[CVE-2026-63941][MODERATE 7.0] KVM: arm64: Correctly cap ZCR_EL2 provided by a guest hypervisor

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

CVE: CVE-2026-63941
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: KVM: arm64: Correctly cap ZCR_EL2 provided by a guest hypervisor
Commit: 742a9b5ccd8c46caf983ca90c94f855466968e34
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=742a9b5ccd8c46caf983ca90c94f855466968e34
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-63941
Analysis date: Sun, 19 Jul 2026 12:25:47 -0400
ActionableScore: 7
ActionableScore lower bound: 4
Actionable bucket: Strong Important candidate / Actionable Moderate at minimum
Manual review required: YES

Summary:
A KVM arm64 nested virtualization flaw allows a guest hypervisor to provide an uncapped ZCR_EL2.LEN value through the ZCR_EL1 path, letting an L2 guest exceed the configured SVE vector length limit and potentially affecting guest state isolation.

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

The original Linux kernel CVE announcement for CVE-2026-63941 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-63941

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
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CVE-2026-63941	MODERATE	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-20;CWE-693;CWE-269;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:C/C:L/I:L/A:L';BEST CVSS score: '7.8';DESCR 'A KVM arm64 nested virtualization bug can allow a guest hypervisor to provide an uncapped ZCR_EL2.LEN value when the value is updated through the ZCR_EL1 path and later restored for an L2 guest. This can let the L2 guest access SVE vector lengths beyond the VM configured limit and bypass the intended virtualization boundary for CPU feature state. For the CVSS the PR:L is used because reliable triggering requires control of a guest or nested guest hypervisor context, but does not require host administrator privileges in a tenant virtualization model. The issue is not network reachable and is triggered through local guest execution and nested virtualization state handling. Impact is at least a virtualization policy bypass with possible guest state isolation impact. For the paranoid score, the concern is that programming hardware with a vector length larger than the vCPU SVE state layout may create a plausible path to incorrect SVE save restore behavior, stale register exposure, or memory corruption, so manual review is preferred over an autoclosed false negative.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate / Actionable Moderate at minimum (with actual score 6)	YES	DANGER VIRT INIT HARDWARE KPANIC  KPANIC INCREASED_TO_HIGH_BASED_ON_GUESSCVSS KPANIC DECREASED_TO_MODERATE70_BASED_ON_ACTIONABLESCORELOWERTHAN7	YES	NO	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=7
ActionableScoreLower=4

## 1. ActionableScore

* Conservative score: 4
* Paranoid score: 7
* Final recommended bucket: **Strong Important candidate / Actionable Moderate at minimum**::

## 2. Signal breakdown

Conservative signals:

* Local unprivileged or tenant-level trigger: +1
  Triggering requires control of a guest or nested guest hypervisor context, not host root.
* Security-boundary / virtualization policy bypass: +2
  The bug allows an L2 guest to access SVE vector lengths beyond the VM-configured limit.
* Confidentiality or integrity concern, limited: +1
  The exposed primitive is a CPU feature-state isolation violation, not a demonstrated arbitrary write or host memory disclosure.
* Rare / configuration-dependent path: -1
  Requires arm64 KVM nested virtualization with SVE handling.

Conservative total: 4

Paranoid additional signals:

* Weak / indirect corruption candidate: +1
  The hardware ZCR_EL2 value can become inconsistent with the vCPU SVE state layout and configured VL limit.
* Memory layout invariant restoration: +1
  The patch restores consistency between the guest-visible ZCR_EL2 state, the physical ZCR_EL2 programming, and the vCPU maximum SVE vector length.
* Availability / host-state concern: +1
  Incorrect SVE save/restore behavior could plausibly cause guest or host-side state corruption or failure, although no concrete crash primitive is shown.
* Security-boundary impact retained at high weight: +2
  This is a KVM nested virtualization boundary issue, not a pure guest-local correctness bug.

