[CVE-2026-99176][MODERATE 7.0] KVM: nVMX: Reset MMU back to the root MMU if loading L2 state fails for !VM-Entry

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

CVE: CVE-2026-99176
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: KVM: nVMX: Reset MMU back to the root MMU if loading L2 state fails for !VM-Entry
Commit: d00d5f534a8238674755e59826a6c7d3c8efb870
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=d00d5f534a8238674755e59826a6c7d3c8efb870
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-99176
Analysis date: Fri, 30 Oct 2026 11:54:56 -0400
ActionableScore: 5
ActionableScore lower bound: 4
Actionable bucket: Actionable Moderate / Strong Important candidate requiring manual review
Manual review required: YES

Summary:
A failed nested-VMX state load can leave a non-root-controlled vCPU using the nested MMU after returning to L1, leading to a host-kernel NULL dereference when the nested state is freed.

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

The original Linux kernel CVE announcement for CVE-2026-99176 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-99176

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
======================================================================

CVE-2026-99176	MODERATE	https://www.kernelcve.org/list/?q=CVE-2026-99176 CHECK WITH IMPACT FROM ORIG NN MODERATE | AEGIS IMPACT MODERATE_KPANIC AEGIS_AS=2 LEGACY=MODERATE_KPANIC RESULT=SAME (final reconciliation agrees; DECISION: both MODERATE_KPANIC)	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H';**CWE-476;CWE-703;CWE-665;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '5.5';DESCR 'A stale nVMX MMU context can remain active when loading L2 state fails outside VM-Entry during RSM emulation or KVM_SET_NESTED_STATE. KVM can then run L1 with the nested EPT MMU. If nested state is disabled and vmcs12 is freed first, nested_ept_get_eptp reads through the missing vmcs12 state and causes a host kernel NULL pointer dereference. For the CVSS the PR:L value is selected because reliable triggering requires control of a KVM vCPU through guest execution or a userspace VMM with access to KVM ioctls. The path is local rather than directly network reachable. The demonstrated primitive is a NULL pointer dereference and host denial of service. The patch does not show an OOB access, UAF dereference, arbitrary write, or information leak, so confidentiality and integrity remain N in both vectors. Automated impact decision path. The original SimpleNet classifier selected MODERATE. The primary LLM estimated paranoid CVSS 5.5 and the production ActionableScore returned 5 with recommendation Actionable Moderate / Strong Important candidate requiring manual review. The production rules then evaluated CVSS thresholds and ActionableScore thresholds. Recorded rule transitions were CVSS and initial triage stage MODERATE to MODERATE_KPANIC because the CVSS score, vector or bug class threshold rule was applied. The legacy KP specialist produced a review artifact and its decision was processed by the applicable KP rules. The legacy FD or DH downgrade specialist has no review artifact for this run. Exact triggered rule markers were INCREASED_TO_HIGH_BASED_ON_GUESSCVSS and LOWERED_FROM_HIGH_BASED_ON_NULLPTR. The independent weaker Aegis shadow evaluated CVSS 4.3 and ActionableScore 2 with primitive class CLASS_C and manual review REQUIRED. Its independently reconciled impact was MODERATE_KPANIC. The shadow agreed with the final production impact and aggregation did not change that impact. The final reported impact is MODERATE_KPANIC.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate / Strong Important candidate requiring manual review (with actual score 5)	MAYBE	NULLPTR VIRT INIT LEAK UAF KERNEL_PANIC_PLUS_UAF HARDWARE  KPANIC INCREASED_TO_HIGH_BASED_ON_GUESSCVSS KPANIC LOWERED_FROM_HIGH_BASED_ON_NULLPTR	-	-	checked

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

ActionableScore=5
ActionableScoreLower=4
PrimitiveClass=CLASS_B
RealUAF=NO
HostAdminRequired=NO
NamespaceScopedPrivilege=UNKNOWN
ImportantCandidate=YES
ManualReview=REQUIRED
AutoDowngradeAllowed=NO
RequiresHostUID0=NO
AuthorizationDecisionBypass=NO
ProtectedResourceImpact=NONE
B4PendingReview=NO
B4PendingConfirmed=NO

1. ActionableScore

* Raw base score: 4.
* Conservative score: 4; no floor or cap applied.
* Raw paranoid score: 5.
* Paranoid score: 5; the extra point reflects the specific guest/VMM-to-host active-MMU-state chain whose non-crash consequences remain unresolved.
* Final recommended bucket: **Actionable Moderate / Strong Important candidate requiring manual review**::

2. Signal breakdown

* +1 — Weak/indirect active-state corruption: failed L2-state loading leaves `vcpu->arch.mmu` using the nested MMU instead of restoring the root MMU.
* +1 — Active semantic-domain transition: KVM attempts to run L1 using an L1-to-L2 nested EPT context; the fix explicitly invokes `nested_ept_uninit_mmu_context()`.
* +1 — Reliable host-kernel crash: freeing nested state leaves `vmcs12` unavailable and `nested_ept_get_eptp()` performs the demonstrated NULL-offset read.
* +1 — Below-host-admin local trigger: the trace shows UID 1000 using the KVM vCPU ioctl path, and supplied evidence grounds guest/VMM control of the relevant vCPU operations.
* Paranoid +1 — The attacker-influenced active MMU context is consumed by privileged host KVM after a credible guest/VMM boundary transition. Only the NULL dereference is demonstrated; broader translation or integrity effects remain unknown.

