[CVE-2026-89911][MODERATE 7.0] KVM: arm64: Correctly cap TLBI Range to the architural limit

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

CVE: CVE-2026-89911
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: KVM: arm64: Correctly cap TLBI Range to the architural limit
Commit: 566b1f08d983615f731cc693f2ad59db302c6b57
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=566b1f08d983615f731cc693f2ad59db302c6b57
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89911
Analysis date: Wed, 16 Sep 2026 12:29:38 -0400
ActionableScore: 5
ActionableScore lower bound: 4
Actionable bucket: Actionable Moderate / Strong Important candidate requiring manual review
Manual review required: YES

Summary:
No one-sentence report phrase was found.

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

The original Linux kernel CVE announcement for CVE-2026-89911 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89911

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 7.0
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-2026-89911	MODERATE	https://www.kernelcve.org/list/?q=CVE-2026-89911 CHECK WITH IMPACT FROM ORIG NN 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:L/I:L/A:H';CWE-190;*CWE-20;CWE-682;Other CVSS 'AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '6.6';DESCR 'A KVM/arm64 nested virtualization bug can allow a guest controlled TLBI Range operation to produce an overflowing address range when the decoded range is added to the base VA. This may cause incorrect TLB invalidation, including stale translations or invalidation of the wrong address interval, which is security sensitive in a hypervisor MMU path. For the CVSS the PR:L is used because the attacker typically needs control of a guest or guest privileged code path, but does not need host administrator privileges. The issue is not directly network reachable and requires access to a VM environment with the affected nested virtualization feature. Impact is at least denial of service or VM isolation correctness failure, and in the paranoid interpretation may allow limited confidentiality or integrity impact through stale translation behavior.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate / Strong Important candidate requiring manual review (with actual score 5) 	MAYBE	OOB VIRT SIMPLEFIX HARDWARE KPANIC 	-	-	checked

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

ActionableScore=5
ActionableScoreLower=4
PrimitiveClass=CLASS_B
RealUAF=NO
HostAdminRequired=NO
NamespaceScopedPrivilege=NO
ImportantCandidate=YES
ManualReview=REQUIRED
AutoDowngradeAllowed=NO

1. ActionableScore

* Raw conservative score: 4
* Conservative score: 4
* Raw paranoid score: 5
* Paranoid score: 5
* Final recommended bucket: **Actionable Moderate / Strong Important candidate requiring manual review**::

No floor or cap changed the score.

2. Signal breakdown

Conservative signals:

* +1 Weak or indirect corruption candidate. Guest controlled TLBI Range decoding can produce an overflowing address range when range is added to base VA.
* +1 Active translation synchronization concern. The affected value controls a TLB invalidation range in a KVM MMU path, so an incorrect range may leave stale translations active or invalidate the wrong interval.
* +1 Below host admin virtualization reachability. The commit explicitly says a guest can generate the overflowing range, so host administrator privilege is not required for the relevant guest side input.
* +1 Security sensitive KVM nested MMU context. The bug is in nested KVM arm64 TLBI handling, where stale or incorrect translation invalidation can affect isolation correctness and not just passive metadata.

Paranoid additional signal:

* +1 Limited confidentiality or integrity concern. Stale translation behavior in a hypervisor MMU path can plausibly affect memory access correctness, but arbitrary host read or write is not demonstrated.

Non awarded signals:

* RealUAF not awarded. The patch does not establish a freed object or stale post lifetime object use.
* Controlled reclaim not demonstrated.
* Arbitrary write not demonstrated.
* Strong guest to host exploitation leverage not awarded. The evidence shows guest influenced invalidation state, not a concrete guest controlled host memory corruption primitive.
* Reliable kernel crash not awarded. The commit does not provide a panic or Oops trace for this bug.
* Remote network reachability not established.
* Local unprivileged host trigger not awarded. The attacker model is guest or VMM related, not an ordinary host user syscall path.
* Extra Availability not awarded because no distinct persistent availability consequence is established.
* Concrete repeated release or refcount signal not applicable.
* No FLOOR 5 from UAF rules because RealUAF=NO.

3. Reachability analysis

The evaluated attacker is a malicious guest or guest privileged code path in an affected KVM arm64 nested virtualization setup. The supplied commit states that a guest can generate the overflow by choosing an arbitrary VA and a large TLBI Range encoding.

Host administrator privilege is not required for the guest supplied input, so HostAdminRequired=NO. This is not a Linux namespace scoped capability issue, so NamespaceScopedPrivilege=NO.