Paranoid total: 7

## 3. Reachability analysis

The bug is not network reachable. It is triggered through local guest execution in an arm64 KVM nested virtualization setup where a VHE guest hypervisor can update ZCR_EL2 indirectly through the ZCR_EL1 path. In a cloud or tenant virtualization model, the attacker may only need control of a guest or nested guest hypervisor, so treating this as PR:L is more realistic than PR:H. The path is configuration-dependent because it requires nested virtualization and SVE support, but it affects a security-sensitive KVM state-restore path.

Call-site confidence: high. The patch shows both the register access path and the SVE restore paths where the uncapped value is written to hardware.

## 4. Severity interpretation

This is not an ordinary Moderate correctness fix. The confirmed impact is a virtualization policy bypass where an L2 guest can access vector lengths outside the configured VM limit. The patch does not prove arbitrary host memory corruption, arbitrary code execution, or a reliable host crash. However, the bug sits in KVM guest state restore logic and can make hardware CPU state inconsistent with the vCPU SVE layout. That makes the paranoid interpretation Important-candidate worthy and requires manual review.

## 5. One-sentence report phrase

A KVM arm64 nested virtualization flaw allows a guest hypervisor to provide an uncapped ZCR_EL2.LEN value through the ZCR_EL1 path, letting an L2 guest exceed the configured SVE vector length limit and potentially affecting guest state isolation.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

Manual review is required because this is a KVM nested virtualization boundary issue involving CPU feature-state restore, not a simple DoS or validation-only bug. The demonstrated impact is a policy bypass, and the plausible worst case includes incorrect SVE state save/restore behavior that could affect isolation.

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

Patch: KVM: arm64: Correctly cap ZCR_EL2 provided by a guest hypervisor
Commit: 742a9b5ccd8c46caf983ca90c94f855466968e34
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=742a9b5ccd8c46caf983ca90c94f855466968e34

Commit description:

ZCR_EL2 can be updated by a VHE guest hypervisor either using ZCR_EL2
(which traps) or ZCR_EL1 (which does not trap). KVM handles both in
different way:

- on ZCR_EL2 trap, ZCR_EL2.LEN is immediately capped at the VM's own
  VL limit. This has the potential to break existing SW that relies
  on the full LEN field to be stateful.

- on ZCR_EL1 access, we do absolutely nothing.

On restoring the SVE context for an L2 guest, we directly restore the
guest hypervisor's view of ZCR_EL2 into the physical ZCR_EL2. If the
guest's view of the register was updated using the ZCR_EL2 accessor,
the value has already been sanitised (with the caveat mentioned above).

But if the guest used ZCR_EL1, the raw value is written into the HW,
and the L2 guest can now access VLs that it shouldn't.

Fix all the above by moving the VL capping to the restore points,
ensuring that:

- the HW is always programmed with a capped value, irrespective of
  the accessor being used,

- the ZCR_EL2.LEN field is always completely stateful, irrespective
  of the accessor being used.

Additionally, move ZCR_EL2 to be a sanitised register, ensuring that
only the LEN field is actually stateful. This requires some creative
construction of the RES0 mask, as the sysreg generation script does
not yet generate RAZ/WI fields.

Fixes: b3d29a8 ("KVM: arm64: nv: Handle ZCR_EL2 traps")
Signed-off-by: Mark Brown <broonie@kernel.org>
Cc: stable@vger.kernel.org
Link: https://patch.msgid.link/20260529-kvm-arm64-fix-zcr-len-nv-v2-1-86cad51992bd@kernel.org
[maz: rewrote commit message, tidy up access_zcr_el2()]
Signed-off-by: Marc Zyngier <maz@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  arch/arm64/include/asm/kvm_host.h
  arch/arm64/kvm/hyp/include/hyp/switch.h
  arch/arm64/kvm/nested.c
  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=742a9b5ccd8c46caf983ca90c94f855466968e34

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

The original Linux kernel CVE announcement for CVE-2026-63941 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-63941

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:C/C:L/I:L/A:L
  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: 4
  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: 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).