[CVE-2026-98164][MODERATE REGULAR] KVM: x86/mmu: Check write tracking in all address spaces

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

CVE: CVE-2026-98164
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: KVM: x86/mmu: Check write tracking in all address spaces
Commit: 09aa68552d2542cc6c23edd1568ac265dc5d886f
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=09aa68552d2542cc6c23edd1568ac265dc5d886f
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98164
Analysis date: Tue, 29 Sep 2026 08:51:37 -0400
ActionableScore: 1
ActionableScore lower bound: unknown
Actionable bucket: Ordinary Moderate
Manual review required: NO

Summary:
No one-sentence report phrase was found.

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

The original Linux kernel CVE announcement for CVE-2026-98164 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98164

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: NO

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

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

CVE-2026-98164	MODERATE	https://www.kernelcve.org/list/?q=CVE-2026-98164 CHECK WITH IMPACT FROM ORIG NN MODERATE	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-362;*CWE-664;CWE-754;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 KVM x86 MMU logic error can miss write tracking for a guest frame when the frame is checked through a different SMM address space. This can allow mmu_try_to_unsync_pages to mark a shared shadow page unsync and eventually trigger a BUG in pte_list_remove, causing a host kernel crash. For the CVSS the PR:L reflects an attacker who can run or control a KVM guest, for example a cloud tenant or a service account with KVM access. The issue is not directly network reachable, but a remote tenant may reach the vulnerable host through a hosted virtual machine service. Impact is denial of service on the host. The patch does not demonstrate a use after free, out of bounds access, information disclosure, arbitrary write, or a supported privilege escalation primitive.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate (with actual score 1)	YES	OOB VIRT HARDWARE  DECREASED_TO_MODERATE_REGULAR_FROM_MODERATE7_BASED_ON_GUESSCVSS_AND_CIANNH	NO	NO	guess

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

ActionableScore=1
PrimitiveClass=CLASS_C
RealUAF=NO
HostAdminRequired=UNKNOWN
NamespaceScopedPrivilege=UNKNOWN
ImportantCandidate=NO
ManualReview=RECOMMENDED
AutoDowngradeAllowed=YES

1. ActionableScore

* Raw conservative score: 1
* Conservative score: 1
* Raw paranoid score: 1
* Paranoid score: 1
* Final recommended bucket: **Ordinary Moderate**::

2. Signal breakdown

* +1 Reliable host kernel DoS. The patch explicitly states that incorrect write tracking can allow mmu_try_to_unsync_pages to mark an upper level shadow page unsync and eventually trigger BUG in pte_list_remove.
* No UAF bonus. The patch does not establish that a KVM object, shadow page, rmap entry, or memslot is freed and subsequently dereferenced.
* No active lifetime or repeated release bonus. No put, free, refcount decrement, double release, or post lifetime mutation is shown.
* No corruption bonus. The demonstrated outcome is an invalid MMU bookkeeping state followed by BUG, not an arbitrary write, controlled overwrite, OOB access, type confusion, or attacker controlled replacement.
* No remote reachability bonus. KVM code alone does not prove direct network reachability.
* No local unprivileged trigger bonus. The supplied patch does not establish that a malicious guest alone can enable the required write tracking state and complete the full trigger sequence.
* No guest to host exploitation leverage bonus. The evidence supports a possible host BUG while processing KVM MMU state, but does not show guest controlled host memory corruption, controlled reclaim, host arbitrary read or write, callback control, or a guest escape primitive.
* No C or I related signal. There is no demonstrated disclosure, integrity violation, protected object modification, or privilege escalation.
* No race penalty. This is an address space consistency and write tracking logic defect. The supplied evidence does not establish a narrow or uncontrollable timing race.
* No niche subsystem penalty. KVM and SMM requirements are relevant reachability conditions, but the evidence does not establish that they are attacker inaccessible or rare enough to justify a negative point.

3. Reachability analysis

The affected path is in host KVM x86 MMU handling and requires a VM using multiple KVM address spaces, specifically the SMM and SMRAM address space model, together with active KVM write tracking. The patch shows that a GFN can be write tracked through one address space while appearing untracked through the other.

