[CVE-2026-98469][MODERATE REGULAR] KVM: arm64: Fix AArch32 DBGBXVR<n> handling

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

CVE: CVE-2026-98469
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: KVM: arm64: Fix AArch32 DBGBXVR<n> handling
Commit: 8530a05b304d312fe7dfbc5df7d3e87cc31b256a
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=8530a05b304d312fe7dfbc5df7d3e87cc31b256a
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98469
Analysis date: Mon, 19 Oct 2026 05:45:54 -0400
ActionableScore: 1
ActionableScore lower bound: unknown
Actionable bucket: Ordinary Moderate
Manual review required: NO

Summary:
An AArch32 guest access to DBGBXVR can hit KVM_BUG_ON because Op2 value 1 is not mapped to dbg_bvr, causing denial of service by marking the affected VM dead without demonstrated host memory corruption or guest-to-host privilege escalation.

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ABOUT THIS REPORT
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The original Linux kernel CVE announcement for CVE-2026-98469 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98469

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: NO

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-2026-98469	MODERATE	https://www.kernelcve.org/list/?q=CVE-2026-98469 CHECK WITH IMPACT FROM ORIG NN MODERATE | AEGIS IMPACT MODERATE AEGIS_AS=1 LEGACY=MODERATE RESULT=SAME V39_AEGIS_ONE_STEP_UP=LOW->MODERATE (final reconciliation agrees; DECISION: both MODERATE)	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H';CWE-697;*CWE-20;CWE-754;Other CVSS 'AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:L';BEST CVSS score: '5.5';DESCR 'An incorrect register demultiplexing path in KVM arm64 fails to map the AArch32 DBGBXVR register with Op2 value 1 to the dbg_bvr backing array. An AArch32 guest access can therefore reach KVM_BUG_ON and mark the affected VM dead, resulting in denial of service for that VM. For the CVSS the PR:L value reflects that reliable triggering may require control of privileged AArch32 code or the guest kernel, while those privileges remain restricted to the guest and do not provide host administrator access. The issue is not network reachable by itself and requires local execution inside a virtual machine. The demonstrated impact is VM termination only, with no supported host panic, memory corruption, information disclosure, or guest to host privilege escalation primitive. The paranoid availability rating treats repeatable termination of the affected VM as a complete availability loss, but it does not extend the vulnerability beyond its DoS class. Automated impact decision path. The original SimpleNet classifier selected MODERATE_KPANIC. The primary LLM estimated paranoid CVSS 5.5 and the production ActionableScore returned 1 with recommendation Ordinary Moderate. The production rules then evaluated CVSS thresholds and ActionableScore thresholds. Recorded rule transitions were CVSS and initial triage stage MODERATE_KPANIC to MODERATE because the CVSS score, vector or bug class threshold rule was applied. ActionableScore thresholds stage MODERATE to LOW because the ActionableScore threshold rule was applied. final reconciliation stage LOW to MODERATE because KP specialist did not establish a qualifying panic and the cross model evidence reconciliation rule was applied. The legacy KP specialist has no review artifact for this run. The legacy FD or DH downgrade specialist has no review artifact for this run. Exact triggered rule markers were DECREASED_TO_MODERATE_REGULAR_FROM_MODERATE7_BASED_ON_GUESSCVSS_AND_CIANNH and DECREASED_TO_LOW_FROM_MODERATE_BASED_ON_ACTIONABLESCORELESSTHAN3 and KPANIC_UNSUPPORTED_REMOVED and V39_AEGIS_ONE_STEP_UP and MANUAL_REVIEW_REQUIRED. The independent weaker Aegis shadow evaluated CVSS 5.5 and ActionableScore 1 with primitive class CLASS_C and manual review RECOMMENDED. Its independently reconciled impact was MODERATE. Cross model aggregation changed the production result under an enabled evidence gated merge rule. The final reported impact is MODERATE.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate (with actual score 1)	MAYBE	DANGER VIRT SIMPLEFIX HARDWARE LINUS DECREASED_TO_MODERATE_REGULAR_FROM_MODERATE7_BASED_ON_GUESSCVSS_AND_CIANNH DECREASED_TO_LOW_FROM_MODERATE_BASED_ON_ACTIONABLESCORELESSTHAN3 KPANIC_UNSUPPORTED_REMOVED V39_AEGIS_ONE_STEP_UP MANUAL_REVIEW_REQUIRED	-	-	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=1
PrimitiveClass=CLASS_C
RealUAF=NO
HostAdminRequired=NO
NamespaceScopedPrivilege=NO
ImportantCandidate=NO
ManualReview=NO
AutoDowngradeAllowed=YES
RequiresHostUID0=NO
AuthorizationDecisionBypass=NO
ProtectedResourceImpact=NONE
B4PendingReview=NO
B4PendingConfirmed=NO

