[CVE-2026-98468][LOW] KVM: arm64: Fix spurious warning for benign stage 2 teardown race

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

CVE: CVE-2026-98468
Priority: LOW
AL-KERNEL base severity: LOW
KPANIC flag: NO
Patch: KVM: arm64: Fix spurious warning for benign stage 2 teardown race
Commit: f44a92633136b409ebb9b7036fceb7e11de60a20
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=f44a92633136b409ebb9b7036fceb7e11de60a20
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98468
Analysis date: Mon, 19 Oct 2026 04:09:53 -0400
ActionableScore: 0
ActionableScore lower bound: unknown
Actionable bucket: Ordinary Moderate / Low-like
Manual review required: NO

Summary:
A benign KVM arm64 stage 2 teardown race can produce a spurious warning after the page tables are safely removed, with no demonstrated UAF, memory corruption, authorization bypass, or guest-to-host security primitive.

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

The original Linux kernel CVE announcement for CVE-2026-98468 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98468

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: LOW
Manual review required: NO

A detailed explanation of the methodology and priority rules is included
at the end of this message.

======================================================================
AL-KERNEL CLASSIFICATION RESULT
======================================================================

CVE-2026-98468	LOW	https://www.kernelcve.org/list/?q=CVE-2026-98468 CHECK WITH IMPACT FROM ORIG NN IMPORTANT | AEGIS IMPACT LOW AEGIS_AS=0 LEGACY=LOW RESULT=SAME (final reconciliation agrees; DECISION: both LOW)	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H';CWE-362;CWE-667;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:L';BEST CVSS score: '4.7';DESCR 'A race can occur when KVM arm64 stage 2 range processing temporarily releases kvm mmu_lock while a concurrent teardown removes the stage 2 page tables and sets mmu pgt to NULL. After the worker reacquires the lock it observes the missing page table and returns EINVAL, which produces a spurious WARN even though the teardown was completed safely. The code does not retain or dereference the old page table pointer after the lock is reacquired, so the analyzed path does not provide a UAF, OOB access, information leak, or write primitive. For the CVSS the PR:L reflects that a local user or process generally needs KVM access and control of a nested guest workload to reach the affected path. AC:H reflects the required timing between MMU notifier processing, stage 2 teardown, lock rescheduling, and host memory pressure. The issue is not network reachable. The normal impact is limited to a warning and log noise, but a host configured with panic_on_warn may convert the warning into a kernel panic and denial of service. Automated impact decision path. The original SimpleNet classifier selected IMPORTANT. The primary LLM estimated paranoid CVSS 4.7 and the production ActionableScore returned 0 with recommendation Ordinary Moderate / Low-like. The production rules then evaluated CVSS thresholds and ActionableScore thresholds. Recorded rule transitions were CVSS and initial triage stage IMPORTANT to MODERATE_KPANIC because the CVSS score, vector or bug class threshold rule was applied. ActionableScore thresholds stage MODERATE_KPANIC to LOW because the ActionableScore threshold rule was applied. final reconciliation stage LOW to LOW because KP specialist did not establish a qualifying panic. The legacy KP specialist has no review artifact for this run. The legacy FD or DH downgrade specialist has no review artifact for this run. Exact triggered rule markers were LOWERED_FROM_HIGH_BASED_ON_GUESSCVSS and DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN4 and DECREASED_TO_LOW_FROM_MODERATE_BASED_ON_ACTIONABLESCORELESSTHAN3 and KPANIC_UNSUPPORTED_REMOVED. The independent weaker Aegis shadow evaluated CVSS 0.0 and ActionableScore 0 with primitive class CLASS_C and manual review NO. Its independently reconciled impact was LOW. The shadow agreed with the final production impact and aggregation did not change that impact. The final reported impact is LOW.';NO MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate / Low-like (with actual score 0)	MAYBE	WRITE OOB VIRT RACE UAF HARDWARE LINUS LOWERED_FROM_HIGH_BASED_ON_GUESSCVSS DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN4 DECREASED_TO_LOW_FROM_MODERATE_BASED_ON_ACTIONABLESCORELESSTHAN3 KPANIC_UNSUPPORTED_REMOVED	-	-	checked

