[CVE-2025-21785][MODERATE REGULAR] arm64: cacheinfo: Avoid out-of-bounds write to cacheinfo array [ Upstream

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

CVE: CVE-2025-21785
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: arm64: cacheinfo: Avoid out-of-bounds write to cacheinfo array [ Upstream
Commit: 4ff25f0b18d1d0174c105e4620428bcdc1213860
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=4ff25f0b18d1d0174c105e4620428bcdc1213860
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2025-21785
Analysis date: Mon, 12 Oct 2026 21:48:16 -0400
ActionableScore: 3
ActionableScore lower bound: 2
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-2025-21785 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2025-21785

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-2025-21785	MODERATE	https://www.kernelcve.org/list/?q=CVE-2025-21785 CHECK WITH IMPACT FROM ORIG NN LOW | AEGIS IMPACT MODERATE_KPANIC AEGIS_AS=1 LEGACY=MODERATE RESULT=DIFF (independent Legacy and Aegis differ; FINAL: MODERATE, Aegis: MODERATE_KPANIC)	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:H/PR:H/UI:N/S:U/C:H/I:H/A:H';*CWE-787;CWE-193;CWE-129;Other CVSS 'AV:L/AC:H/PR:H/UI:N/S:U/C:L/I:L/A:H';BEST CVSS score: '6.4';DESCR 'An indexing error in arm64 cache discovery can write beyond the allocated cacheinfo array when a cache level contains separate data and instruction cache leaves. The affected initialization path advances the destination for two leaves while the original bounds counter advances only once per cache level. This provides a structured out-of-bounds write in kernel memory and can cause a kernel crash. Depending on the adjacent allocation and later use of corrupted data, confidentiality or integrity impact and local privilege escalation cannot be excluded without further analysis. For the CVSS the PR:H reflects that a reliable trigger normally requires administrative control of CPU hotplug or of the platform cache topology. The issue is not network reachable and an ordinary local user cannot directly invoke cache topology population. The paranoid score uses C:H I:H A:H because the confirmed primitive is an out-of-bounds kernel write, although control over the written values appears limited.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline / manual review recommended (with actual score 3)	MAYBE	WRITE OOB INIT SIMPLEFIX HARDWARE LINUS  DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN4	-	-	checked

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

ActionableScore=3
ActionableScoreLower=2
PrimitiveClass=CLASS_B
RealUAF=NO
HostAdminRequired=UNKNOWN
NamespaceScopedPrivilege=NO
ImportantCandidate=NO
ManualReview=RECOMMENDED
AutoDowngradeAllowed=YES
RequiresHostUID0=UNKNOWN
AuthorizationDecisionBypass=NO
ProtectedResourceImpact=NONE
B4PendingReview=NO
B4PendingConfirmed=NO

1. ActionableScore

* Raw conservative score: 2
* Conservative score: 2
* Raw paranoid score: 3
* Paranoid score: 3
* No floor or cap was applied.
* Final recommended bucket: **Borderline / manual review recommended**::

2. Signal breakdown

Conservative signals:

* +1 Constrained kernel buffer overwrite. `ci_leaf_init()` performs a confirmed write beyond the allocated `info_list` array when the leaf index does not account for both entries of a separate data and instruction cache level.
* +1 Broadly exercised path. ARM64 cache discovery is a normal CPU initialization path, and separate data and instruction cache leaves are an ordinary architecture feature.

Paranoid-only signal:

* +1 Realistic kernel crash. Writing a complete `struct cacheinfo` beyond a heap array can corrupt an adjacent allocation and plausibly produce an Oops or panic, although no specific crash or adjacent victim is demonstrated.

No negative points were applied:

* Host-admin privilege was not positively established, so no privilege penalty was applied.
* No rare-hardware penalty was applied merely because the code is ARM64-specific.
* No timing penalty applies because this is not a race.