Not awarded: no heap OOB write, UAF dereference, controlled reclaim, arbitrary write, disclosure, authorization bypass, or LPE. The freed `vmcs12` leads to missing/NULL state, not a demonstrated access to reclaimed storage. B5-R, B3-PENDING, and B4-PENDING do not apply.

3. Reachability analysis

The evidenced actor is either guest code controlling nested-VMX transitions or a non-root VMM process with `/dev/kvm` access; UID 1000 is shown. Host-administrator powers and host UID 0 are not required for that established path. Namespace capability scope is not documented and is irrelevant to ordinary KVM-fd ownership. No special hardware is required beyond nested VMX support. The exact division between guest-triggered RSM/state changes and VMM-issued `KVM_SET_NESTED_STATE` can vary, but the supplied evidence grounds a below-host-admin complete crash path.

4. Primitive and severity reading

This is an active structured-state consistency breach, not a real UAF: failed nested-state loading leaves the host vCPU’s MMU pointer in the wrong semantic domain. A later privileged KVM MMU load consumes that state after nested backing state may be freed, producing the shown host NULL dereference.

The demonstrated effect is host DoS. Host-memory corruption, unauthorized translation, or guest escape is unproven. Nevertheless, the concrete attacker-influenced active state, privileged consumer, and credible guest/VMM-to-host boundary satisfy the mandatory trust-boundary review rule. It is therefore CLASS_B, ImportantCandidate=YES, with no automatic downgrade, but it is not proof of Important severity.

5. One-sentence report phrase

A failed nested-VMX state load can leave a non-root-controlled vCPU using the nested MMU after returning to L1, leading to a host-kernel NULL dereference when the nested state is freed.

6. Manual review advice

MANUAL CHECK REQUIRED

Review whether any execution between the failed transition and the demonstrated NULL fault consumes stale nested translations in a security-relevant host context.

7. Call-site confidence

Call-site confidence: high

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

Patch: KVM: nVMX: Reset MMU back to the root MMU if loading L2 state fails for !VM-Entry
Commit: d00d5f534a8238674755e59826a6c7d3c8efb870
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=d00d5f534a8238674755e59826a6c7d3c8efb870

Commit description:

Reset the vCPU's MMU pointer back to the root MMU if loading L2 state fails
in the "not from VM-Entry" case, i.e. when emulating RSM or handling
KVM_SET_NESTED_STATE.  Failure to unwind the MMU pointer results in KVM
trying to run L1 with a nested MMU, and is especially problematic if the
vCPU manages to free its nested state before the VM crashes.  Constructing
the root for a nested EPT MMU goes through vmcs12, which is dynamically
allocated/freed when nVMX is enabled/disabled.  And so if the nested state
is freed with the MMU still pointing at the nested MMU, KVM will hit a NULL
pointer deref when attempting to get L1=>L2 EPTP.

  BUG: kernel NULL pointer dereference, address: 0000000000000078
  #PF: supervisor read access in kernel mode
  #PF: error_code(0x0000) - not-present page
  PGD 114a2d067 P4D 111743067 PUD 11141f067 PMD 0
  Oops: Oops: 0000 [#1] SMP
  CPU: 20 UID: 1000 PID: 1272 Comm: nested_tdp_faul Not tainted 7.3.0-rc4-8c89f11d1a99b-sink-vm #1914 PREEMPTLAZY
  Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 0.0.0 02/06/2015
  RIP: 0010:nested_ept_get_eptp+0xb/0x10 [kvm_intel]
  Call Trace:
   <TASK>
   kvm_mmu_load+0xcd/0x870 [kvm]
   kvm_arch_vcpu_ioctl_run+0x18f5/0x1f80 [kvm]
   kvm_vcpu_ioctl+0x2c7/0x970 [kvm]
   __x64_sys_ioctl+0x90/0xd0
   do_syscall_64+0x9c/0x4d0
   entry_SYSCALL_64_after_hwframe+0x4b/0x53
  RIP: 0033:0x47491b
   </TASK>
  Modules linked in: kvm_intel kvm irqbypass
  CR2: 0000000000000078
  ---[ end trace 0000000000000000 ]---
  RIP: 0010:nested_ept_get_eptp+0xb/0x10 [kvm_intel]

Cc: David Woodhouse <dwmw2@infradead.org>
Fixes: 72e9cbd ("KVM: nVMX: fix SMI injection in guest mode")
Fixes: 8fcc4b5 ("kvm: nVMX: Introduce KVM_CAP_NESTED_STATE")
Cc: stable@vger.kernel.org
Signed-off-by: Amirmohammad Eftekhar <amirmohammadeftekhar@gmail.com>
[sean: write changelog]
Signed-off-by: Sean Christopherson <seanjc@google.com>
Link: https://patch.msgid.link/20261001214356.3825100-2-seanjc@google.com
Signed-off-by: Paolo Bonzini <pbonzini@redhat.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=d00d5f534a8238674755e59826a6c7d3c8efb870

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

The original Linux kernel CVE announcement for CVE-2026-99176 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-99176

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:H
  The Best / paranoid CVSS vector: AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS score: 5.5

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).