[CVE-2026-53108][MODERATE REGULAR] powerpc/64s: Fix unmap race with PMD migration entries [ Upstream

[CVE-2026-53108][MODERATE REGULAR] powerpc/64s: Fix unmap race with PMD migration entries [ Upstream

[AL-KERNEL]

CVE: CVE-2026-53108
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: powerpc/64s: Fix unmap race with PMD migration entries [ Upstream
Commit: 829367e55012c053738ebe7db20c4a90d6609ece
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=829367e55012c053738ebe7db20c4a90d6609ece
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-53108
Analysis date: Fri, 26 Jun 2026 11:15:43 -0400
ActionableScore: 3
ActionableScore lower bound: 2
Actionable bucket: Borderline, manual review recommended
Manual review required: YES

Summary:
A local race in powerpc/64s huge PMD migration and munmap handling can expose a non-present migration PMD to the huge PMD clear path, causing a kernel BUG or WARN and resulting in local denial of service.

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

The original Linux kernel CVE announcement for CVE-2026-53108 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-53108

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-2026-53108	MODERATE	CHECK	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:H';CWE-362;CWE-617;CWE-754;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '5.8';DESCR 'A race in powerpc 64s huge PMD handling can occur when PMD migration entries or device private THP entries are present while munmap() is unmapping the same address range. The unmap path may reach pmdp_huge_get_and_clear_full() with a non present PMD migration entry and hit a VM_BUG_ON or WARN in the architecture huge PMD clear path, causing a local kernel crash or severe warning condition. For the CVSS the PR:L is used because a local user process can plausibly trigger the relevant move_pages() and munmap() activity on its own mappings, although reliable triggering depends on THP migration and a narrow race window. The issue is not network reachable and requires local execution on affected powerpc 64s systems. Impact is at least denial of service via kernel crash. In the paranoid interpretation, limited confidentiality or integrity impact is kept for manual review sensitivity because the race is in page table migration and unmap handling, but the patch does not show a strong arbitrary write or proven privilege escalation primitive.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline, manual review recommended (with actual score 3) 	MAYBE	READ OOB DEADLOCK INIT RACE HARDWARE LINUS TEST 	-	-	checked

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

ActionableScore=3
ActionableScoreLower=2

## 1. ActionableScore

* Conservative score: 2
* Paranoid score: 3
* Final recommended bucket: **Borderline, manual review recommended**

## 2. Signal breakdown

* Local unprivileged trigger: +1
  A local process can plausibly race `move_pages()` or device-private THP migration activity with `munmap()` on its own mappings.

* Reliable kernel crash / strong DoS: +1
  The reported failure is a `VM_BUG_ON()` or WARN in powerpc huge PMD handling, with a demonstrated kernel BUG path.

* Weak LPE concern / sensitive page-table race: +1 in paranoid score only
  The bug is in PMD migration and unmap handling, which is security-sensitive memory-management code. However, the patch shows an invalid state assertion and non-present PMD handling issue, not a demonstrated UAF, arbitrary write, stale mapping takeover, or page-cache corruption primitive.

* Hard or unreliable race / special timing required: -1
  Triggering requires a narrow race between PMD THP migration and concurrent unmap.

* Rare AND difficult-to-reach subsystem/configuration: -1
  The issue is powerpc/64s specific and depends on THP PMD migration or device-private PMD entries, so exposure is narrower than common x86 or generic mm paths.

## 3. Reachability analysis

The bug is locally reachable, not network reachable. A local user may be able to influence the relevant path through `move_pages()` and `munmap()` on suitable mappings, but reliable triggering depends on powerpc/64s, THP PMD migration behavior, and timing. Device-private PMD migration may involve HMM or accelerator-related configurations, which further narrows practical exposure. Namespaces do not appear to create a remote or container breakout primitive by themselves, but local unprivileged reachability should not be dismissed if the syscall and memory layout conditions are available.

## 4. Severity interpretation

This behaves primarily like a local DoS issue, not an Important-class memory corruption issue. The realistic demonstrated impact is a kernel BUG/WARN during huge PMD unmap. The theoretical concern is that the race occurs in page-table migration/unmap code, so manual review is reasonable to confirm there is no stale mapping, refcount, or PMD lifetime corruption beyond the visible crash. The patch does not support scoring this as strong memory corruption or likely privilege escalation.

## 5. One-sentence report phrase

A local race in powerpc/64s huge PMD migration and munmap handling can expose a non-present migration PMD to the huge PMD clear path, causing a kernel BUG or WARN and resulting in local denial of service.

## 6. Manual review recommendation

MANUAL CHECK RECOMMENDED

Manual review is recommended because this is a race in architecture-specific page-table handling, but the available patch evidence supports DoS as the realistic impact and does not show a concrete privilege-escalation primitive.

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

Patch: powerpc/64s: Fix unmap race with PMD migration entries [ Upstream
Commit: 829367e55012c053738ebe7db20c4a90d6609ece
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=829367e55012c053738ebe7db20c4a90d6609ece

Changed files:
  arch/powerpc/mm/book3s64/pgtable.c
  arch/powerpc/mm/book3s64/hash_pgtable.c
  arch/powerpc/include/asm/book3s/64/pgtable.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=829367e55012c053738ebe7db20c4a90d6609ece

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

The original Linux kernel CVE announcement for CVE-2026-53108 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-53108

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:N/I:N/A:H
  The Best / paranoid CVSS vector: AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:H
  The Best / paranoid CVSS score: 5.8

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: 2
  Paranoid ActionableScore: 3

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).