[CVE-2026-64130][MODERATE REGULAR] mm/page_alloc: fix initialization of tags of the huge zero folio with init_on_free

[CVE-2026-64130][MODERATE REGULAR] mm/page_alloc: fix initialization of tags of the huge zero folio with init_on_free

[AL-KERNEL]

CVE: CVE-2026-64130
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: mm/page_alloc: fix initialization of tags of the huge zero folio with init_on_free
Commit: 738d18f1da3513d17b6f7bf30146cc4ac2480ffd
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=738d18f1da3513d17b6f7bf30146cc4ac2480ffd
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64130
Analysis date: Sun, 19 Jul 2026 13:58:39 -0400
ActionableScore: 2
ActionableScore lower bound: 1
Actionable bucket: Ordinary Moderate / Low-like, with manual review recommended for MTE metadata exposure
Manual review required: YES

Summary:
An arm64 MTE initialization bug can leave stale allocation tags on the huge zero folio when init_on_free is enabled, allowing a local user to observe non zero tag metadata instead of the documented zero tag state.

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ABOUT THIS REPORT
======================================================================

The original Linux kernel CVE announcement for CVE-2026-64130 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64130

The original announcement does not normally provide a security severity
estimate, CVSS assessment, or enough information to determine whether the
reported kernel bug represents a practically relevant security issue.

This report was generated by AL-KERNEL, an AI-assisted Linux kernel
vulnerability analysis system developed by Alexander Larkin. It combines
an autonomous classifier with LLM-assisted technical analysis and a
separate ActionableScore mechanism.

The purpose of this report is to prioritize Linux kernel CVEs before
manual review, identify cases that require prompt investigation, and
support automatic closure of issues that are unlikely to have meaningful
security impact.

Published priority for this report: MODERATE REGULAR
Manual review required: YES

A detailed explanation of the methodology and priority rules is included
at the end of this message.

======================================================================
AL-KERNEL CLASSIFICATION RESULT
======================================================================

CVE-2026-64130	MODERATE	CHECK WITH IMPACT FROM ORIG NN LOW	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:L';CWE-665;CWE-908;*CWE-200;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:N/A:L';BEST CVSS score: '4.5';DESCR 'An arm64 MTE initialization bug can leave stale allocation tags on the huge zero folio when init_on_free is enabled. The huge zero folio is mapped through a special PMD, so the normal first user mapping path does not initialize tags with set_pte_at and userspace may observe non zero tags instead of the documented zero tag state. For the CVSS the PR:L is used because a local unprivileged process is needed to create and access the relevant user mappings on an affected arm64 MTE system. The issue is not network reachable and does not provide a demonstrated arbitrary write or UAF primitive. Impact is mainly limited metadata exposure and possible local process disruption from MTE tag faults, with a conservative integrity impact because MTE tag state is security relevant metadata.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate / Low-like, with manual review recommended for MTE metadata exposure (with actual score 2)  SKIP CVE-2026-64130 UNKNOWN SKIP The Fixes patch not applied yet, so unlikely that actual: adfb6609c6809e107ded9a1cd46f519c882e64ea YES  NO NO unknown 	MAYBE	READ KASAN INIT HARDWARE  INCREASED_FROM_LOW_BASED_ON_REQUIREMANUALCHECK DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN5	-	-	checked

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ACTIONABLESCORE ANALYSIS
======================================================================

ActionableScore=2
ActionableScoreLower=1

## 1. ActionableScore

* Conservative score: 1
* Paranoid score: 2
* Final recommended bucket: **Ordinary Moderate / Low-like, with manual review recommended for MTE metadata exposure**::

## 2. Signal breakdown

Conservative signals:

* Local unprivileged trigger: +1
  A local process can create and access the relevant user mappings on an affected arm64 MTE system.

* Confidentiality impact plausible: +1
  Userspace may observe stale non zero MTE allocation tags from the huge zero folio. This is metadata exposure, not direct page-content disclosure.

* Rare AND difficult-to-reach subsystem/configuration: -1
  The issue requires arm64 MTE, init_on_free, huge zero folio behavior, and a mapping path through a special PMD.

Paranoid additional signal:

* Integrity impact plausible: +1
  MTE tags are security-relevant metadata. Incorrect initial tag state may weaken expected memory-tagging semantics, but no arbitrary write, UAF, or privilege escalation primitive is shown.

Not awarded:

* No generic memory corruption. The patch shows stale tag metadata, not UAF, OOB write, double free, or type confusion.
* No reliable kernel crash or strong DoS.
* No network reachability.
* No privilege escalation signal.
* No page-cache or Dirty-Pipe-like ownership/COW bypass.

