CVE-2026-64368: mm/slab: do not limit zeroing to orig_size when only red zoning is enabled

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

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

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

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

mm/slab: do not limit zeroing to orig_size when only red zoning is enabled

When init (zeroing) on allocation is requested, for kmalloc() we
generally have to zero the full object size even if a smaller size is
requested, in order to provide krealloc()'s __GFP_ZERO guarantees.

But if we track the requested size, krealloc() uses that information to
do the right thing, so we can zero only the requested size. With red
zoning also enabled, any extra size became part of the red zone, so it
must not be zeroed and thus we must zero only the requested size.

However the current check is imprecise, and will trigger also when only
SLAB_RED_ZONE is enabled without SLAB_STORE_USER (which enables tracking
the requested size). This means enabling red zoning alone can compromise
krealloc()'s __GFP_ZERO contract.

Fix this by using slub_debug_orig_size() instead, which is the exact
check for whether the requested size is tracked. We don't need to care
if red zoning is also enabled or not. Also update and expand the
comment accordingly.

The Linux kernel CVE team has assigned CVE-2026-64368 to this issue.


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

	Issue introduced in 6.2 with commit 9ce67395f5a0cdec6ce152d26bfda13b98b25c01 and fixed in 6.6.145 with commit 6256899c3a34674bba6076884aedbba49fc695e4
	Issue introduced in 6.2 with commit 9ce67395f5a0cdec6ce152d26bfda13b98b25c01 and fixed in 6.12.96 with commit 7e706d50fa119eead6376bf0ef973e8d73a96030
	Issue introduced in 6.2 with commit 9ce67395f5a0cdec6ce152d26bfda13b98b25c01 and fixed in 6.18.39 with commit 2382971aaaef5bf85a651234c64906f59580b8be
	Issue introduced in 6.2 with commit 9ce67395f5a0cdec6ce152d26bfda13b98b25c01 and fixed in 7.1.4 with commit 0d18ccef142f04433dfb2a0c120cf223d2b8a42c
	Issue introduced in 6.2 with commit 9ce67395f5a0cdec6ce152d26bfda13b98b25c01 and fixed in 7.2-rc1 with commit 648927ceb84021a25a0fbd5673740956f318d534

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-2026-64368
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:
	mm/slub.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/6256899c3a34674bba6076884aedbba49fc695e4
	https://git.kernel.org/stable/c/7e706d50fa119eead6376bf0ef973e8d73a96030
	https://git.kernel.org/stable/c/2382971aaaef5bf85a651234c64906f59580b8be
	https://git.kernel.org/stable/c/0d18ccef142f04433dfb2a0c120cf223d2b8a42c
	https://git.kernel.org/stable/c/648927ceb84021a25a0fbd5673740956f318d534