[CVE-2026-46317][MODERATE 7.0] KVM: arm64: Reassign nested_mmus array behind mmu_lock

[CVE-2026-46317][MODERATE 7.0] KVM: arm64: Reassign nested_mmus array behind mmu_lock

[AL-KERNEL]

CVE: CVE-2026-46317
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: KVM: arm64: Reassign nested_mmus array behind mmu_lock
Commit: 918450ad6010df6ecd2efde12a1409e011da22d6
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=918450ad6010df6ecd2efde12a1409e011da22d6
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-46317
Analysis date: Tue, 09 Jun 2026 09:02:18 -0400
ActionableScore: 5
ActionableScore lower bound: 4
Actionable bucket: Strong Important candidate / Actionable Moderate at minimum
Manual review required: YES

Summary:
A race in KVM arm64 nested virtualization can free and replace the `nested_mmus` array while MMU notifier code still walks the old array, creating a race dependent UAF risk in nested Stage 2 MMU state that can cause host DoS and warrants manual review for possible stronger impact.

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

The original Linux kernel CVE announcement for CVE-2026-46317 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-46317

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-2026-46317	MODERATE	CHECK	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-416;CWE-362;CWE-667;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:H';BEST CVSS score: '7';DESCR 'A race in KVM arm64 nested virtualization can allow the nested_mmus array to be reallocated and the old buffer to be freed while MMU notifier code is still walking it under mmu_lock. This is a use after free candidate because the walker may dereference entries from the freed array during nested stage 2 unmap processing. For the CVSS the PR:L is used because a local user or virtualization process with access to KVM can potentially trigger the path without full host administrator privileges. The issue is not directly network reachable and requires local KVM access plus a timing window, so AC:H is used. Impact is at least local denial of service via host kernel crash and in the paranoid case may include confidentiality or integrity impact due to memory corruption in KVM MMU state.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate / Actionable Moderate at minimum (with actual score 5)	YES	VIRT INIT NOMEMCHK UAF HARDWARE LINUS KPANIC 	YES	NO	checked

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

ActionableScore=5
ActionableScoreLower=4

## 1. ActionableScore

* Conservative score: 4
* Paranoid score: 5
* Final recommended bucket: **Strong Important candidate / Actionable Moderate at minimum**

## 2. Signal breakdown

Conservative signals:

* Local unprivileged trigger: +1. A local user or VM management process with access to KVM can potentially reach nested vCPU initialization paths without full host root.
* Memory corruption, weak or race dependent primitive: +1. The patch describes a stale reference to a freed `nested_mmus` array, but does not show attacker controlled reclaim, overwrite, callback dispatch, or type confusion.
* Real lifetime corruption: +1. This is a real UAF style lifetime bug where `kvrealloc()` can free the old array while another path still walks it.
* Reliable kernel crash / strong DoS: +1. A walker dereferencing a freed KVM MMU array can realistically crash the host kernel.
* Privileged kernel MMU state corruption path: +1. The stale object is KVM nested Stage 2 MMU state, which is high value kernel virtualization metadata.
* Hard or unreliable race / special timing required: -1. The issue depends on concurrent reallocation and MMU notifier traversal.
* Conservative total: 4.

Paranoid adjustment:

* Paranoid score raises the result to 5 because KVM MMU lifetime corruption can be security relevant even when the patch does not demonstrate a concrete LPE primitive.
* No extra full LPE bonus is awarded because the patch does not show attacker controlled reclaim, stale object replacement, arbitrary write, or callback control.
* Paranoid total: 5.

## 3. Reachability analysis

The bug is reachable from local KVM usage on arm64 systems with nested virtualization support. A likely attacker model is a local user, service account, or virtualization stack component that can access `/dev/kvm` and create or initialize nested vCPU state.

This is not network reachable by itself. Namespaces and containers may matter if KVM access is delegated into a container or to a reduced privilege service account. In that case PR should not be treated as full host root, because KVM access can exist without full administrative privileges.

The affected path is not a general default kernel path. It depends on KVM arm64 nested virtualization and concurrent MMU notifier activity, so exploitation requires a specific virtualization configuration and a timing window.

## 4. Severity interpretation

This is more than an ordinary Moderate because it is a real KVM lifetime bug in MMU related state. The realistic demonstrated impact is host kernel crash or DoS. The theoretical memory corruption concern is higher because a freed `nested_mmus` array may still be traversed by KVM MMU notifier code, but the patch does not show a strong exploit primitive such as controlled reclaim, arbitrary write, or function pointer control.

Overall this should be treated as Actionable Moderate at minimum and a possible Important candidate for manual review, not auto closed.

## 5. One-sentence report phrase

A race in KVM arm64 nested virtualization can free and replace the `nested_mmus` array while MMU notifier code still walks the old array, creating a race dependent UAF risk in nested Stage 2 MMU state that can cause host DoS and warrants manual review for possible stronger impact.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED.

Reason: this is a KVM MMU lifetime bug with UAF characteristics in virtualization memory management state. The conservative evidence supports DoS, while the subsystem and object type justify manual review for possible confidentiality, integrity, or LPE implications even though a strong exploitation primitive is not demonstrated by the patch.

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

Patch: KVM: arm64: Reassign nested_mmus array behind mmu_lock
Commit: 918450ad6010df6ecd2efde12a1409e011da22d6
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=918450ad6010df6ecd2efde12a1409e011da22d6

Commit description:

kvm->arch.nested_mmus[] is walked under kvm->mmu_lock, including from the
MMU notifier path (kvm_unmap_gfn_range() -> kvm_nested_s2_unmap()), which
can run at any time. kvm_vcpu_init_nested() reallocates the array and frees
the old buffer while holding only kvm->arch.config_lock, so such a walker
can reference the freed array.

Allocate the new array outside of mmu_lock, as the allocation can sleep.
Under the lock, copy the existing entries, fix up the back pointers and
reassign the array. Free the old buffer after dropping the lock, as
kvfree() can sleep as well.

Fixes: 4f128f8 ("KVM: arm64: nv: Support multiple nested Stage-2 mmu structures")
Signed-off-by: Hyunwoo Kim <imv4bel@gmail.com>
Reviewed-by: Oliver Upton <oupton@kernel.org>
Link: https://patch.msgid.link/aiKIVVeIr1aAB1yp@v4bel
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/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=918450ad6010df6ecd2efde12a1409e011da22d6

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

The original Linux kernel CVE announcement for CVE-2026-46317 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-46317

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).