[CVE-2026-89775][MODERATE 7.0] KVM: arm64: Handle negative S1 walk levels in VNCR TLB size evaluation

[CVE-2026-89775][MODERATE 7.0] KVM: arm64: Handle negative S1 walk levels in VNCR TLB size evaluation

[AL-KERNEL]

CVE: CVE-2026-89775
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: KVM: arm64: Handle negative S1 walk levels in VNCR TLB size evaluation
Commit: be54a70067c7dbe1381157e8a238fa4b16ea9c05
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=be54a70067c7dbe1381157e8a238fa4b16ea9c05
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89775
Analysis date: Wed, 16 Sep 2026 05:21:25 -0400
ActionableScore: 5
ActionableScore lower bound: 4
Actionable bucket: Actionable Moderate / Strong Important candidate requiring manual review
Manual review required: YES

Summary:
A KVM arm64 nested virtualization bug can mis-handle negative S1 walk levels for disabled MMU state, producing an invalid or zero TLB invalidation size and potentially leaving stale translations that affect VM correctness or host-managed virtualization state.

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

The original Linux kernel CVE announcement for CVE-2026-89775 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89775

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.

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

CVE-2026-89775	MODERATE	https://www.kernelcve.org/list/?q=CVE-2026-89775 CHECK WITH IMPACT FROM ORIG NN MODERATE	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:H';CWE-682;CWE-755;CWE-119;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:H';BEST CVSS score: '7';DESCR 'A KVM arm64 nested virtualization bug can mis-handle a negative S1 walk level used when the stage 1 MMU is disabled. The value can be converted into an invalid TTL and can make the computed TLB invalidation size become zero or otherwise wrong. This may leave stale translations after a TLB invalidation path and can affect memory consistency in the virtualization context. For the CVSS the PR:L is used because triggering requires a local KVM capable process or a guest and VMM setup that can exercise nested virtualization paths, but it does not require full host administrator privileges in typical KVM access models. The issue is not directly network reachable. Impact is at least denial of service or guest memory inconsistency, and the paranoid score treats stale TLB mappings as a possible confidentiality and integrity risk that should be reviewed manually.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate / Strong Important candidate requiring manual review (with actual score 5)  SKIP CVE-2026-89775 UNKNOWN SKIP The Fixes patch not applied yet, so unlikely that actual: url YES  NO NO unknown 	MAYBE	VIRT HARDWARE LINUS  KPANIC INCREASED_TO_MODERATE7_BASED_ON_GUESSCVSS	-	-	checked

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

ActionableScore=5
ActionableScoreLower=4
PrimitiveClass=CLASS_B
RealUAF=NO
HostAdminRequired=UNKNOWN
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:

* +2: CLASS_B active translation invalidation primitive. The patch fixes a KVM arm64 nested MMU case where a negative S1 walk level, used to represent disabled S1 MMU, was converted as if it were a normal level. This could produce an invalid TTL and a zero or wrong TLB invalidation size.
* +1: Active semantic reinterpretation. A sentinel negative level representing S1_MMU_DISABLED was consumed as an unsigned mapping level and then converted into TTL state.
* +1: Realistic availability or correctness impact. A wrong or zero invalidation size can leave stale translations and break VM or nested virtualization memory consistency.
* 0: No confirmed kernel panic, arbitrary host memory read/write, controlled reclaim, or demonstrated guest-to-host escape is established.

Paranoid additional signal:

* +1: Stale TLB state remains operational outside the immediate software structure. In KVM MMU code this is a meaningful unresolved translation-synchronization issue, even though it is not proof of arbitrary host memory access.

Negative and non-awarded signals:

* RealUAF=NO. No use-after-free or stale heap object lifetime violation is directly established.
* No concrete repeated release/refcount operation exists.
* Controlled reclaim is not demonstrated.
* Arbitrary or directed kernel write is not demonstrated.
* Confidentiality and integrity are not directly awarded as established consequences.
* Remote reachability is not established.
* KVM strong guest-to-host exploitation leverage is not awarded because the evidence does not show guest-controlled host memory corruption, callback control, arbitrary read/write, or escape.
* No race penalty is applied. The issue is a deterministic invalid level/TTL conversion, not a proven narrow timing race.
* No host-admin penalty is applied because host-admin-equivalent privilege is not positively established by the supplied evidence.

## 3. Reachability analysis

The affected code is in KVM arm64 nested virtualization TLB invalidation handling. The likely attacker models are:

* malicious guest code influencing guest S1 state and TLBI behavior;
* local VMM or host userspace process with `/dev/kvm` access creating and running the nested virtualization scenario;
* host administrator only if the deployment restricts KVM access to host root.