Important non-awarded signals:

* No attacker-controlled target or meaningfully controlled payload is established. The written fields are derived from cache type and level.
* No arbitrary or directed kernel write is established.
* No controlled reclaim, object substitution, callback influence, or privilege escalation is demonstrated.
* No confidentiality or integrity consequence involving a protected object is established.
* No reliable below-host-admin trigger is established.
* No remote reachability is established.
* No extra availability point was awarded for persistence or a second independent availability mechanism.
* B5-R does not apply. There is no retry-accumulated index, attacker-influenced retry sequence, or demonstrated later consumption of retry-corrupted operation state.
* B3-PENDING does not apply because no shared backing or COW ownership violation exists.
* B4-PENDING does not apply because this is not an authorization decision defect.

3. Reachability analysis

The vulnerable function populates per-CPU cache information during ARM64 CPU cache discovery. The malformed write depends on the relationship between `num_levels`, `num_leaves`, and levels represented by separate data and instruction cache entries.

No ordinary userspace interface that directly controls these values is shown. CPU hotplug can cause cache initialization to run again, but the supplied evidence does not establish the complete privilege requirements or show that a local actor can select the affected topology. Cache topology is primarily platform, firmware, CPU, or potentially hypervisor supplied.

The operation is global CPU state and is not scoped to a user or network namespace. There is no network trigger, retry loop, or timing race. A malicious virtual platform might influence presented cache topology, but that attacker model and the relevant ARM64 virtualization behavior are not established here.

HostAdminRequired remains UNKNOWN because the supplied evidence establishes neither an ordinary below-host-admin trigger nor a complete host-admin-only attack sequence.

4. Primitive and severity interpretation

The exact primitive is a heap out-of-bounds write of one or more `struct cacheinfo` entries. The old loop advanced the destination once for each populated leaf but advanced its checked index once per cache level. A separate-cache level could therefore consume two array entries without the bound tracking both writes.

This is structured, constrained kernel memory corruption rather than a UAF. The destination is adjacent to the allocated cacheinfo array, but the attacker is not shown to select the victim object. The values written by `ci_leaf_init()` are also constrained by cache type and level.

The demonstrated consequence is corruption outside the array. A kernel crash is plausible but not directly reported. Confidentiality impact, protected-state modification, and privilege escalation remain speculative because no adjacent object, read-back path, controlled target, or subsequent security-sensitive use is identified.

PrimitiveClass is CLASS_B because a real kernel overwrite exists but strong attacker-controlled exploitation leverage is not established. ImportantCandidate is NO because the evidence does not satisfy B5-R or otherwise establish controlled corruption, attacker reachability, or a security-boundary consequence. AutoDowngradeAllowed is YES because the issue may remain below Important after review, although the confirmed heap write warrants manual inspection rather than immediate dismissal.

Likely operational severity is Low-like to Moderate, with higher concern on platforms where an untrusted hypervisor or another below-host-admin actor can control cache topology and repeatedly invoke CPU initialization.

5. One-sentence report phrase

An ARM64 cache leaf indexing error causes a constrained heap out-of-bounds write during CPU cache discovery, but attacker control over topology, the overwritten object, and subsequent exploitation is not established.

6. Manual review recommendation

MANUAL CHECK RECOMMENDED

Review the allocation layout following `info_list`, affected cache topologies, CPU hotplug call paths, and whether an untrusted ARM64 virtual machine host can supply a topology that reliably reaches and repeats the overwrite.

7. Call-site confidence

Call-site confidence: medium

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

Patch: arm64: cacheinfo: Avoid out-of-bounds write to cacheinfo array [ Upstream
Commit: 4ff25f0b18d1d0174c105e4620428bcdc1213860
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=4ff25f0b18d1d0174c105e4620428bcdc1213860

Changed files:
  arch/arm64/kernel/cacheinfo.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=4ff25f0b18d1d0174c105e4620428bcdc1213860

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

The original Linux kernel CVE announcement for CVE-2025-21785 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2025-21785

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:H/UI:N/S:U/C:L/I:L/A:H
  The Best / paranoid CVSS vector: AV:L/AC:H/PR:H/UI:N/S:U/C:H/I:H/A:H
  The Best / paranoid CVSS score: 6.4

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).