[CVE-2026-64106][MODERATE REGULAR] KVM: arm64: vgic-its: Reject restored DTE with out-of-range num_eventid_bits

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

CVE: CVE-2026-64106
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: KVM: arm64: vgic-its: Reject restored DTE with out-of-range num_eventid_bits
Commit: 1716b7fea2ead941a0dfac06c4504a3437cdf00d
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=1716b7fea2ead941a0dfac06c4504a3437cdf00d
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64106
Analysis date: Sun, 19 Jul 2026 17:59:03 -0400
ActionableScore: 4
ActionableScore lower bound: 3
Actionable bucket: Borderline, manual review recommended / Actionable Moderate at minimum
Manual review required: YES

Summary:
A malformed restored KVM arm64 VGIC ITS Device Table Entry can set num_eventid_bits beyond the virtual ITS limit, causing an oversized ITT scan during restore and potentially leading to host-side denial of service for userspace with KVM restore access.

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

The original Linux kernel CVE announcement for CVE-2026-64106 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64106

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
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CVE-2026-64106	MODERATE	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:N/I:N/A:H';*CWE-20;CWE-834;**CWE-400;Other CVSS 'AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:L';BEST CVSS score: '5.5';DESCR 'A malformed KVM arm64 VGIC ITS restore state can provide a Device Table Entry with num_eventid_bits larger than the virtual ITS supports. The live MAPD path rejects this state, but the restore path could accept it and later use the value to build an oversized scan_its_table range during ITT restore. For the CVSS the PR:L because a local user or VM management process with access to KVM restore ioctls can supply the restored device state, and full administrator privileges are not necessarily required when /dev/kvm access is delegated. The issue is not network reachable and is triggered through the local KVM control plane rather than guest packet or interrupt traffic. Impact is mainly denial of service through excessive kernel work or a stalled restore path on the host. No direct C or I impact is assigned because the observable primitive is an oversized scan range rather than UAF, OOB write, or information leak.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline, manual review recommended / Actionable Moderate at minimum (with actual score 4)	YES	OOB VIRT SIMPLEFIX HARDWARE LINUS  DECREASED_TO_MODERATE_REGULAR_FROM_MODERATE7_BASED_ON_GUESSCVSS_AND_CIANNH	NO	NO	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=4
ActionableScoreLower=3

## 1. ActionableScore

* Conservative score: 3
* Paranoid score: 4
* Final recommended bucket: **Borderline, manual review recommended / Actionable Moderate at minimum**::

## 2. Signal breakdown

Conservative signals:

* Local non-root KVM control-plane trigger: +1
  A userspace VMM or VM management process with access to KVM restore ioctls can provide the malformed restored ITS DTE state. This is PR:L in many deployments where /dev/kvm is delegated to a kvm group or virtualization service, not necessarily full host root.

* Reliable host availability concern: +1
  The unchecked num_eventid_bits can reach vgic_its_restore_itt() and expand scan_its_table() into an oversized range, creating a realistic host-side DoS or stalled restore path.

* Common virtualization host attack surface: +1
  KVM is a common subsystem on virtualization hosts, and ITS state restore is relevant to VM migration or snapshot restore on arm64 systems.

Paranoid additional signal:

* System-wide or operational availability impact: +1
  In a virtualization host, a stuck or excessive KVM restore path can affect VM management operations and potentially host responsiveness, so a higher availability interpretation is defensible.

Not counted:

* No generic memory corruption bonus. The patch shows input validation and oversized scanning, not UAF, OOB write, double free, refcount misuse, or type confusion.
* No privilege escalation bonus. There is no demonstrated LPE primitive.
* No confidentiality or integrity impact. The observable primitive is excessive iteration or DoS, not disclosure or modification of protected data.
* No remote/network trigger. This is a local KVM control-plane restore path.

## 3. Reachability analysis

The bug is triggerable by userspace that can restore VGIC ITS state through KVM interfaces. In typical systems this means a VMM such as QEMU, a VM migration stack, or a management process with access to /dev/kvm and the relevant VM device ioctls. It is not directly reachable from the network and does not appear to be triggerable by ordinary guest interrupt traffic alone.

Namespaces or containers matter only if /dev/kvm is delegated into the container or service context. A container root or reduced-capability service with KVM access should be treated closer to PR:L than PR:H, because it is not equivalent to full host-root control. The path is configuration-dependent on arm64 KVM with VGIC ITS restore, but it is not an exotic legacy mode.

## 4. Severity interpretation

This behaves like an actionable Moderate or borderline manual-review issue, not a strong Important candidate. The realistic impact is host availability degradation during VM state restore due to an oversized scan range. Theoretical escalation is not supported by the patch because the fix only rejects an out-of-range restored value and does not show memory lifetime corruption, attacker-controlled overwrite, stale pointer reuse, or information disclosure.

The conservative score is 3 because the issue is local and control-plane based. The paranoid score is 4 because KVM host availability impact in VM restore paths is operationally important and worth manual review.

## 5. One-sentence report phrase

A malformed restored KVM arm64 VGIC ITS Device Table Entry can set num_eventid_bits beyond the virtual ITS limit, causing an oversized ITT scan during restore and potentially leading to host-side denial of service for userspace with KVM restore access.

## 6. Manual review recommendation

MANUAL CHECK RECOMMENDED

Manual review is recommended because this is a KVM host-kernel restore path with plausible host availability impact, but the patch does not support memory corruption, privilege escalation, or remote reachability.

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

Patch: KVM: arm64: vgic-its: Reject restored DTE with out-of-range num_eventid_bits
Commit: 1716b7fea2ead941a0dfac06c4504a3437cdf00d
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=1716b7fea2ead941a0dfac06c4504a3437cdf00d

Commit description:

Userspace can restore an ITS Device Table Entry whose Size field encodes
more EventID bits than the virtual ITS supports.  The live MAPD path
rejects that state, but vgic_its_restore_dte() accepts it and stores the
out-of-range value in dev->num_eventid_bits.

Reject restored DTEs with num_eventid_bits > VITS_TYPER_IDBITS before
allocating the device.  This mirrors the MAPD check and prevents the
restored state from reaching vgic_its_restore_itt(), where the unchecked
value can be converted into an oversized scan_its_table() range.

Fixes: 57a9a11 ("KVM: arm64: vgic-its: Device table save/restore")
Assisted-by: Claude:claude-opus-4-7
Signed-off-by: Michael Bommarito <michael.bommarito@gmail.com>
Link: https://lore.kernel.org/r/20260519132519.2142458-1-michael.bommarito@gmail.com
Signed-off-by: Marc Zyngier <maz@kernel.org>
Cc: stable@vger.kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  arch/arm64/kvm/vgic/vgic-its.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=1716b7fea2ead941a0dfac06c4504a3437cdf00d

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

The original Linux kernel CVE announcement for CVE-2026-64106 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64106

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: 3
  Paranoid ActionableScore: 4

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).