Three attacker roles must remain separate. A malicious guest can control guest CPU state, guest memory accesses, and SMM execution if SMM is enabled for that VM. A local VMM process with access to `/dev/kvm` controls VM construction, memslots, and potentially configuration related to tracking. A host administrator can create or configure the overall virtualization deployment. The supplied patch does not prove that guest controlled activity alone can establish all prerequisites for write tracking and trigger the faulty unsync path without VMM cooperation.

HostAdminRequired is therefore UNKNOWN. It is not positively established that host admin privilege is required, but below host admin reachability is also not fully proven. NamespaceScopedPrivilege is UNKNOWN because no namespace scoped capability chain is supplied and KVM VM configuration is not demonstrated to be completable inside an attacker controlled namespace.

4. Primitive and severity interpretation

The patch establishes a KVM MMU state consistency defect. Write tracking is evaluated only in the supplied memslot even though write tracking is effectively relevant across both SMM related address spaces and shadow pages are shared. This can incorrectly allow an upper level shadow page to become unsynchronized and later trigger BUG in pte_list_remove.

This is not an established UAF. There is no shown object free followed by stale dereference, no stale rmap traversal after a freed object lifetime, and no missing zap or drop before replacement shown in the supplied diff. The exact demonstrated primitive is invalid shadow page and write tracking state leading to an assertion style kernel failure. It is therefore CLASS_C rather than CLASS_B.

The patch supports a possible host kernel crash or DoS. It does not support a practical guest to host corruption primitive, privilege escalation, confidentiality impact, integrity impact, arbitrary host kernel read or write, or controlled host object replacement. A malicious guest may be involved in reaching the path, but the complete guest only trigger chain remains unresolved because write tracking and VM configuration may be controlled by the VMM.

ImportantCandidate is NO because the established behavior is crash oriented invalid state handling without a concrete lifetime, ownership, corruption, authorization, or boundary bypass primitive. AutoDowngradeAllowed is YES because no unresolved active UAF, repeated refcount mutation, controlled corruption, or security boundary bypass is established from the supplied evidence.

Likely operational severity is ordinary Moderate when the affected KVM SMM and write tracking configuration is reachable. It should still receive targeted KVM deployment review because a host crash caused from a tenant controlled guest would be operationally significant if a guest only trigger is confirmed.

5. One-sentence report phrase

A KVM x86 MMU write tracking inconsistency across SMM address spaces can mark a shared shadow page unsync and eventually trigger BUG in pte_list_remove, causing a potential host kernel denial of service without demonstrated guest to host memory corruption or privilege escalation.

6. Manual review recommendation

MANUAL CHECK RECOMMENDED

Confirm whether an untrusted guest can independently reach the required SMM plus write tracking configuration and trigger the pte_list_remove BUG without VMM controlled setup.

7. Call-site confidence

Call-site confidence: medium

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

Patch: KVM: x86/mmu: Check write tracking in all address spaces
Commit: 09aa68552d2542cc6c23edd1568ac265dc5d886f
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=09aa68552d2542cc6c23edd1568ac265dc5d886f

Commit description:

kvm_gfn_is_write_tracked() checks only the supplied memslot, but page
tracking is per-address-space and shadow pages are shared across all
address spaces.  With SMM, a GFN can therefore be write-tracked in one
address space and appear untracked through the other.

Check the supplied slot first, then the slot for the other address space.
This ensures all callers honor write tracking regardless of the active
address space.  In particular, it prevents mmu_try_to_unsync_pages() from
marking an upper-level shadow page unsync and eventually triggering the
BUG in pte_list_remove().

Fixes: 699023e ("KVM: x86: add SMM to the MMU role, support SMRAM address space")
Assisted-by: Codex:GPT-5
Signed-off-by: Jinu Kim <kimjw04271234@gmail.com>
Message-ID: <20260721103512.2136240-2-kimjw04271234@gmail.com>
[invert direction of the conditional. - Paolo]
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
[ artem: adapt to 6.1's kvm_slot_page_track_is_active() and mode-indexed
  gfn_track[]; KVM_ADDRESS_SPACE_NUM is unconditionally 2 on x86 in 6.1,
  so the peer-slot lookup needs no guard ]
Signed-off-by: Artem Dinaburg <artem@trailofbits.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>

Changed files:
  arch/x86/kvm/mmu/page_track.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=09aa68552d2542cc6c23edd1568ac265dc5d886f

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

The original Linux kernel CVE announcement for CVE-2026-98164 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98164

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: unknown
  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: 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).