CVE-2026-74544: net/sched: cls_u32: validate offshift to prevent shift-out-of-bounds

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

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

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

net/sched: cls_u32: validate offshift to prevent shift-out-of-bounds

u32_change() copies the user-provided tc_u32_sel.offshift (unsigned char,
0-255) into the kernel knode object without bounds validation. When a
packet later hits u32_classify() with TC_U32_VAROFFSET set, it evaluates
`ntohs(offmask & *data) >> offshift` where the left operand is a 16-bit
value promoted to a 32-bit int. Any offshift >= 32 is undefined behavior
per C11 6.5.7p3, triggerable by an unprivileged user via user/network
namespaces.

UBSAN: shift-out-of-bounds in net/sched/cls_u32.c:236:43
shift exponent 32 is too large for 32-bit type int

Fix this by rejecting offshift >= 16 during filter creation in
u32_change().

The Linux kernel CVE team has assigned CVE-2026-74544 to this issue.


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

	Issue introduced in 2.6.12 with commit 1da177e4c3f41524e886b7f1b8a0c1fc7321cac2 and fixed in 7.1.8 with commit 313cb9ffc4101a885d791facf4b5ea3d5e06144d
	Issue introduced in 2.6.12 with commit 1da177e4c3f41524e886b7f1b8a0c1fc7321cac2 and fixed in 7.2-rc6 with commit aef96eead2860cbfa371e4471d4f04412213b958

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-2026-74544
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/cls_u32.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/313cb9ffc4101a885d791facf4b5ea3d5e06144d
	https://git.kernel.org/stable/c/aef96eead2860cbfa371e4471d4f04412213b958