[CVE-2026-99120][MODERATE REGULAR] KVM: x86/mmu: Cope with the SS bit being set in #NPF exit codes [ Upstream

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

CVE: CVE-2026-99120
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: KVM: x86/mmu: Cope with the SS bit being set in #NPF exit codes [ Upstream
Commit: 0caeefed99b37f03251a00eb1fdb403a1cf2cb68
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=0caeefed99b37f03251a00eb1fdb403a1cf2cb68
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-99120
Analysis date: Wed, 28 Oct 2026 22:53:16 -0400
ActionableScore: 3
ActionableScore lower bound: unknown
Actionable bucket: Borderline / manual review recommended
Manual review required: YES

Summary:
A nested guest can induce a host KVM permissions-array OOB read whose value affects MMU permission handling, with WARN/UBSAN demonstrated but no OOB write, host authorization bypass, or reliable crash proven.

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

The original Linux kernel CVE announcement for CVE-2026-99120 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-99120

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 REGULAR
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-2026-99120	MODERATE	https://www.kernelcve.org/list/?q=CVE-2026-99120 CHECK WITH IMPACT FROM ORIG NN MODERATE | AEGIS IMPACT MODERATE_KPANIC AEGIS_AS=1 LEGACY=MODERATE RESULT=DIFF (final Legacy and Aegis differ; FINAL: MODERATE, Aegis: 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:L/I:L/A:H';CWE-125;CWE-129;*CWE-20;Other CVSS 'AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:L/A:L';BEST CVSS score: '6.6';DESCR 'A nested guest can cause KVM permission_fault to use an SS bearing nested page fault error code as an index into a 16 entry permissions array. Values such as SS plus USER produce an out of bounds kernel read and the resulting value participates in the MMU permission decision. For the CVSS the PR:L value reflects the need to control a guest or nested hypervisor rather than mere network reachability. The demonstrated primitive is an out of bounds read with WARN or UBSAN reports. There is no supported evidence of an out of bounds write or UAF, but incorrect permission decisions and a host denial of service remain plausible consequences requiring review. Automated impact decision path. The original SimpleNet classifier selected MODERATE. The primary LLM estimated paranoid CVSS 6.6 and the production ActionableScore returned 3 with recommendation Borderline / manual review recommended. 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. ActionableScore thresholds stage MODERATE_KPANIC to MODERATE because the ActionableScore threshold rule was applied. 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 INCREASED_TO_MODERATE7_BASED_ON_GUESSCVSS and DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN4 and V41_INHERITED_KPANIC_PROMOTION_BLOCKED and MANUAL_REVIEW_REQUIRED. The independent weaker Aegis shadow evaluated CVSS 0.0 and ActionableScore 1 with primitive class CLASS_C and manual review REQUIRED. Its independently reconciled impact was MODERATE_KPANIC. The models disagreed but the aggregation rules retained the production result. The final reported impact is MODERATE.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline / manual review recommended (with actual score 3)	MAYBE	WRITE KASAN OOB VIRT INIT LEAK HARDWARE TEST  INCREASED_TO_MODERATE7_BASED_ON_GUESSCVSS DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN4 V41_INHERITED_KPANIC_PROMOTION_BLOCKED MANUAL_REVIEW_REQUIRED	-	-	checked

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

ActionableScore=3
PrimitiveClass=CLASS_C
RealUAF=NO
HostAdminRequired=NO
NamespaceScopedPrivilege=UNKNOWN
ImportantCandidate=NO
ManualReview=REQUIRED
AutoDowngradeAllowed=YES
RequiresHostUID0=NO
AuthorizationDecisionBypass=NO
ProtectedResourceImpact=NONE
B4PendingReview=NO
B4PendingConfirmed=NO

1. ActionableScore

* Raw base score: 3.
* Conservative score: 3; no floor or cap applied.
* Raw paranoid score: 3.
* Paranoid score: 3; no additional effect is sufficiently established.
* Final recommended bucket: **Borderline / manual review recommended**::

2. Signal breakdown

* +1 — Confirmed kernel out-of-bounds read: an SS-bearing fault code produces an index such as 34 into `fmt->permissions[16]`.
* +1 — Below-host-admin guest reachability: supplied evidence grounds triggering from a nested guest or nested hypervisor after nested virtualization is exposed.
* +1 — Security-relevant downstream consumption: the OOB-derived value participates in KVM's MMU permission-fault decision rather than being merely read and discarded.
* No negative privilege penalty: host-administrator powers are not required by the evidenced guest trigger.
* No reliable-crash point: only WARN and UBSAN reports are demonstrated; panic or host-wide DoS is plausible but not established.
* No write, disclosure, UAF, controlled replacement, arbitrary access, or host permission bypass is shown.
* NEW52/B5-R does not apply because the access is read-only, not a retry-accumulated heap OOB write.
* B4 does not apply: this is not an evidenced mandatory authorization guard changing DENY to ALLOW.

3. Reachability analysis

A nested guest controls the accesses that cause the NPF, while hardware/KVM supplies the SS-bearing exit code. The VMM or host administrator must initially expose KVM and nested virtualization, but no per-trigger host-admin operation is shown. The trigger is therefore guest-side after provisioning, not a compound guest-plus-VMM lifecycle operation. User-namespace capability scope is irrelevant and unresolved. No special physical hardware beyond AMD nested-virtualization support is identified, and no timing race is required.

4. Primitive and severity reading

The exact primitive is an attacker-reachable host-kernel array OOB read whose result affects an MMU permission calculation. It is neither active-object corruption nor stale storage, and no repeated release exists. The demonstrated outcome is invalid-state processing with WARN/UBSAN diagnostics; incorrect guest permission handling and host DoS remain possible but unproven. This is CLASS_C because no separately useful write, disclosure, host-resource access, or guest-to-host corruption primitive is established. ImportantCandidate is NO and auto-downgrade remains allowed, but manual review is required to determine the concrete effect of the malformed permission result.

5. One-sentence report phrase

A nested guest can induce a host KVM permissions-array OOB read whose value affects MMU permission handling, with WARN/UBSAN demonstrated but no OOB write, host authorization bypass, or reliable crash proven.

6. Manual review advice

MANUAL CHECK REQUIRED

Determine whether any reachable OOB value can grant unintended mappings or reliably crash the host rather than only producing diagnostics or guest-local permission errors.

7. Call-site confidence

Call-site confidence: high

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

Patch: KVM: x86/mmu: Cope with the SS bit being set in #NPF exit codes [ Upstream
Commit: 0caeefed99b37f03251a00eb1fdb403a1cf2cb68
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=0caeefed99b37f03251a00eb1fdb403a1cf2cb68

Changed files:
  arch/x86/kvm/mmu.h
  arch/x86/kvm/mmu/paging_tmpl.h

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=0caeefed99b37f03251a00eb1fdb403a1cf2cb68

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

The original Linux kernel CVE announcement for CVE-2026-99120 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-99120

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:L/PR:L/UI:N/S:U/C:N/I:L/A:L
  The Best / paranoid CVSS vector: AV:L/AC:L/PR:L/UI:N/S:U/C:L/I:L/A:H
  The Best / paranoid CVSS score: 6.6

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: 3

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: NO
Published priority for this report (same as in Subject): MODERATE REGULAR

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).