[CVE-2026-64287][IMPORTANT] KVM: arm64: Bound used_lrs when flushing the pKVM hyp vCPU

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

CVE: CVE-2026-64287
Priority: IMPORTANT
AL-KERNEL base severity: IMPORTANT
KPANIC flag: YES
Patch: KVM: arm64: Bound used_lrs when flushing the pKVM hyp vCPU
Commit: 2c5e72b9fbf83fdfa724e9f1af0f418ccf8739b8
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=2c5e72b9fbf83fdfa724e9f1af0f418ccf8739b8
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64287
Analysis date: Sat, 25 Jul 2026 07:15:21 -0400
ActionableScore: 7
ActionableScore lower bound: 5
Actionable bucket: Strong Important candidate / Actionable Moderate at minimum
Manual review required: YES

Summary:
KVM arm64 pKVM copies host vGIC state into the hyp vCPU without bounding `used_lrs`, allowing local host-side virtualization paths to drive EL2 out-of-bounds list-register handling that can cause hypervisor DoS and may threaten pKVM isolation in the worst case.

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ABOUT THIS REPORT
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The original Linux kernel CVE announcement for CVE-2026-64287 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64287

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: IMPORTANT
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-2026-64287	IMPORTANT	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:C/C:H/I:H/A:H';CWE-129;*CWE-787;CWE-125;Other CVSS 'AV:L/AC:H/PR:H/UI:N/S:C/C:L/I:L/A:H';BEST CVSS score: '7.8';DESCR 'KVM arm64 pKVM copies host vGIC state into the private hyp vCPU and can trust used_lrs as a loop bound for vgic_lr and GIC list register handling at EL2. If used_lrs is larger than the implemented number of list registers, EL2 code may index beyond the valid vgic_lr array or access invalid ICH_LR registers. For the CVSS the PR:L is used for the paranoid score because a local VMM or host side process with KVM device access may influence vCPU or vGIC state in some deployments, while the base case may require stronger host control. The issue is not network reachable and is triggered through local host side virtualization control paths. Impact is at least denial of service via EL2 fault or host hypervisor crash. Because the bug is an EL2 out of bounds access across the pKVM protection boundary, worst case impact may include confidentiality and integrity impact against protected virtualization isolation.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate / Actionable Moderate at minimum (with actual score 7)	YES	DANGER VIRT INIT SIMPLEFIX HARDWARE LINUS KPANIC  KPANIC INCREASED_TO_HIGH_BASED_ON_GUESSCVSS	YES	NO	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=7
ActionableScoreLower=5

## 1. ActionableScore

* Conservative score: 5
* Paranoid score: 7
* Final recommended bucket: **Strong Important candidate / Actionable Moderate at minimum**::

## 2. Signal breakdown

Conservative signals:

* Local KVM/VMM-side trigger: +1. The affected path is local host-side KVM execution, not network traffic.
* Memory corruption, weak or constrained corruption candidate: +1. `used_lrs` is used as a loop bound for `vgic_lr[]` and GIC LR handling, but the patch does not demonstrate an arbitrary write primitive.
* Privileged kernel/device-management memory corruption path: +1. The bug occurs in arm64 pKVM EL2 hyp/vGIC state handling.
* Cross-boundary isolation concern: +1. The value is copied from host vCPU state into the protected hyp vCPU path, crossing the pKVM host-to-EL2 trust boundary.
* Reliable crash / strong DoS plausible: +1. Out-of-bounds EL2 access or invalid ICH_LR access can plausibly crash or destabilize the hypervisor/host.
* Rare/configuration-dependent pKVM/GICv3 exposure: -1. This requires arm64 KVM with pKVM/nVHE and GICv3 list register handling.

Paranoid additional/stronger interpretation:

* Generic memory corruption: +2 instead of weak +1. If save/restore paths write beyond `vgic_lr[]`, this becomes an EL2 OOB write candidate.
* Weak LPE / isolation-break concern: +1. Hypervisor memory corruption can plausibly undermine pKVM isolation, although no concrete exploit primitive is shown.
* Confidentiality impact plausible: +1. EL2 OOB access may expose protected virtualization state in the worst case.
* Integrity impact plausible: +1. EL2 OOB write or invalid state manipulation may corrupt hyp/vCPU state.
* Constrained primitive / no demonstrated attacker-chosen target: -1. The patch does not show controlled payload, controlled target selection, or a demonstrated reclaim/object replacement primitive.

