CVE-2026-74696: tcp: fix TFO max_qlen accounting across reuseport migration

From: Greg Kroah-Hartman <[email protected]>

Description
===========

In the Linux kernel, the following vulnerability has been resolved:

tcp: fix TFO max_qlen accounting across reuseport migration

A listener's TCP_FASTOPEN max_qlen stops being accurate and lets through
far more pending Fast Open requests than it was configured for.

This only shows up with SO_REUSEPORT listener migration, where closing a
listener hands its still-pending TFO children over to a surviving one.

fastopenq.qlen is charged in tcp_fastopen_create_child() when the child
is created and uncharged in reqsk_fastopen_remove() when the handshake
completes.  The uncharge follows rsk_listener of the request the child
points at, and inet_reqsk_clone() has repointed the child at a new
request owned by the new listener, so the ++ and the -- land on two
different sockets.  The new listener's qlen drifts negative and its
limit no longer binds.

Charge the new listener during migration, like reqsk_queue_migrated()
already does for queue->young and queue->qlen.

The Linux kernel CVE team has assigned CVE-2026-74696 to this issue.


Affected and fixed versions
===========================

	Issue introduced in 5.14 with commit 54b92e84193749c9968aff2dd46e3b0f42643e18 and fixed in 5.15.216 with commit e98f0d80b9cccb5f828425d2004f9686e7d1ae24
	Issue introduced in 5.14 with commit 54b92e84193749c9968aff2dd46e3b0f42643e18 and fixed in 6.1.183 with commit b6247e0f96bd825ffb2005257f6177b5e642dee6
	Issue introduced in 5.14 with commit 54b92e84193749c9968aff2dd46e3b0f42643e18 and fixed in 6.6.152 with commit 585fc5247d14939a561056aa2addd9b7c2b1f670
	Issue introduced in 5.14 with commit 54b92e84193749c9968aff2dd46e3b0f42643e18 and fixed in 6.12.104 with commit 6e10ee56524a26b250229ad348637825646ddb88
	Issue introduced in 5.14 with commit 54b92e84193749c9968aff2dd46e3b0f42643e18 and fixed in 6.18.45 with commit a66e869cf0c90c1e47ae75f72b6482acbfc808ff
	Issue introduced in 5.14 with commit 54b92e84193749c9968aff2dd46e3b0f42643e18 and fixed in 7.1.9 with commit d974618b2097453778389d385e3741629c40e0a3
	Issue introduced in 5.14 with commit 54b92e84193749c9968aff2dd46e3b0f42643e18 and fixed in 7.2 with commit a0ab2ba83e35159d81cec830a92e885ecf8139be

Please see https://www.kernel.org for a full list of currently supported
kernel versions by the kernel community.

Unaffected versions might change over time as fixes are backported to
older supported kernel versions.  The official CVE entry at
	https://cve.org/CVERecord/?id=CVE-2026-74696
will be updated if fixes are backported, please check that for the most
up to date information about this issue.


Affected files
==============

The file(s) affected by this issue are:
	net/ipv4/inet_connection_sock.c


Mitigation
==========

The Linux kernel CVE team recommends that you update to the latest
stable kernel version for this, and many other bugfixes.  Individual
changes are never tested alone, but rather are part of a larger kernel
release.  Cherry-picking individual commits is not recommended or
supported by the Linux kernel community at all.  If however, updating to
the latest release is impossible, the individual changes to resolve this
issue can be found at these commits:
	https://git.kernel.org/stable/c/e98f0d80b9cccb5f828425d2004f9686e7d1ae24
	https://git.kernel.org/stable/c/b6247e0f96bd825ffb2005257f6177b5e642dee6
	https://git.kernel.org/stable/c/585fc5247d14939a561056aa2addd9b7c2b1f670
	https://git.kernel.org/stable/c/6e10ee56524a26b250229ad348637825646ddb88
	https://git.kernel.org/stable/c/a66e869cf0c90c1e47ae75f72b6482acbfc808ff
	https://git.kernel.org/stable/c/d974618b2097453778389d385e3741629c40e0a3
	https://git.kernel.org/stable/c/a0ab2ba83e35159d81cec830a92e885ecf8139be