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

ActionableScore=0
PrimitiveClass=NONE
RealUAF=NO
HostAdminRequired=UNKNOWN
NamespaceScopedPrivilege=UNKNOWN
ImportantCandidate=NO
ManualReview=NO
AutoDowngradeAllowed=YES
RequiresHostUID0=UNKNOWN
AuthorizationDecisionBypass=NO
ProtectedResourceImpact=NONE
B4PendingReview=NO
B4PendingConfirmed=NO

1. ActionableScore

* Raw conservative score: 0
* Conservative score: 0
* Raw paranoid score: 0
* Paranoid score: 0
* No floor or cap was applied.
* Final recommended bucket: **Ordinary Moderate / Low-like**::

2. Signal breakdown

* +0 WARN_ON behavior. The demonstrated result is a spurious warning, not an established kernel crash.
* +0 race. The race is benign because teardown and range processing remain serialized by `kvm->mmu_lock`.
* +0 memory corruption. No UAF, stale dereference, OOB access, write primitive, or secondary-index inconsistency is established.
* +0 guest-to-host impact. Guest activity helps create the workload, but no guest-controlled host corruption or confirmed host DoS is shown.
* +0 authorization impact. No security check, protected resource, or DENY-to-ALLOW transition is involved.
* No race penalty was applied because the score has no positive security signal and timing difficulty is not needed to dismiss the issue.
* A hypothetical panic through `panic_on_warn` was not scored because the supplied evidence only demonstrates WARN continuation.
* B5-R does not apply. There is no retry-accumulated heap OOB write.
* B3-PENDING does not apply. There is no shared-backing write.
* V32, V33, and V34 do not apply. No attacker-influenced SKB or active structured-state corruption exists.
* B4-PENDING does not apply. There is no authorization-state preservation defect.

3. Reachability analysis

The observed sequence requires arm64 KVM nested virtualization, concurrent stage-2 teardown, an MMU-notifier invalidation caused by host migration or compaction, and rescheduling while `kvm->mmu_lock` is temporarily released.

A malicious guest can influence guest workload and shutdown behavior, but the supplied evidence does not show that guest code alone controls host compaction, the host MMU-notifier sequence, or the complete trigger. A local VMM with `/dev/kvm` access controls VM creation and teardown, while host memory management supplies the competing operation. This is therefore a compound VMM and host-kernel scheduling condition rather than a confirmed guest-only trigger.

The evidence does not establish whether the complete reproducer requires host administration or merely delegated KVM access, so `HostAdminRequired=UNKNOWN`. No user-namespace or network-namespace capability path is involved. There is no remote network trigger.

4. Primitive and severity interpretation

`stage2_apply_range()` reads `mmu->pgt` while holding `kvm->mmu_lock`. When `cond_resched_rwlock_write()` temporarily releases that lock, teardown may acquire it, free the stage-2 page-table hierarchy, and set `mmu->pgt` to `NULL`. After reacquiring the lock, the range walker reloads `mmu->pgt`, observes `NULL`, and does not dereference the former page-table pointer.

The old translation state is retired through the expected teardown protocol. No active replacement bypasses invalidation, no stale reverse mapping is demonstrated, and no old translation remains operational after teardown. The only defect is interpreting this legitimate post-reschedule state as `-EINVAL`, which causes `__unmap_stage2_range()` to emit a warning.

The patch changes that result to successful early termination only when the lock was previously dropped. It retains `-EINVAL` when `mmu->pgt` is unexpectedly absent before any rescheduling. This is a control-flow correction for a benign race, not a lifetime or corruption fix.

Consequently, `PrimitiveClass=NONE`, `RealUAF=NO`, and there is no supported confidentiality, integrity, privilege-escalation, or guest-to-host isolation impact. Automatic downgrade is safe.

5. One-sentence report phrase

A benign KVM arm64 stage 2 teardown race can produce a spurious warning after the page tables are safely removed, with no demonstrated UAF, memory corruption, authorization bypass, or guest-to-host security primitive.

6. Manual review recommendation

NO MANUAL CHECK NEEDED

The locking and pointer reload mechanics directly show that the removed page-table pointer is not reused after teardown, and the observed consequence is limited to an erroneous warning.