## 3. Reachability analysis

The bug is not remotely reachable. It is triggered through local host-side virtualization control paths during pKVM vCPU run/flush handling. A fully privileged malicious host kernel would trivially be able to influence host vCPU state, but the more security-relevant scenario is a local VMM or host-side process with KVM access influencing vCPU/vGIC state in a pKVM deployment.

Namespaces do not directly make this reachable unless `/dev/kvm` or an equivalent VMM interface is delegated into the namespace/container. The path is not a generic default kernel path, but it is relevant on arm64 systems using protected KVM with GICv3. Realistic exploitation requires a pKVM-capable system and a way to drive or corrupt the relevant vGIC state so that `used_lrs` exceeds the implemented LR count.

Call-site confidence: medium. The patch clearly shows the vulnerable copy and clamp site, and the commit explains the downstream use as a loop bound, but the exact LR save/restore call sites are not included in the pasted diff.

## 4. Severity interpretation

This is not an ordinary Moderate bookkeeping issue. It is at least Actionable Moderate because the bug is an EL2 bounds violation in protected virtualization state handling. The realistic, conservative impact is hypervisor/host DoS from invalid EL2 access. The theoretical worst case is more serious because an out-of-bounds access in pKVM hyp context may affect confidentiality and integrity of protected virtualization state.

The patch does not prove arbitrary code execution, attacker-controlled overwrite target, or a reliable privilege escalation primitive. However, because the affected context is EL2 and the bug crosses the pKVM host-to-hyp boundary, manual review should not auto-close it as a simple DoS.

## 5. One-sentence report phrase

KVM arm64 pKVM copies host vGIC state into the hyp vCPU without bounding `used_lrs`, allowing local host-side virtualization paths to drive EL2 out-of-bounds list-register handling that can cause hypervisor DoS and may threaten pKVM isolation in the worst case.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: this is an EL2 out-of-bounds access candidate in pKVM hyp/vGIC state handling, with plausible DoS and possible isolation-boundary impact even though a concrete arbitrary-write or LPE primitive is not demonstrated.

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UPSTREAM PATCH SUMMARY
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Patch: KVM: arm64: Bound used_lrs when flushing the pKVM hyp vCPU
Commit: 2c5e72b9fbf83fdfa724e9f1af0f418ccf8739b8
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=2c5e72b9fbf83fdfa724e9f1af0f418ccf8739b8

Commit description:

flush_hyp_vcpu() copies the host vGIC state into the hyp's private vCPU
on every run. The vGIC list register save and restore use used_lrs as
their loop bound and expect it to stay within the number of implemented
list registers. While this is generally the case, flush_hyp_vcpu()
copies vgic_v3 verbatim and does not enforce this, so a value provided
by the host is used at EL2 to index vgic_lr[] and access ICH_LR<n>_EL2
(host -> EL2).

Fix by clamping used_lrs to the number of implemented list registers
after the copy, as the trusted path already does in
vgic_flush_lr_state(). The number of implemented list registers is
constant after init, so it is replicated once from
kvm_vgic_global_state.nr_lr into hyp_gicv3_nr_lr rather than read on
every entry.

Cc: stable@vger.kernel.org
Fixes: be66e67 ("KVM: arm64: Use the pKVM hyp vCPU structure in handle___kvm_vcpu_run()")
Signed-off-by: Hyunwoo Kim <imv4bel@gmail.com>
Reviewed-by: Fuad Tabba <tabba@google.com>
Tested-by: Fuad Tabba <tabba@google.com>
Link: https://patch.msgid.link/20260606175614.83273-3-imv4bel@gmail.com
Signed-off-by: Marc Zyngier <maz@kernel.org>
[ tabba: adjust context in flush_hyp_vcpu() and kvm_hyp.h ]
Signed-off-by: Fuad Tabba <fuad.tabba@linux.dev>
Signed-off-by: Sasha Levin <sashal@kernel.org>

Changed files:
  arch/arm64/include/asm/kvm_hyp.h
  arch/arm64/kvm/arm.c
  arch/arm64/kvm/hyp/nvhe/hyp-main.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=2c5e72b9fbf83fdfa724e9f1af0f418ccf8739b8

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

The original Linux kernel CVE announcement for CVE-2026-64287 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64287

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:C/C:L/I:L/A:H
  The Best / paranoid CVSS vector: AV:L/AC:H/PR:L/UI:N/S:C/C:H/I:H/A:H
  The Best / paranoid CVSS score: 7.8

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: 5
  Paranoid ActionableScore: 7

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: IMPORTANT
KPANIC detected for this report: YES
Published priority for this report (same as in Subject): IMPORTANT

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).