1. ActionableScore

* Raw conservative score: 1
* Conservative score: 1
* Raw paranoid score: 1
* Paranoid score: 1
* No floor or cap was applied.
* Final recommended bucket: **Ordinary Moderate**::

2. Signal breakdown

* +1 Reliable target VM denial of service. An AArch32 guest access to DBGBXVR with Op2 value 1 reaches KVM_BUG_ON and marks that VM dead.
* +0 Local unprivileged trigger. Guest-only reachability is established, but access by an ordinary unprivileged guest process is not established. It may require privileged AArch32 guest code.
* +0 Guest-to-host impact. The failure terminates the initiating VM. No host panic, host memory corruption, or effect on other VMs is established.
* +0 Memory corruption. The patch corrects register demultiplexing and provides no UAF, OOB access, stale reference, invalid write, or corrupted active object.
* +0 Authorization bypass. No DENY-to-ALLOW security decision or omitted capability check exists.
* +0 Extra availability point. Repeatability of the same VM termination is not an independent consequence.
* Controlled reclaim, arbitrary write, disclosure, integrity impact, privilege escalation, and remote reachability are not demonstrated.
* NEW52 B5-R does not apply because there is no heap OOB write or retry-accumulated index.
* NEW53 B3-PENDING does not apply because there is no shared-backing write.
* V33 and V34 do not apply because no attacker-influenced active structured state is corrupted and consumed downstream.

3. Reachability analysis

The evaluated attacker is guest-only. An AArch32 guest can access the affected debug register path and cause its own VM to be marked dead. No host VMM lifecycle operation or host administrator action is required for the vulnerable access after the VM exists, so this is not a compound guest plus VMM trigger.

The guest controls the register access, but no object creation, free, replacement, stale traversal, or host-memory mutation is involved. Guest privilege may be required to execute the relevant debug-register access, but guest administrator privilege is not host-administrator privilege. No host UID 0, Linux namespace capability, special physical hardware, race, or network path is involved. Prior host setup is limited to creating and running an AArch32-capable VM and does not create an additional security boundary bypass.

4. Primitive and severity interpretation

The exact primitive is an invalid register-selector path that reaches KVM_BUG_ON. It is a logic and invalid-state failure classified as CLASS_C.

There is no stale storage, active-object UAF, repeated release, refcount mutation, authorization bypass, or semantic transition involving live mappings. The demonstrated consequence is termination of the affected VM. A host crash, guest-to-host corruption primitive, VM escape, confidentiality loss, and integrity loss are unsupported.

ImportantCandidate is NO because the patch establishes only VM-local DoS without an independently useful corruption or boundary-bypass primitive. AutoDowngradeAllowed is YES because no unresolved Class A or Class B mechanism remains.

5. One-sentence report phrase

An AArch32 guest access to DBGBXVR can hit KVM_BUG_ON because Op2 value 1 is not mapped to dbg_bvr, causing denial of service by marking the affected VM dead without demonstrated host memory corruption or guest-to-host privilege escalation.

6. Manual review recommendation

NO MANUAL CHECK NEEDED

The source establishes a narrow VM-termination path, and no host-wide failure, memory corruption, authorization bypass, or unresolved exploitation primitive is present.

7. Call-site confidence

Call-site confidence: high

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

Patch: KVM: arm64: Fix AArch32 DBGBXVR<n> handling
Commit: 8530a05b304d312fe7dfbc5df7d3e87cc31b256a
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=8530a05b304d312fe7dfbc5df7d3e87cc31b256a

Commit description:

The consolidation of the breakpoint and watchpoint register accessors
switched DBGBXVR<n> from trap_bvr() to trap_dbg_wb_reg(). The latter
selects backing storage with demux_wb_reg(), which only handles Op2 values
4 through 7. Since DBGBXVR<n> uses Op2 1, an AArch32 guest access hits
KVM_BUG_ON() and marks the VM dead.

DBGBXVR<n> aliases DBGBVR<n>_EL1[63:32], and its AA32(HI) descriptor
already selects the upper half. Map Op2 1 to dbg_bvr[] alongside Op2 4,
restoring the pre-regression behavior.

Fixes: 3ce9f33 ("KVM: arm64: Fold DBGxVR/DBGxCR accessors into common set")
Cc: stable@vger.kernel.org
Assisted-by: Claude:claude-opus-5
Signed-off-by: Karl Mehltretter <kmehltretter@gmail.com>
Reviewed-by: Marc Zyngier <maz@kernel.org>
Link: https://patch.msgid.link/20260810005616.13227-1-kmehltretter@gmail.com
Signed-off-by: Oliver Upton <oupton@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  arch/arm64/kvm/sys_regs.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=8530a05b304d312fe7dfbc5df7d3e87cc31b256a

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

The original Linux kernel CVE announcement for CVE-2026-98469 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98469

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:L/PR:L/UI:N/S:U/C:N/I:N/A:L
  The Best / paranoid CVSS vector: AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS score: 5.5

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: 1

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).