## 3. Reachability analysis

The bug is locally reachable by a user process that can create the relevant MTE-enabled mappings on arm64. It is not network reachable. Containers do not fundamentally change the primitive unless the container can access arm64 MTE userspace features and create the required mappings, but the impact remains metadata exposure rather than host memory corruption.

The affected path is configuration dependent. It requires arm64 MTE support and init_on_free. The huge zero folio special PMD path is important because ordinary PTE mappings would initialize tags through set_pte_at and related paths.

Call-site confidence: high. The relevant allocation hook, tag clearing helper, and huge zero folio mapping behavior are directly described by the patch and commit message.

## 4. Severity interpretation

This behaves more like an ordinary Moderate or Low-like issue than an Important vulnerability. The theoretical concern is that stale MTE allocation tags are exposed to userspace and violate documented MTE initialization semantics. The realistic evidence supports limited metadata exposure and possible local process-level disruption from tag faults, not kernel memory corruption or LPE.

Paranoid scoring raises the issue slightly because MTE tags are security metadata, but the patch does not support a strong confidentiality, integrity, or availability impact.

## 5. One-sentence report phrase

An arm64 MTE initialization bug can leave stale allocation tags on the huge zero folio when init_on_free is enabled, allowing a local user to observe non zero tag metadata instead of the documented zero tag state.

## 6. Manual review recommendation

MANUAL CHECK RECOMMENDED

Manual review is recommended because the issue exposes MTE security metadata and depends on architecture-specific memory-tagging semantics. It does not currently justify Important handling because there is no demonstrated UAF, arbitrary write, privilege escalation, or system-wide DoS.

======================================================================
UPSTREAM PATCH SUMMARY
======================================================================

Patch: mm/page_alloc: fix initialization of tags of the huge zero folio with init_on_free
Commit: 738d18f1da3513d17b6f7bf30146cc4ac2480ffd
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=738d18f1da3513d17b6f7bf30146cc4ac2480ffd

Commit description:

__GFP_ZEROTAGS semantics are currently a bit weird, but effectively this
flag is only ever set alongside __GFP_ZERO and __GFP_SKIP_KASAN.

If we run with init_on_free, we will zero out pages during
__free_pages_prepare(), to skip zeroing on the allocation path.

However, when allocating with __GFP_ZEROTAG set, post_alloc_hook() will
consequently not only skip clearing page content, but also skip clearing
tag memory.

Not clearing tags through __GFP_ZEROTAGS is irrelevant for most pages that
will get mapped to user space through set_pte_at() later: set_pte_at() and
friends will detect that the tags have not been initialized yet
(PG_mte_tagged not set), and initialize them.

However, for the huge zero folio, which will be mapped through a PMD
marked as special, this initialization will not be performed, ending up
exposing whatever tags were still set for the pages.

The docs (Documentation/arch/arm64/memory-tagging-extension.rst) state
that allocation tags are set to 0 when a page is first mapped to user
space.  That no longer holds with the huge zero folio when init_on_free is
enabled.

Fix it by decoupling __GFP_ZEROTAGS from __GFP_ZERO, passing to
tag_clear_highpages() whether we want to also clear page content.

Invert the meaning of the tag_clear_highpages() return value to have
clearer semantics.

Reproduced with the huge zero folio by modifying the check_buffer_fill
arm64/mte selftest to use a 2 MiB area, after making sure that pages have
a non-0 tag set when freeing (note that, during boot, we will not actually
initialize tags, but only set KASAN_TAG_KERNEL in the page flags).

	$ ./check_buffer_fill
	1..20
	...
	not ok 17 Check initial tags with private mapping, sync error mode and mmap memory
	not ok 18 Check initial tags with private mapping, sync error mode and mmap/mprotect memory
	...

This code needs more cleanups; we'll tackle that next, like
decoupling __GFP_ZEROTAGS from __GFP_SKIP_KASAN.

[akpm@linux-foundation.org: s/__GPF_ZERO/__GFP_ZERO/, per David]
Link: https://lore.kernel.org/20260421-zerotags-v2-1-05cb1035482e@kernel.org
Fixes: adfb660 ("mm/huge_memory: initialise the tags of the huge zero folio")
Signed-off-by: David Hildenbrand (Arm) <david@kernel.org>
Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
Tested-by: Lance Yang <lance.yang@linux.dev>
Cc: Brendan Jackman <jackmanb@google.com>
Cc: Dev Jain <dev.jain@arm.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Liam Howlett <liam@infradead.org>
Cc: Lorenzo Stoakes (Oracle) <ljs@kernel.org>
Cc: Mark Brown <broonie@kernel.org>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Ryan Roberts <ryan.roberts@arm.com>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Will Deacon <will@kernel.org>
Cc: Zi Yan <ziy@nvidia.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  arch/arm64/memory-tagging-extension.rst
  arch/arm64/include/asm/page.h
  arch/arm64/mm/fault.c
  include/linux/gfp_types.h
  include/linux/highmem.h
  mm/page_alloc.c

Diff excerpt:

Not included in this email. See the upstream URL for the full patch.

