[CVE-2026-63875][IMPORTANT] arm64: tlb: Flush walk cache when unsharing PMD tables [ Upstream

[AL-KERNEL <alkernel@kernelcve.org>]

CVE: CVE-2026-63875
Priority: IMPORTANT
AL-KERNEL base severity: IMPORTANT
KPANIC flag: YES
Patch: arm64: tlb: Flush walk cache when unsharing PMD tables [ Upstream
Commit: dced308d7d6a0de1c09d2058f38f1aaaf5cbb914
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=dced308d7d6a0de1c09d2058f38f1aaaf5cbb914
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-63875
Analysis date: Sun, 19 Jul 2026 17:16:31 -0400
ActionableScore: 7
ActionableScore lower bound: 5
Actionable bucket: Strong Important candidate / Actionable Moderate at minimum
Manual review required: YES

Summary:
An arm64 TLB maintenance flaw can leave stale PMD table state in the walk cache after huge_pmd_unshare(), allowing later page-table walks to use outdated non-leaf translation state and potentially causing denial of service or memory isolation impact on systems using affected hugepage mappings.

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ABOUT THIS REPORT
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The original Linux kernel CVE announcement for CVE-2026-63875 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-63875

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: IMPORTANT
Manual review required: YES

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

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AL-KERNEL CLASSIFICATION RESULT
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CVE-2026-63875	IMPORTANT	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:H/I:H/A:H';*CWE-664;*CWE-416;CWE-693;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:H';BEST CVSS score: '7';DESCR 'An arm64 TLB maintenance bug can leave a stale PMD table entry in the walk cache after huge_pmd_unshare unshares a PMD table. A later page table walk may use outdated non-leaf translation state, which can produce incorrect address translations and may affect memory isolation or crash the system. For the CVSS the PR:L is used because triggering requires local code execution and control over mappings that can reach PMD unshare paths, but it does not necessarily require full administrator privileges when hugetlb access is available. The issue is not network reachable and depends on arm64 hardware behavior plus hugepage or hugetlb mapping patterns. Impact is at least local denial of service and in the paranoid case may include confidentiality and integrity impact due to stale page table state.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate / Actionable Moderate at minimum (with actual score 7)  SKIP CVE-2026-63875 UNKNOWN SKIP No affected files built, so skip this CVE NO  - - unknown 	MAYBE	HARDWARE  KPANIC INCREASED_TO_MODERATE_FROM_LOW_BASED_ON_GUESSCVSS KPANIC INCREASED_TO_HIGH_BASED_ON_ACTIONABLESCOREHIGHEREQTHAN7	-	-	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=7
ActionableScoreLower=5

## 1. ActionableScore

* Conservative score: 5
* Paranoid score: 7
* Final recommended bucket: **Strong Important candidate / Actionable Moderate at minimum**::

## 2. Signal breakdown

Conservative score signals:

* Local unprivileged trigger: +1. A local process may be able to exercise hugetlb or huge PMD unshare paths if hugepage access is available.
* Weak or indirect corruption candidate: +1. The stale walk-cache PMD entry can cause incorrect page-table walks, but the patch does not show an attacker-controlled overwrite or reclaim primitive.
* Weak LPE concern: +1. Incorrect translations in MMU/page-table state can plausibly affect memory isolation, but practical privilege escalation is not demonstrated.
* Availability impact realistic: +1. Incorrect page-table walks can plausibly crash or destabilize the system.
* Broad/default security-sensitive subsystem: +1. This is arm64 MMU/TLB maintenance code, which is a high-value memory-management path.
* Rare or configuration-dependent trigger: -1. Practical triggering depends on arm64 plus hugepage/hugetlb PMD unshare behavior.

Paranoid additional interpretation:

* Confidentiality impact plausible: +1. Stale non-leaf translation state may expose memory that should no longer be reachable through that page-table context.
* Integrity impact plausible: +1. Incorrect translations may allow writes through stale mappings in a worst-case interpretation.

## 3. Reachability analysis

The bug is local, not network reachable. Triggering requires execution on an affected arm64 system and a mapping pattern that reaches huge_pmd_unshare() and PMD table unsharing. In typical deployments this likely requires hugetlb or hugepage availability, which may be restricted by configuration, limits, filesystem permissions, or administrative setup. However, it does not necessarily require full host root if hugepage use is delegated to local users or workloads. Namespaces and containers may matter if hugepage resources are exposed to containerized workloads.

Call-site confidence: medium. The direct patch is small, but the changed flag is in the architecture TLB flush path and the commit clearly ties it to huge_pmd_unshare() and stale walk-cache state.

## 4. Severity interpretation

This is not an ordinary validation bug or simple DoS-only issue. It restores a missing walk-cache invalidation condition for unshared PMD tables, and stale page-table walk state is security-sensitive because it can affect address translation correctness. Realistic exploitation evidence is limited, so the conservative score stays in Actionable Moderate territory. The paranoid score reaches Strong Important candidate because stale MMU/page-table state can plausibly affect confidentiality and integrity, even without a demonstrated arbitrary write primitive.

## 5. One-sentence report phrase

An arm64 TLB maintenance flaw can leave stale PMD table state in the walk cache after huge_pmd_unshare(), allowing later page-table walks to use outdated non-leaf translation state and potentially causing denial of service or memory isolation impact on systems using affected hugepage mappings.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: this is a memory-management correctness issue involving stale page-table walk-cache state. Even though no concrete exploit primitive is demonstrated, the affected path can influence address translation and memory isolation, so it should not be auto-closed.

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UPSTREAM PATCH SUMMARY
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Patch: arm64: tlb: Flush walk cache when unsharing PMD tables [ Upstream
Commit: dced308d7d6a0de1c09d2058f38f1aaaf5cbb914
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=dced308d7d6a0de1c09d2058f38f1aaaf5cbb914

Changed files:
  arch/arm64/include/asm/tlb.h

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=dced308d7d6a0de1c09d2058f38f1aaaf5cbb914

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

The original Linux kernel CVE announcement for CVE-2026-63875 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-63875

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:L/A:H
  The Best / paranoid CVSS vector: AV:L/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:H
  The Best / paranoid CVSS score: 7

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: 5
  Paranoid ActionableScore: 7

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: IMPORTANT
KPANIC detected for this report: YES
Published priority for this report (same as in Subject): IMPORTANT

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).