                    1460          stalls, times out
  7.3-rc7 + this revert      1416          OK

The asymmetric case the commit was written for does not need a new
default: the advertised MSS can already be set per route with the
advmss metric, which takes precedence over default_advmss(). With this
revert applied, on an asymmetric path where srv learned PMTU 1456
towards cli while cli->srv is 1500 end to end:

  srv route to cli           SYN-ACK MSS   largest cli->srv segment
  default                    1416          1404
  advmss 1460                1460          1448

So DSR-style setups can opt in with "ip route ... advmss <mss>", while
hosts whose peers cannot complete PMTU discovery keep working by
default.

Fixes: 2640e6419594 ("net: advertise TCP MSS from the configured MTU, not the learned PMTU")
Reported-by: Esko Mobius <echoowall@gmail.com>
Closes: https://lore.kernel.org/regressions/4F0FAEB6-A13B-47F3-BFF1-C706686C6F64@gmail.com/
Cc: stable@vger.kernel.org
Signed-off-by: Esko Mobius <echoowall@gmail.com>
---
 include/net/ip.h        | 25 -------------------------
 include/net/ip6_route.h | 37 -------------------------------------
 net/ipv4/route.c        |  4 ++--
 net/ipv6/route.c        |  2 +-
 4 files changed, 3 insertions(+), 65 deletions(-)

diff --git a/include/net/ip.h b/include/net/ip.h
index 618b9806c..0acc4ba5e 100644
--- a/include/net/ip.h
+++ b/include/net/ip.h
@@ -513,31 +513,6 @@ 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 5f47cef0b..95f936be3 100644
--- a/include/net/ip6_route.h
+++ b/include/net/ip6_route.h
@@ -396,43 +396,6 @@ 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 595a3b7f1..93b9f4189 100644
--- a/net/ipv4/route.c
+++ b/net/ipv4/route.c
@@ -1365,8 +1365,8 @@ 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, ip_dst_mtu_configured(dst) - header_size,
-		       net->ipv4.ip_rt_min_advmss);
+	advmss = max_t(unsigned int, ipv4_mtu(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 94e5ef1c4..5760a357e 100644
--- a/net/ipv6/route.c
+++ b/net/ipv6/route.c
@@ -3239,7 +3239,7 @@ 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 = ip6_dst_mtu_configured(dst);
+	unsigned int mtu = dst6_mtu(dst);
 	struct net *net;
 
 	mtu -= sizeof(struct ipv6hdr) + sizeof(struct tcphdr);

-- 
Git-157)