7. Call-site confidence

Call-site confidence: high

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

Patch: KVM: arm64: Fix spurious warning for benign stage 2 teardown race
Commit: f44a92633136b409ebb9b7036fceb7e11de60a20
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=f44a92633136b409ebb9b7036fceb7e11de60a20

Commit description:

kvmtool was used to establish an L1 guest with 8 CPUs and 8 GiB of RAM, an
L2 guest with 4 CPUs and 4 GiB of RAM and an L3 guest with 2 CPUs and 2 GiB
of RAM, all of which was then exited.

Under memory pressure in the L0 host warnings were observed due to
migration triggered by compaction:

WARNING: arch/arm64/kvm/mmu.c:336 at __unmap_stage2_range+0x64/0x80,
CPU#5: kcompactd0/66

Which was, in turn, triggered by an MMU notifier for the host invalidation:

mmu_notifier_invalidate_range_start()
  -> ... -> kvm_mmu_notifier_invalidate_range_start()
    -> kvm_mmu_unmap_gfn_range()
      -> kvm_unmap_gfn_range()
        -> kvm_nested_s2_unmap()
          -> kvm_stage2_unmap_range()
            -> __unmap_stage2_range()
               -> stage2_apply_range()
       	       <- -EINVAL, triggering a WARN_ON()

Racing with L0's teardown of stage 2 page tables:

exit_mm()
  -> mmput()
    -> __mmput()
      -> exit_mmap()
        -> mmu_notifier_release()
	  -> ... -> kvm_mmu_notifier_release()
            -> kvm_flush_shadow_all()
              -> kvm_arch_flush_shadow_all()
	        -> kvm_free_stage2_pgd()
		  -> [ acquire kvm->mmu_lock for write ]
		  -> mmu->pgt = NULL [ among other tasks ]
		  -> [ release kvm->mmu_lock for write ]

It turns out there is a benign race resulting in a spurious warning:

	Thread A - notify: migration   | Thread B - notify: release
	-------------------------------|---------------------------------
	< kvm->mmu_lock held >         |
	stage2_apply_range()           |
	  get mmu->pgt, check !NULL    |
	  ...                          | kvm_arch_flush_shadow_all()
	  cond_resched_rwlock_write(); |   < contend, sleep kvm->mmu_lock >
	< drop kvm->mmu_lock >         |   < acquire kvm->mmu_lock>
                                       |   ...
				       |   kvm_free_stage2_pgd()
                                       |     mmu->pgt = NULL
				       |   < invalidate MMU >
				       |   ...
				       |   < release kvm->mmu_lock >
	[ scheduled ]		       |
	stage2_apply_range()           |
	  < loop to next >             |
	  get, mmu->pgt, check !NULL   |
	  is NULL, return -EINVAL      |
        __unmap_stage2_range()         |
	  WARN_ON(-EINVAL) <--- entirely spurious - the race was handled
                                 correctly.

Fix the spurious warning by updating stage2_apply_range() to no longer
treat concurrent PGT teardown on lock release as an error - whether the
walker is tearing down page tables or doing something else this is a
legitimate reason to abort the operation without error.

This keeps the warning in place for all other circumstances.

In practice only __unmap_stage2_range() actually does anything with the
error so this only impacts that.

Fixes: ec14c27 ("KVM: arm64: nv: Unmap/flush shadow stage 2 page tables")
Cc: stable@vger.kernel.org
Reviewed-by: Yuan Yao <yaoyuan@linux.alibaba.com>
Reviewed-by: Marc Zyngier <maz@kernel.org>
Signed-off-by: Lorenzo Stoakes (ARM) <ljs@kernel.org>
Link: https://patch.msgid.link/20260901-kvm-arm-nested-virt-fix-v3-1-b154676f7e4c@kernel.org
Signed-off-by: Oliver Upton <oupton@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  arch/arm64/kvm/mmu.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=f44a92633136b409ebb9b7036fceb7e11de60a20

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

The original Linux kernel CVE announcement for CVE-2026-98468 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98468

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:N/I:N/A:L
  The Best / paranoid CVSS vector: AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS score: 4.7

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: unknown
  Paranoid ActionableScore: 0

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: LOW
KPANIC detected for this report: NO
Published priority for this report (same as in Subject): LOW

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).