CVE-2026-90075: net/sched: fq_codel: clamp default quantum and mtu

CVE-2026-90075: net/sched: fq_codel: clamp default quantum and mtu

[Greg Kroah-Hartman]

From: Greg Kroah-Hartman <gregkh@kernel.org>

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

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

net/sched: fq_codel: clamp default quantum and mtu

fq_codel_init() sets q->quantum = psched_mtu(qdisc_dev(sch)) without
clamping. A device with a huge MTU (e.g. dummy with max_mtu == 0
accepting MTU 2147483634) makes psched_mtu() return 0x80000000, which
overflows the signed flow->deficit to INT_MIN in fq_codel_dequeue(),
causing an infinite loop and soft lockup. Emulate fq_codel_change()
and constrain to [256, FQ_CODEL_QUANTUM_MAX].

The same unclamped psched_mtu() is assigned to q->cparams.mtu a bit
below, and fq_codel_change() never updates it. codel_should_drop()
tests "*backlog <= params->mtu"; with mtu == 0x80000000 (~2 GiB) and
the default 32 MiB memory_limit, the test is always true, so CoDel is
silently and completely disabled (no drops, no ECN). Declare a single
clamped mtu and assign both q->quantum and q->cparams.mtu from it,
which also removes the double psched_mtu() call.

Conditions to recreate the bug: a device whose MTU (plus
hard_header_len) wraps psched_mtu() into the sign bit (e.g. a dummy
device with max_mtu == 0 accepting MTU 2147483634). Requires
CAP_NET_ADMIN in a user namespace.

The Linux kernel CVE team has assigned CVE-2026-90075 to this issue.


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

	Issue introduced in 3.5 with commit 4b549a2ef4bef9965d97cbd992ba67930cd3e0fe and fixed in 5.10.270 with commit d315ee8a07fd1810880227319cf60e6cd925ef22
	Issue introduced in 3.5 with commit 4b549a2ef4bef9965d97cbd992ba67930cd3e0fe and fixed in 5.15.221 with commit a9a5b2943a00df2ab81a2209f31dd9e87016f120
	Issue introduced in 3.5 with commit 4b549a2ef4bef9965d97cbd992ba67930cd3e0fe and fixed in 6.1.188 with commit 3782067ec0d485628b6f1f9ede3eeeb4f611fcf2
	Issue introduced in 3.5 with commit 4b549a2ef4bef9965d97cbd992ba67930cd3e0fe and fixed in 6.6.157 with commit 397e2b1f71d9f15b8b4e47d24eb620e4dff8878d
	Issue introduced in 3.5 with commit 4b549a2ef4bef9965d97cbd992ba67930cd3e0fe and fixed in 6.12.110 with commit 324f86806673ae4a55f66d28db84577f46f615e1
	Issue introduced in 3.5 with commit 4b549a2ef4bef9965d97cbd992ba67930cd3e0fe and fixed in 6.18.52 with commit dfb4b61db886917244284b18b44b23d2254b82c2
	Issue introduced in 3.5 with commit 4b549a2ef4bef9965d97cbd992ba67930cd3e0fe and fixed in 7.2.6 with commit 9f499e5827fdb6d7fdb46a7ce731852f6b1a1bb9
	Issue introduced in 3.5 with commit 4b549a2ef4bef9965d97cbd992ba67930cd3e0fe and fixed in 7.3-rc1 with commit d9ebd8f9aa8b2773235889cb903fafd61f2d8585

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-2026-90075
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/sched/sch_fq_codel.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/d315ee8a07fd1810880227319cf60e6cd925ef22
	https://git.kernel.org/stable/c/a9a5b2943a00df2ab81a2209f31dd9e87016f120
	https://git.kernel.org/stable/c/3782067ec0d485628b6f1f9ede3eeeb4f611fcf2
	https://git.kernel.org/stable/c/397e2b1f71d9f15b8b4e47d24eb620e4dff8878d
	https://git.kernel.org/stable/c/324f86806673ae4a55f66d28db84577f46f615e1
	https://git.kernel.org/stable/c/dfb4b61db886917244284b18b44b23d2254b82c2
	https://git.kernel.org/stable/c/9f499e5827fdb6d7fdb46a7ce731852f6b1a1bb9
	https://git.kernel.org/stable/c/d9ebd8f9aa8b2773235889cb903fafd61f2d8585