[CVE-2026-93239][MODERATE REGULAR] arm64: mm: Fix the lockless page-table walk in show_pte()

[CVE-2026-93239][MODERATE REGULAR] arm64: mm: Fix the lockless page-table walk in show_pte()

[AL-KERNEL]

CVE: CVE-2026-93239
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: arm64: mm: Fix the lockless page-table walk in show_pte()
Commit: 08d931a0a850ad16a64f2cd23a19f40795cdbdf7
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=08d931a0a850ad16a64f2cd23a19f40795cdbdf7
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-93239
Analysis date: Thu, 24 Sep 2026 11:51:15 -0400
ActionableScore: 3
ActionableScore lower bound: unknown
Actionable bucket: Borderline / manual review recommended
Manual review required: YES

Summary:
No one-sentence report phrase was found.

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

The original Linux kernel CVE announcement for CVE-2026-93239 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-93239

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-2026-93239	MODERATE	https://www.kernelcve.org/list/?q=CVE-2026-93239 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:H';*CWE-416;CWE-362;CWE-367;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '5.8';DESCR 'An arm64 use after free race can occur in the diagnostic show_pte page table walk when concurrent address space teardown clears and frees a lower level page table after show_pte has begun walking it. The stale parent entry may then be used to derive a lower level pointer, and the diagnostic path can dereference a released page table. For the CVSS the PR:L because a local user context is required to participate in the task teardown scenario, while reliable triggering also requires a kernel fault that invokes show_pte and a narrow race window. The issue is not network reachable. The most likely impact is a local denial of service through a kernel fault or panic. The paranoid score includes limited confidentiality and integrity impact because the patch fixes an actual freed memory dereference in page table management, but privilege escalation is not demonstrated by this diagnostic read only path.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline / manual review recommended (with actual score 3)	YES	SIMPLEFIX HARDWARE LINUS MEMORY  DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN4	NO	NO	guess

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

ActionableScore=3
PrimitiveClass=CLASS_B
RealUAF=YES
HostAdminRequired=UNKNOWN
NamespaceScopedPrivilege=UNKNOWN
ImportantCandidate=NO
ManualReview=RECOMMENDED
AutoDowngradeAllowed=YES

1. ActionableScore

* Raw conservative score: 3
* Conservative score: 3
* Raw paranoid score: 3
* Paranoid score: 3
* Final recommended bucket: **Borderline / manual review recommended**::

2. Signal breakdown

* +2 Real UAF / lifetime violation. `show_pte()` can dereference a page-table page after concurrent teardown has cleared the parent entry and RCU-deferred freeing has released the lower-level table page.
* +1 Reliable kernel crash / strong DoS. The commit explicitly describes dereferencing a released table page or deriving a bogus lower-level pointer, which can cause a further kernel fault or panic in the fault diagnostic path.
* +0 No controlled reclaim, attacker-controlled replacement, arbitrary write, callback control, or refcount mutation is established.
* +0 No confidentiality or integrity signal is awarded. The stale operation is a diagnostic page-table read and no disclosure path or write primitive is shown.
* +0 No local-unprivileged-trigger point is awarded. The supplied evidence does not establish that an ordinary user can independently invoke the required kernel-fault diagnostic path.
* +0 No remote-reachability point is awarded. This is not network reachable.
* +0 No structured stale-object leverage is awarded. Page tables are active kernel structures, but the demonstrated stale operation only reads entries for diagnostic output and then faults.
* +0 No hard-race penalty is applied. The issue is concurrent, but the supplied patch does not establish a specifically rare or externally uncontrollable timing condition beyond the required concurrent teardown.
* +0 No privilege penalty is applied. Host-admin-equivalent privilege is not positively established, but below-host-admin reachability is also not proven.

The conservative and paranoid scores are identical. The patch establishes a real stale page-table dereference, but it does not establish a post-free write, controlled reuse, disclosure, or a security-boundary primitive that would justify an additional paranoid point.

3. Reachability analysis

`show_pte()` is an arm64 diagnostic helper used while reporting a kernel page-fault condition. The vulnerable sequence requires both a kernel fault that reaches this diagnostic path and concurrent teardown of the relevant task address space.

The supplied context does not establish that an ordinary local user can directly and reliably cause the required kernel-side fault without relying on another defect, a faulty kernel component, or privileged diagnostic activity. Therefore, the complete attacker privilege boundary is unresolved.

