[CVE-2026-98713][LOW] KVM: arm64: Apply the fine-grained UNDEFs without FEAT_FGT [ Upstream

[CVE-2026-98713][LOW] KVM: arm64: Apply the fine-grained UNDEFs without FEAT_FGT [ Upstream

[AL-KERNEL]

CVE: CVE-2026-98713
Priority: LOW
AL-KERNEL base severity: LOW
KPANIC flag: NO
Patch: KVM: arm64: Apply the fine-grained UNDEFs without FEAT_FGT [ Upstream
Commit: 2f4a15c52a552c5244ec27f4ca8bda855594f918
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=2f4a15c52a552c5244ec27f4ca8bda855594f918
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98713
Analysis date: Sat, 24 Oct 2026 05:21:00 -0400
ActionableScore: 1
ActionableScore lower bound: 0
Actionable bucket: Ordinary Moderate
Manual review required: NO

Summary:
A malicious arm64 nested guest can reach an unexpected KVM TLBI handler and trigger a spurious WARN, with host panic possible only under fatal-WARN configuration.

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

The original Linux kernel CVE announcement for CVE-2026-98713 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98713

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-2026-98713	LOW	https://www.kernelcve.org/list/?q=CVE-2026-98713 CHECK WITH IMPACT FROM ORIG NN LOW | AEGIS IMPACT LOW AEGIS_AS=1 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:L/PR:L/UI:N/S:U/C:N/I:N/A:H';CWE-670;CWE-754;Other CVSS 'AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:L';BEST CVSS score: '5.5';DESCR 'KVM arm64 incorrectly gated fine grained UNDEF trap configuration on hardware FEAT_FGT support even though the FGU behavior can be applied without that feature. On a non FEAT_FGT host, a guest running at vEL1 without FEAT_TLBIOS can execute a TLBI OS instruction that reaches handle_tlbi_el1 through the coarse HCR_EL2 TTLBOS trap and triggers a spurious WARN instead of receiving UNDEF. For the CVSS the PR:L value is selected because exploitation requires control of guest code or local access to a KVM virtual machine rather than network reachability alone. The demonstrated primitive is an unexpected handler path and WARN with no evidence of UAF, OOB access, information disclosure, arbitrary write, or privilege escalation. Base availability impact is low, while the paranoid availability impact is high only for systems configured to panic on WARN. Automated impact decision path. The original SimpleNet classifier selected LOW. The primary LLM estimated paranoid CVSS 5.5 and the production ActionableScore returned 1 with recommendation Ordinary Moderate. The production rules then evaluated CVSS thresholds and ActionableScore thresholds. Recorded rule transitions were CVSS and initial triage stage LOW 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. 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_FROM_LOW_BASED_ON_HIGHCVSSSCORE and DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN4 and DECREASED_TO_LOW_FROM_MODERATE_BASED_ON_ACTIONABLESCORELESSTHAN3. 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 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 (with actual score 1)	MAYBE	VIRT WARNONLY HARDWARE LINUS  INCREASED_FROM_LOW_BASED_ON_HIGHCVSSSCORE DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN4 DECREASED_TO_LOW_FROM_MODERATE_BASED_ON_ACTIONABLESCORELESSTHAN3	-	-	checked

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

ActionableScore=1
ActionableScoreLower=0
PrimitiveClass=CLASS_C
RealUAF=NO
HostAdminRequired=NO
NamespaceScopedPrivilege=NO
ImportantCandidate=NO
ManualReview=NO
AutoDowngradeAllowed=YES
RequiresHostUID0=NO
AuthorizationDecisionBypass=NO
ProtectedResourceImpact=NONE
B4PendingReview=NO
B4PendingConfirmed=NO

1. ActionableScore

* Raw base score: 0.
* Conservative score: 0; WARN and incorrect exception handling alone receive no availability point.
* Raw paranoid score: 1 from conditional host crash when WARN is configured as fatal.
* Paranoid score: 1; no floor or cap applied.
* Final recommended bucket: **Ordinary Moderate**::

2. Signal breakdown

* Base +0: The missing trap configuration sends a guest TLBI OS instruction to `handle_tlbi_el1()` and produces a spurious WARN rather than injecting UNDEF.
* Paranoid +1: `panic_on_warn` can convert this reachable WARN into a host-wide crash. This is configuration-dependent, not a guaranteed panic.
* No UAF, OOB access, write primitive, stale structured state, authorization bypass, protected-resource access, disclosure, or privilege escalation is shown.
* No KVM guest-to-host corruption leverage is awarded: the demonstrated cross-boundary effect is only a host WARN, conditionally fatal.
* NEW52/B5-R and NEW53/B3-PENDING do not apply; there is no heap write or shared-backing write.

3. Reachability analysis

Guest code executing at vEL1 can issue the relevant TLBI OS instruction on a non-FEAT_FGT host when the guest lacks FEAT_TLBIOS. This is below host-administrator privilege once the VM and nested-virtualization environment exist, so host UID 0 is not required for the immediate instruction trigger. The VMM controls VM creation and feature exposure, but no host-admin action is needed for each trigger. No user/net namespace capability is involved. There is no race.

4. Primitive and severity reading

The exact defect is incorrect KVM arm64 trap routing: omitted fine-grained UNDEF configuration allows an instruction to reach an handler that does not expect it, resulting in WARN instead of guest UNDEF injection. This is CLASS_C invalid-state/availability behavior, not memory corruption or active-state consumption. The shown effect is a WARN; host panic is conditional on fatal-WARN policy. ImportantCandidate is NO and auto-downgrade is allowed because no material non-crash primitive or unresolved security boundary exists.

5. One-sentence report phrase

A malicious arm64 nested guest can reach an unexpected KVM TLBI handler and trigger a spurious WARN, with host panic possible only under fatal-WARN configuration.

6. Manual review advice

NO MANUAL CHECK NEEDED

The patch supports only constrained WARN/conditional-DoS behavior and no Important-preserving primitive.

7. Call-site confidence

Call-site confidence: high

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

Patch: KVM: arm64: Apply the fine-grained UNDEFs without FEAT_FGT [ Upstream
Commit: 2f4a15c52a552c5244ec27f4ca8bda855594f918
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=2f4a15c52a552c5244ec27f4ca8bda855594f918

Changed files:
  arch/arm64/kvm/emulate-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=2f4a15c52a552c5244ec27f4ca8bda855594f918

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

The original Linux kernel CVE announcement for CVE-2026-98713 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98713

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:N/A:L
  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: 0
  Paranoid ActionableScore: 1

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).