The trigger is not directly network reachable. It requires access to a VM environment using the affected nested virtualization TLBI path. Prior host setup is practically required to run such a VM, but that is environmental exposure rather than the attacker needing host admin at exploit time.

No special physical hardware requirement is established beyond arm64 KVM support for the affected feature. No hard race or timing window is involved. This is a deterministic arithmetic range handling issue.

KVM actor distinction:

* Malicious guest: can influence guest VA and TLBI Range input.
* Local VMM or /dev/kvm process: may provide the VM environment but is not shown to be needed for the exact overflowing TLBI operand once the guest is running.
* Host administrator: needed to provision some deployments, but not positively required as the attacking principal.

4. Primitive and severity interpretation

The exact primitive is an integer or range overflow in decode_range_tlbi() for KVM arm64 nested TLBI Range handling. The decoded invalidation range was not capped to the architectural VA half selected by bit 48, so a large guest controlled range near the boundary could wrap or exceed the valid half of the address space.

This is not a UAF and not a direct OOB write. It is best treated as CLASS_B because it affects active hypervisor translation invalidation state. The active state is not merely diagnostic. It can affect which TLB translations are retired and which stale translations may remain usable.

Demonstrated consequence: incorrect TLB invalidation range calculation in host KVM processing of guest supplied TLBI Range input.

Speculative consequence: stale translations or wrong interval invalidation could lead to limited confidentiality or integrity impact, but the patch does not establish arbitrary host read, arbitrary host write, controlled reclaim, callback control, or a complete guest to host escape.

ImportantCandidate=YES because this is a KVM nested MMU translation correctness bug with guest controlled input and unresolved stale translation impact. It should not be automatically downgraded as ordinary Moderate based only on absence of a crash trace.

AutoDowngradeAllowed=NO because the primitive is a concrete active translation state violation in a virtualization boundary path, and the security impact beyond DoS remains unresolved enough to require manual review.

Likely operational severity: Actionable Moderate at minimum, with Important review warranted for affected arm64 KVM nested virtualization environments.

5. One-sentence report phrase

A KVM arm64 nested TLBI Range overflow can let a guest supplied VA and range produce incorrect TLB invalidation, potentially leaving stale translations active or invalidating the wrong address interval in a security sensitive hypervisor MMU path.

6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: the bug is not a proven UAF or arbitrary write, but it is a guest influenced KVM MMU translation invalidation flaw where stale translation effects may have security boundary consequences that cannot be safely auto closed from the small patch alone.

7. Call-site confidence

Call-site confidence: medium

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

Patch: KVM: arm64: Correctly cap TLBI Range to the architural limit
Commit: 566b1f08d983615f731cc693f2ad59db302c6b57
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=566b1f08d983615f731cc693f2ad59db302c6b57

Commit description:

TLB Invalidation by Range has a fairly powerful way of encoding pretty
large ranges in a small number of bits. This range can be based on an
arbitrary VA, which means it is pretty easy for a guest to generate an
overflow should the hypervisor be naive enough to add the range to the
base...

Make sure the range is capped to the limit dictated by the address bit
that determines the VA range. For an IPA invalidation, this is further
corrected down the line to ignore the upper range.

Fixes: 4ffa72a ("KVM: arm64: nv: Add S1 TLB invalidation primitive for VNCR_EL2")
Reported-by: Wei-Lin Chang <weilin.chang@arm.com>
Link: https://lore.kernel.org/r/yifz3wn5gk5sr6mapi32trgk5m5kp33bquctsjmkifebnsnndt@fix6u4rthx4g
Signed-off-by: Marc Zyngier <maz@kernel.org>
Cc: stable@vger.kernel.org
Reviewed-by: Wei-Lin Chang <weilin.chang@arm.com>
Link: https://patch.msgid.link/20260810170616.746100-1-maz@kernel.org
Signed-off-by: Oliver Upton <oupton@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  arch/arm64/include/asm/kvm_nested.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=566b1f08d983615f731cc693f2ad59db302c6b57

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

The original Linux kernel CVE announcement for CVE-2026-89911 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89911

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:H
  The Best / paranoid CVSS vector: AV:L/AC:L/PR:L/UI:N/S:U/C:L/I:L/A:H
  The Best / paranoid CVSS score: 6.6

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: 4
  Paranoid ActionableScore: 5

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: YES
Published priority for this report (same as in Subject): MODERATE 7.0

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).