Full patch:
https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=738d18f1da3513d17b6f7bf30146cc4ac2480ffd

======================================================================
DETAILED REPORT METHODOLOGY
======================================================================

The original Linux kernel CVE announcement for CVE-2026-64130 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64130

The original CVE announcement normally does not include a security-level
estimate. In particular, it may not contain a CVSS assessment, an impact
level, or enough information to determine whether the reported bug is a
practically relevant security issue. One purpose of this parallel CVE list
is to provide that missing technical and prioritization information.

The original goal of the AL-KERNEL project was to prioritize Linux kernel
CVE analysis automatically before manual review. The system can also help
identify non-security issues that may be suitable for automatic closure.

This report was generated by AL-KERNEL, an AI-assisted Linux kernel
vulnerability analysis system developed by Alexander Larkin.

The first analysis stage combines an autonomous classifier with additional
LLM-based analysis. The autonomous classifier runs locally on a CPU and is
based on a backpropagation neural network. Together, these mechanisms
produce a technical vulnerability description, identify likely weakness
types, estimate CVSS severity, and provide input for ActionableScore.

Two CVSS estimates are retained because incomplete kernel vulnerability
information often permits more than one defensible interpretation:

  Conservative CVSS vector: AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:N/A:L
  The Best / paranoid CVSS vector: AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:L
  The Best / paranoid CVSS score: 4.5

The conservative vector represents a lower-impact interpretation.
The Best/paranoid vector intentionally represents a plausible upper-bound
interpretation and should not automatically be treated as demonstrated
real-world impact.

CVSS may also need to be adjusted for a particular Linux deployment,
because actual reachability, privileges, enabled kernel configuration,
hardware, namespaces, exposed device nodes, and other environmental
conditions can differ significantly between systems.

A separate ActionableScore mechanism evaluates practical remediation
urgency. Its analysis may include reachability, attack prerequisites,
subsystem exposure, memory-corruption characteristics, denial-of-service
reliability, and possible confidentiality, integrity, or
privilege-escalation impact.

  Conservative ActionableScore: 1
  Paranoid ActionableScore: 2

The final base severity is taken directly from the second tab-separated
field of the AL-KERNEL classification result. ActionableScore does not
replace or independently override that final AL-KERNEL decision, and
if ActionableScore adjusted impact level of ALKERNEL, then you would see
self-readable flags above like INCREASED_TO_HIGH_BASED_ON_ACTIONABLESCOREHIGHEREQTHAN7.

For an AL-KERNEL result of MODERATE, this report uses the following
additional presentation split:

  ActionableScore below 5   -> MODERATE REGULAR
  ActionableScore 5 or more -> MODERATE 7.0

The distinction between MODERATE REGULAR and MODERATE 7.0 makes it
possible to identify Moderate issues that should receive manual analysis
and fixes before lower-priority MODERATE REGULAR issues. In many cases,
MODERATE REGULAR fixes may wait for a later rebase or routine update.

There is one override in which MODERATE REGULAR becomes MODERATE 7.0
even when the ActionableScore is below 5. When the AL-KERNEL result
contains the KPANIC flag, a MODERATE result is always presented as
MODERATE 7.0. The KPANIC flag selected with few regexps without
usage of AI at all, so it helps to detect cases when Kernel Crash happens
and similar (to filter False-Negative results from the LLM usage).

KPANIC indicates that a reliable kernel crash, kernel panic, or similarly
serious kernel availability impact was identified by the classification
workflow.

AL-KERNEL base severity for this report: MODERATE
KPANIC detected for this report: NO
Published priority for this report (same as in Subject): MODERATE REGULAR

These results are intended to support engineering triage. They are
machine-generated estimates, and cases marked for manual review should
be validated by a human security engineer before final disposition.
For more info read docs linked from here: https://kernelcve.org/
(and you can submit you own patch there to generate such a report
for non-existant CVE-id yet).

Note that in many cases this AI tool selects higher severity, than
real is (means you can expect Importants instead of Moderate 7.0 or
Moderates 7.0 instead of regular Moderates). If you see such cases,
please use reply email interface to add additional manual analyses
info to this particular CVE.
And please, please, let me know when you see Lows instead of Importants
or Important instead of Low (because particular for such cases I
need to tune this AI tool to make it better for this one and next similar).
My contact email for such notifications is alexanjelausa@gmail.com (and both
send reply to CVE record itself too and see "reply" button below for howto reply).