[CVE-2026-53109][MODERATE REGULAR] powerpc/pgtable-frag: Fix bad page state in pte_frag_destroy [ Upstream

[CVE-2026-53109][MODERATE REGULAR] powerpc/pgtable-frag: Fix bad page state in pte_frag_destroy [ Upstream

[AL-KERNEL]

CVE: CVE-2026-53109
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: powerpc/pgtable-frag: Fix bad page state in pte_frag_destroy [ Upstream
Commit: c8b710655012a2993a9567873fb71a8a51f8459c
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=c8b710655012a2993a9567873fb71a8a51f8459c
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-53109
Analysis date: Wed, 24 Jun 2026 13:38:23 -0400
ActionableScore: 3
ActionableScore lower bound: 2
Actionable bucket: Borderline Moderate / Manual review recommended
Manual review required: YES

Summary:
A local powerpc process may trigger a page table fragment cleanup corner case where `pte_frag_destroy()` frees a folio with the active flag still set, causing `BUG: Bad page state` and local denial of service, with manual review recommended because the affected code is page table lifetime management.

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

The original Linux kernel CVE announcement for CVE-2026-53109 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-53109

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.

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

CVE-2026-53109	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:N/I:N/A:H';CWE-459;*CWE-664;CWE-703;BEST CVSS score: '4.7';DESCR 'A bad page state condition can occur in the powerpc page table fragment cleanup path because pte_frag_destroy can free a folio while the active flag is still set. The corner case is tied to pte_free_defer and RCU deferred PTE table freeing after MADV_COLLAPSE on shmem, followed by process exit with the folio still cached as a pte_frag in mm context. For the CVSS the PR:L is used because a local user process can potentially exercise madvise and exit paths on its own mappings, although reliable triggering depends on a specific powerpc configuration and timing. The issue is not network reachable. Impact is local denial of service through BUG: Bad page state or kernel crash. No confidentiality or integrity impact is assigned because the observed primitive is an invalid page state at free time rather than a demonstrated UAF, OOB access, or arbitrary write.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline Moderate / Manual review recommended (with actual score 3)	YES	READ DANGER INIT UAF HARDWARE LINUS MEMORY  DECREASED_TO_MODERATE_REGULAR_FROM_MODERATE7_BASED_ON_GUESSCVSS_AND_CIANNH	YES	NO	checked

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

ActionableScoreLower=2
ActionableScore=3

## 1. ActionableScore

* Conservative score: **2**
* Paranoid score: **3**
* Final recommended bucket: **Borderline Moderate / Manual review recommended**::

## 2. Signal breakdown

Conservative signals:

* Local unprivileged trigger: **+1**. A local process can potentially exercise `madvise(MADV_COLLAPSE)` on shmem mappings and then exit.
* Real lifetime corruption: **+1**. The bug is in page table fragment lifetime cleanup with RCU deferred freeing and stale `folio active` state.
* Reliable kernel crash / strong DoS: **+1**. The reported failure is `BUG: Bad page state` during `pte_frag_destroy()` from `do_exit()`.
* Hard or unreliable timing / corner case: **-1**. Trigger depends on a specific interaction between `pte_free_defer()`, cached PTE fragments, process exit, and page table fragment reference accounting.
* Rare architecture / configuration: **-1**. The path is powerpc specific and especially tied to Hash MMU with 64K pages and PTE fragments.

Paranoid additional signal:

* Weak/indirect corruption candidate: **+1**. This is page table memory management and RCU lifetime code, so the stale state at free time deserves manual review, but the patch does not show attacker controlled reclaim, write-after-free, type confusion, or arbitrary write.

## 3. Reachability analysis

The bug is locally reachable, not network reachable. A local user process may be able to trigger the relevant paths through shmem mappings, `MADV_COLLAPSE`, and process exit, so PR is effectively low in CVSS terms. Namespaces do not appear to be the main privilege boundary here, since the relevant operations are normal local memory management behavior rather than an admin-only ioctl or netlink control path.

The exposure is limited by architecture and configuration. It affects powerpc page table fragment handling and appears most relevant to Hash MMU with 64K page size. The trigger also depends on a narrow cleanup corner case involving RCU deferred PTE table freeing and cached `mm->context` PTE fragments.

## 4. Severity interpretation

This behaves like an ordinary to borderline Moderate kernel DoS, not an Important-class issue based on the supplied evidence. The demonstrated impact is a bad page state and likely kernel crash during process exit. Theoretical concern exists because the bug is in page table lifetime management, but there is no demonstrated UAF reclaim, stale object reuse, attacker-controlled overwrite, type confusion, or information disclosure primitive.

The paranoid score is raised only to keep page table and RCU lifetime code from being auto-closed without review. It should not be treated as a strong LPE candidate from this patch alone.

## 5. One-sentence report phrase

A local powerpc process may trigger a page table fragment cleanup corner case where `pte_frag_destroy()` frees a folio with the active flag still set, causing `BUG: Bad page state` and local denial of service, with manual review recommended because the affected code is page table lifetime management.

## 6. Manual review recommendation

**MANUAL CHECK RECOMMENDED**

Manual review is recommended because this is page table memory management with RCU deferred freeing, but the current evidence supports DoS only and does not show a practical privilege escalation primitive.

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

Patch: powerpc/pgtable-frag: Fix bad page state in pte_frag_destroy [ Upstream
Commit: c8b710655012a2993a9567873fb71a8a51f8459c
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=c8b710655012a2993a9567873fb71a8a51f8459c

Changed files:
  arch/powerpc/mm/pgtable-frag.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=c8b710655012a2993a9567873fb71a8a51f8459c

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

The original Linux kernel CVE announcement for CVE-2026-53109 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-53109

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: Not available
  The Best / paranoid CVSS vector: AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS score: 4.7

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: 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).