The complete trigger chain is not proven to be guest-only from the supplied patch. Guest code can plausibly influence S1 MMU state, but the VMM controls VM creation and nested virtualization configuration. Therefore the attacker model is best treated as unresolved or compound guest plus VMM unless additional advisory evidence proves a pure guest-to-host trigger.

HostAdminRequired is UNKNOWN because the patch does not prove host-root-only reachability, but also does not prove ordinary unprivileged reachability. NamespaceScopedPrivilege is NO because this is not a namespace-scoped capability path.

No network trigger is present. Prior setup may include KVM availability, arm64 nested virtualization support, and a VMM configuration that exercises VNCR/S1 invalidation paths.

## 4. Primitive and severity interpretation

The exact primitive is incorrect TLB invalidation range computation due to semantic misinterpretation of a negative S1 walk level. This is not a heap UAF, not a double free, and not a direct OOB access. It is an active translation-state consistency flaw.

The issue is CLASS_B because stale or insufficiently invalidated translation state in KVM MMU code can be security-relevant and needs exploitation-leverage review. It is not CLASS_A because there is no demonstrated controlled host memory corruption, arbitrary read/write, controlled reclaim, callback hijack, or proven guest-to-host escape.

The demonstrated consequence is incorrect or missing TLB invalidation, potentially causing stale translation behavior and VM correctness or availability problems. The speculative consequence is confidentiality or integrity impact from stale translations, but the patch alone does not establish a concrete primitive sufficient to score that as proven.

ImportantCandidate=YES because KVM MMU stale-translation defects can affect a virtualization boundary and the practical impact is not safely reducible to ordinary crash-only behavior from the patch alone. AutoDowngradeAllowed=NO because the bug involves active translation invalidation semantics, and automatic closure would risk missing a boundary-relevant KVM MMU issue.

Operationally, this is at least Actionable Moderate. It is not yet a clear Strong Important issue without stronger evidence of guest-only reachability or concrete host memory corruption.

## 5. One-sentence report phrase

A KVM arm64 nested virtualization bug can mis-handle negative S1 walk levels for disabled MMU state, producing an invalid or zero TLB invalidation size and potentially leaving stale translations that affect VM correctness or host-managed virtualization state.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: the bug is in KVM MMU/TLB invalidation logic and may leave stale translations, but the supplied patch does not fully establish whether the practical impact is limited to VM malfunction/DoS or can affect confidentiality or integrity across the virtualization boundary.

## 7. Call-site confidence

Call-site confidence: medium

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

Patch: KVM: arm64: Handle negative S1 walk levels in VNCR TLB size evaluation
Commit: be54a70067c7dbe1381157e8a238fa4b16ea9c05
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=be54a70067c7dbe1381157e8a238fa4b16ea9c05

Commit description:

Computing the effects of a TLB invalidation involves looking at
the size of the mapping cached by the TLB. For S1 mappings such as
VNCR, this is deducted from the combination of the base granule size
and the mapping level.

However, this implies that the S1 MMU is *on*. When the MMU is off,
we indicate this with the level being set to a "creative" value of
-127 (S1_MMU_DISABLED).

This ends-up being misinterpreted by pgshift_level_to_ttl() as it
doesn't handle negative levels at all (the level is immediately cast
to a u8 and only the bottom two bits considered), leading to an
invalidation size of 0. Not helpful.

Tidy-up pgshift_level_to_ttl() to handle these negative levels, and
ttl_to_size() to always return SZ_1G when no valid TTL is present.
This allows the removal of open-coded checks for similar situations.

Note that the check for a negative value not explicitely checking for
S1_MMU_DISABLED is deliberate, so that actual negative levels introduced
with LVA2 and D128 can take the same path if we ever support them.

Fixes: 7270cc9 ("KVM: arm64: nv: Handle VNCR_EL2 invalidation from MMU notifiers")
Reported-by: Hyunwoo Kim <imv4bel@gmail.com>
Link: https://lore.kernel.org/r/ameGoxbn2wzBq2kL@v4bel
Signed-off-by: Marc Zyngier <maz@kernel.org>
Cc: stable@vger.kernel.org
Link: https://patch.msgid.link/20260806091026.620700-3-maz@kernel.org
Signed-off-by: Oliver Upton <oupton@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  arch/arm64/kvm/nested.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=be54a70067c7dbe1381157e8a238fa4b16ea9c05

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

The original Linux kernel CVE announcement for CVE-2026-89775 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89775

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