No namespace-scoped capability path is shown. A user namespace, network namespace, container, or delegated capability does not appear relevant to the page-table teardown and diagnostic-fault sequence. No special device, firmware, network service, boot option, or prior host configuration is identified as necessary by the patch.

The evaluated attacker model is unresolved rather than proven ordinary-user or host-administrator. The issue is local-only and is not directly network-triggerable.

4. Primitive and severity interpretation

The exact primitive is a real RCU/lifetime violation: a lockless diagnostic walk can retain and use a lower-level page-table pointer after another CPU has torn down the address space, cleared an ancestor entry, and freed the table page through the arm64 MMU-gather RCU free path.

This is an active structured kernel object, namely page-table storage, but the stale use is constrained to `show_pte()` diagnostic traversal. The affected path reads page-table entries for logging. It does not modify the freed object, execute callbacks from it, perform a second release operation, manipulate refcounts, or consume attacker-shaped fields as an active operational page-table walk would.

There is no concrete repeated release or refcount mutation. The patch does not show a first release followed by a second `put`, `free`, or refcount decrement through the same dangling reference.

The demonstrated consequence is a kernel fault, recursive fault, Oops, or panic while printing page-table diagnostics. The speculative consequence is limited to the generic possibility that any kernel UAF may have more serious consequences under unknown allocator conditions. That possibility is not sufficient here to establish controlled reclaim, arbitrary kernel read or write, information disclosure, integrity impact, or privilege escalation.

The issue is therefore CLASS_B because a real stale post-lifetime dereference exists. It is not CLASS_A because no independently useful exploitation primitive is established. ImportantCandidate is NO because the stale read is diagnostic-only, the demonstrated impact is crash/DoS, and meaningful post-free reuse leverage is not shown. AutoDowngradeAllowed is YES because the available evidence supports a constrained crash-oriented UAF rather than unresolved active corruption leverage.

Likely operational severity is Borderline Moderate. It merits review because it is a genuine arm64 MM lifetime race, but the patch does not support classifying it as an Important-grade exploitation candidate.

5. One-sentence report phrase

An arm64 race in the diagnostic show_pte page-table walk can dereference an RCU-freed page-table page during concurrent task address-space teardown, causing a local kernel fault or panic without a demonstrated information disclosure, write, or privilege-escalation primitive.

6. Manual review recommendation

MANUAL CHECK RECOMMENDED

The patch establishes a real page-table UAF race in kernel MM code, but the affected operation is diagnostic and read-only. Manual confirmation is appropriate to determine whether any downstream caller can make the required kernel-fault and concurrent-teardown sequence attacker-reachable without a separate vulnerability.

7. Call-site confidence

Call-site confidence: medium

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

Patch: arm64: mm: Fix the lockless page-table walk in show_pte()
Commit: 08d931a0a850ad16a64f2cd23a19f40795cdbdf7
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=08d931a0a850ad16a64f2cd23a19f40795cdbdf7

Commit description:

show_pte() walks page tables locklessly and can run with interrupts
enabled. A concurrent teardown can free a table page while it is being
walked. It can also clear a parent entry after show_pte() checked it; the
regular pXd_offset() helpers then reread the cleared entry and can derive a
bogus lower-level pointer and fault again.

Use the lockless offset helpers with the saved parent entries, as
gup_fast() does, and pass the saved PMD to pte_offset_map().

For task page tables, arm64 selects MMU_GATHER_RCU_TABLE_FREE. Disable
local interrupts around the walk to hold off RCU-deferred table frees and
block the tlb_remove_table_sync_one() IPI until the walk is finished.

Place the IRQ guard after the header print. This does not make the output a
consistent snapshot, but prevents the task page-table walk from
dereferencing a released table page or deriving a pointer from a different
parent value.

Fixes: 1d18c47 ("arm64: MMU fault handling and page table management")
Cc: stable@vger.kernel.org
Assisted-by: LLM
Signed-off-by: Karl Mehltretter <kmehltretter@gmail.com>
Signed-off-by: Will Deacon <will@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  arch/arm64/mm/fault.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=08d931a0a850ad16a64f2cd23a19f40795cdbdf7

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

The original Linux kernel CVE announcement for CVE-2026-93239 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-93239

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: unknown
  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).