[CVE-2026-64286][MODERATE 7.0] KVM: arm64: Clear __hyp_running_vcpu when flushing the pKVM hyp vCPU

[CVE-2026-64286][MODERATE 7.0] KVM: arm64: Clear __hyp_running_vcpu when flushing the pKVM hyp vCPU

[AL-KERNEL]

CVE: CVE-2026-64286
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: KVM: arm64: Clear __hyp_running_vcpu when flushing the pKVM hyp vCPU
Commit: 477145860dba4c30f0b4e36f02f4c5291c1c888b
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=477145860dba4c30f0b4e36f02f4c5291c1c888b
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64286
Analysis date: Sat, 25 Jul 2026 10:18:04 -0400
ActionableScore: 7
ActionableScore lower bound: 4
Actionable bucket: Strong Important candidate / Actionable Moderate at minimum
Manual review required: YES

Summary:
A host-supplied `__hyp_running_vcpu` pointer could be copied into the pKVM hyp guest context and dereferenced at EL2, allowing at least a local denial of service and requiring manual review for possible protected hypervisor boundary impact.

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ABOUT THIS REPORT
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The original Linux kernel CVE announcement for CVE-2026-64286 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64286

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-2026-64286	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-822;*CWE-20;CWE-754;**CWE-476;Other CVSS 'AV:L/AC:H/PR:H/UI:N/S:C/C:N/I:N/A:H';BEST CVSS score: '7.8';DESCR 'KVM arm64 pKVM copies the host vCPU context into the private hyp vCPU on each run, but the copied guest context could retain a non NULL __hyp_running_vcpu pointer. ctxt_to_vcpu expects this field to be NULL for guest contexts, so a value supplied by the host may be dereferenced at EL2 and can crash or corrupt the protected hypervisor context. For the CVSS the PR:L is used in the paranoid score because some deployments expose KVM control to non root service accounts or virtualization management processes, although the base case is closer to PR:H because direct control of the host context is highly privileged. The issue is local and not network reachable by itself. Impact is at least denial of service against pKVM or the host, and in the worst still defensible interpretation it may affect confidentiality or integrity across the host to EL2 boundary, so manual review is recommended.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate / Actionable Moderate at minimum (with actual score 6)  SKIP CVE-2026-64286 UNKNOWN SKIP No affected files built, so skip this CVE NO  - - unknown 	MAYBE	DANGER VIRT SIMPLEFIX HARDWARE LINUS KPANIC  KPANIC INCREASED_TO_HIGH_BASED_ON_GUESSCVSS KPANIC LOWERED_FROM_HIGH_BASED_ON_NULLPTR	-	-	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**::

The conservative score treats this as a local pKVM EL2 invalid pointer dereference with reliable DoS and hypervisor-boundary relevance. The paranoid score is higher because the bug crosses the host to protected hypervisor boundary and involves a host-supplied pointer being interpreted inside EL2 context, which is worth manual review even without a demonstrated arbitrary write or reclaim primitive.

## 2. Signal breakdown

Conservative signals:

* Local low-privileged KVM trigger: +1
  KVM workloads may be runnable by non-root users or service accounts with access to `/dev/kvm`, depending on deployment.

* Memory corruption, weak or indirect corruption candidate: +1
  A non-NULL `__hyp_running_vcpu` pointer from the host context can be copied into a guest hyp context and later dereferenced at EL2. This is a bogus pointer / invalid object interpretation pattern, but not a demonstrated arbitrary write.

* Reliable kernel or hypervisor crash / strong DoS: +1
  Dereferencing an invalid host-provided pointer at EL2 can plausibly fault or crash the protected KVM path.

* Cross-boundary isolation / policy bypass relevance: +1
  The bug crosses the host to EL2 pKVM boundary, which is more security-sensitive than ordinary in-kernel state corruption.

* Privileged kernel / hypervisor context corruption path: +1
  The affected state is in the pKVM hyp vCPU context, a high-value protected execution environment.

* Rare or configuration-dependent mode: -1
  The issue is specific to arm64 pKVM / nVHE protected virtualization, not a universal KVM path on all systems.

Paranoid-only additions:

* Weak LPE / boundary compromise concern: +1
  The primitive is not a proven LPE, but a host-controlled pointer dereference at EL2 is a plausible corruption sink across a privileged isolation boundary.

* Confidentiality impact plausible: +1
  In the worst still-defensible interpretation, corrupting or misresolving protected hyp context could affect isolation-sensitive state.

* Integrity impact plausible: +1
  In the worst still-defensible interpretation, EL2 context misuse could affect protected hypervisor state integrity.

## 3. Reachability analysis

The issue is local, not network reachable by itself. A realistic trigger requires running or controlling KVM/pKVM vCPU execution on an affected arm64 system. In many deployments this is restricted to root or a virtualization service, but some systems delegate KVM access to non-root users or service accounts, so PR:L is defensible for paranoid triage.

Namespaces and containers matter only if they expose KVM access or a virtualization management path to a reduced-privilege actor. This is not a packet-processing or remote protocol bug. The path is also configuration-dependent because it requires arm64 pKVM / protected KVM behavior.

Call-site confidence: medium. The provided patch shows the vulnerable copy in `flush_hyp_vcpu()` and the commit explains the downstream `ctxt_to_vcpu()` dereference behavior, but the full caller chain is not included.

## 4. Severity interpretation

This is not an ordinary Moderate cleanup because it involves a host-provided pointer being dereferenced at EL2 in protected KVM context. The conservative interpretation is local DoS with a weak corruption primitive. The paranoid interpretation is a Strong Important candidate because the affected boundary is pKVM host to protected hypervisor and the pointer misuse may have confidentiality or integrity implications even though no arbitrary write, controlled reclaim, or concrete privilege escalation primitive is shown.

Realistic exploitation evidence supports crash / DoS more strongly than full compromise. Manual review is still justified due to the isolation boundary and hypervisor context.

## 5. One-sentence report phrase

A host-supplied `__hyp_running_vcpu` pointer could be copied into the pKVM hyp guest context and dereferenced at EL2, allowing at least a local denial of service and requiring manual review for possible protected hypervisor boundary impact.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: this is a KVM arm64 pKVM host-to-EL2 pointer dereference across a protected hypervisor boundary, with at least reliable DoS and a plausible but unproven confidentiality or integrity impact.

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

Patch: KVM: arm64: Clear __hyp_running_vcpu when flushing the pKVM hyp vCPU
Commit: 477145860dba4c30f0b4e36f02f4c5291c1c888b
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=477145860dba4c30f0b4e36f02f4c5291c1c888b

Commit description:

flush_hyp_vcpu() copies the host vCPU context into the hyp's private
vCPU on every run. ctxt_to_vcpu() expects a guest context to have a
NULL __hyp_running_vcpu, which is only ever set on the host context, so
that it resolves the vCPU via container_of(). While this is generally
the case, flush_hyp_vcpu() copies the context verbatim and does not
enforce this, so a value provided by the host is dereferenced at EL2
(host -> EL2).

Fix by clearing __hyp_running_vcpu after the copy.

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/20260606175614.83273-2-imv4bel@gmail.com
Signed-off-by: Marc Zyngier <maz@kernel.org>
[ tabba: adjust flush_hyp_vcpu() context ]
Signed-off-by: Fuad Tabba <fuad.tabba@linux.dev>
Signed-off-by: Sasha Levin <sashal@kernel.org>

Changed files:
  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=477145860dba4c30f0b4e36f02f4c5291c1c888b

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

The original Linux kernel CVE announcement for CVE-2026-64286 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64286

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:N/I:N/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: 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).