[CVE-2026-89932][IMPORTANT] KVM: nVMX: Always flush vpid02 on first use

[CVE-2026-89932][IMPORTANT] KVM: nVMX: Always flush vpid02 on first use

[AL-KERNEL]

CVE: CVE-2026-89932
Priority: IMPORTANT
AL-KERNEL base severity: IMPORTANT
KPANIC flag: YES
Patch: KVM: nVMX: Always flush vpid02 on first use
Commit: 26de0d2d9a8d14c03e5ebb25fd68b5bfcd5ac366
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=26de0d2d9a8d14c03e5ebb25fd68b5bfcd5ac366
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89932
Analysis date: Wed, 16 Sep 2026 13:14:40 -0400
ActionableScore: 6
ActionableScore lower bound: 5
Actionable bucket: Actionable Moderate / Strong Important candidate requiring manual review
Manual review required: YES

Summary:
No one-sentence report phrase was found.

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

The original Linux kernel CVE announcement for CVE-2026-89932 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89932

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.

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

CVE-2026-89932	IMPORTANT	https://www.kernelcve.org/list/?q=CVE-2026-89932 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-665;*CWE-664;*CWE-200;CWE-284;BEST CVSS score: '7.8';DESCR 'KVM nVMX can reuse vpid02 after a VMXOFF VMXON sequence without forcing a first use TLB flush because nested last_vpid was not reset when a new vpid02 was allocated. Stale TLB entries from a previous VPID lifetime may then be used by a later L2 run, and the old entries may belong to a different vCPU or even a different VM. For the CVSS the PR:L is used because reliable triggering normally requires control of a guest or KVM user context with nested virtualization, but not full host administrator privileges. The issue is not directly network reachable and is triggered through nested virtualization control flow rather than packet traffic. Impact may include confidentiality and integrity loss across VM isolation boundaries and possible availability impact if stale translations corrupt execution state.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate / Strong Important candidate requiring manual review (with actual score 6) 	MAYBE	VIRT INIT UAF HARDWARE KPANIC  KPANIC INCREASED_TO_HIGH_BASED_ON_GUESSCVSS	-	-	checked

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

ActionableScore=6
ActionableScoreLower=5
PrimitiveClass=CLASS_B
RealUAF=NO
HostAdminRequired=NO
NamespaceScopedPrivilege=NO
ImportantCandidate=YES
ManualReview=REQUIRED
AutoDowngradeAllowed=NO

1. ActionableScore

* Raw conservative score: 5
* Conservative score: 5
* Raw paranoid score: 6
* Paranoid score: 6
* Final recommended bucket: **Actionable Moderate / Strong Important candidate requiring manual review**::

No floor or cap changed the score.

2. Signal breakdown

Positive conservative signals:

* +2: CLASS_B active translation lifetime and consistency violation. A newly allocated vpid02 can be used without forcing first-use TLB retirement because nested.last_vpid is not reset after VMXOFF and later VMXON.
* +1: Active translation synchronization failure. The fix explicitly restores the first-use TLB flush, meaning stale translations can remain operational in CPU TLB state.
* +1: Virtualization isolation concern. The commit states stale TLB entries may belong to a different vCPU or a completely different VM.
* +1: Below-host-admin reachability. The relevant trigger can be driven by a nested virtualization actor such as an L1 guest or local VMM context when nested VMX is exposed, rather than requiring full host administrator control.

Additional paranoid signal:

* +1: Plausible confidentiality or integrity concern from stale translations crossing VM or vCPU lifetime boundaries. This is not scored as arbitrary read or write, but it is stronger than a pure crash-only stale state bug.

Important non-awarded signals:

* RealUAF not awarded. This is stale CPU translation state, not a heap object use-after-free.
* CLASS_A not awarded. No controlled reclaim, arbitrary host read or write, callback control, credential manipulation, or demonstrated guest-to-host exploit is established.
* Strong guest-to-host exploitation leverage +2 not awarded. The patch supports stale translation use, but not a concrete attacker-directed host memory access primitive.
* Reliable kernel crash +1 not awarded as a separate signal. The supplied evidence focuses on stale TLB entries and possible isolation impact, not an observed panic.
* Remote reachability not awarded. This is not packet-triggered or network-reachable.
* Race penalty not applied. The bug requires a VMXOFF -> VMXON -> L2 sequence and VPID reuse, but the patch does not establish a narrow externally uncontrollable timing race.
* No repeated release/refcount signal. There is no put/free/refcount mutation on a dangling kernel object.

3. Reachability analysis

The evaluated attacker is primarily a nested virtualization actor: an L1 guest capable of executing VMXOFF, VMXON, and running L2, or a local VMM or KVM user context that can arrange nested VMX execution. This is below host administrator privilege when nested virtualization is intentionally exposed to guests or userspace virtualization workloads.

This is not a guest-only primitive in the strongest sense of proven host memory access. The guest can drive the nested VMX control flow, but VPID allocation, reuse, CPU scheduling, and physical TLB residency are controlled by KVM and hardware behavior. The trigger therefore depends on KVM nested VMX being enabled and on vpid02 reuse while stale entries still exist on the physical CPU.

HostAdminRequired is NO because the supplied context supports a below-host-admin virtualization attacker model. NamespaceScopedPrivilege is NO because Linux namespaces are not the relevant privilege boundary. Prior host setup may be required in practice to expose nested VMX to the VM, but that is a deployment prerequisite, not proof that exploitation itself requires host administrator privileges.

No direct network vector is established. No rare physical hardware penalty is applied beyond ordinary Intel VMX and VPID support used by KVM nested virtualization.

4. Primitive and severity interpretation

The exact primitive is stale active TLB translation reuse across VPID lifetime boundaries. The stale state is not fixed passive metadata. It is live CPU translation state that may affect later memory accesses under a newly allocated vpid02.

This is an active translation synchronization and lifetime bug, not a heap UAF. It is also not a simple WARN-only issue, because the purpose of the patch is to force retirement of stale translations before the reused VPID becomes active again.

PrimitiveClass is CLASS_B. The issue has a concrete active-state lifetime and synchronization primitive with unresolved security impact, but the supplied evidence does not establish CLASS_A strength such as arbitrary host memory read/write, controlled corruption, controlled replacement, or demonstrated guest-to-host escape.

The demonstrated or directly supported consequence is potential use of stale TLB entries from a previous VPID lifetime. The speculative consequence is cross-vCPU or cross-VM confidentiality and integrity impact if stale translations point into memory belonging to another security domain. That consequence is plausible enough for manual review, but not proven enough to score as a controlled guest-to-host exploitation primitive.

ImportantCandidate is YES because stale translations from a different VM are a serious virtualization boundary concern and should not be automatically treated as ordinary Moderate. AutoDowngradeAllowed is NO because the unresolved primitive is an active translation-state isolation issue, not a constrained crash-only cleanup bug.

Likely operational severity is Actionable Moderate at minimum, with Strong Important consideration depending on whether further analysis shows a practical way for an L1 guest or VMM actor to make stale entries usable for cross-VM memory access.

5. One-sentence report phrase

KVM nVMX may reuse vpid02 after VMXOFF and VMXON without a first-use TLB flush, allowing stale TLB entries from a previous vCPU or VM lifetime to be used and creating an unresolved VM isolation risk requiring manual review.

6. Manual review recommendation

MANUAL CHECK REQUIRED

The patch directly concerns stale TLB entries that may belong to another vCPU or another VM, which is a virtualization security-boundary issue even though arbitrary host read/write or a complete exploit chain is not demonstrated.

7. Call-site confidence

Call-site confidence: high

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

Patch: KVM: nVMX: Always flush vpid02 on first use
Commit: 26de0d2d9a8d14c03e5ebb25fd68b5bfcd5ac366
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=26de0d2d9a8d14c03e5ebb25fd68b5bfcd5ac366

Commit description:

Make sure vpid02 is always flushed on first use by setting last_vpid=0
when allocating vpid02.  nested_vmx_transition_tlb_flush() will always
detect a VPID change on first VM-Enter after VMXON, because VPID=0 in
vmcs12 is not allowed if L1 enables VPID.

This avoids using stale TLB entries from a previous lifetime of the
VPID, that might have been associated with a different vCPU (or a
completely different VM).

Note that last_vpid is already being initialized as 0 when the vCPU is
created, but it is not reset when vpid02 is freed on VMXOFF. Hence, the
problem can only occur if L1 does VMXOFF -> VMXON, runs an L2, and KVM
happens to reuse a VPID that has TLB entries on the physical CPU.

Cc: stable@vger.kernel.org
Signed-off-by: Yosry Ahmed <yosry@kernel.org>
Reviewed-by: Kai Huang <kai.huang@intel.com>
Reviewed-by: Jim Mattson <jmattson@google.com>
Link: https://patch.msgid.link/20260616214652.2157032-2-yosry@kernel.org
Signed-off-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  arch/x86/kvm/vmx/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=26de0d2d9a8d14c03e5ebb25fd68b5bfcd5ac366

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

The original Linux kernel CVE announcement for CVE-2026-89932 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89932

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: Not available
  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: 6

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).