[CVE-2026-64561][IMPORTANT] KVM: x86: Check for invalid/obsolete root *after* making MMU pages available

[CVE-2026-64561][IMPORTANT] KVM: x86: Check for invalid/obsolete root *after* making MMU pages available

[AL-KERNEL]

CVE: CVE-2026-64561
Priority: IMPORTANT
AL-KERNEL base severity: IMPORTANT
KPANIC flag: YES
Patch: KVM: x86: Check for invalid/obsolete root *after* making MMU pages available
Commit: 35e77467610c4a37cb0ff54ee56b85f73b1f5700
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=35e77467610c4a37cb0ff54ee56b85f73b1f5700
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64561
Analysis date: Tue, 04 Aug 2026 02:51:57 -0400
ActionableScore: 7
ActionableScore lower bound: 5
Actionable bucket: Strong Important candidate / Actionable Moderate at minimum
Manual review required: YES

Summary:
A KVM x86 MMU ordering bug can allow a stale page fault to populate mappings below an invalid root after shadow page reclaim, corrupting KVM MMU lifetime invariants and potentially causing host DoS with guest-to-host memory-corruption concerns.

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

The original Linux kernel CVE announcement for CVE-2026-64561 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64561

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-2026-64561	IMPORTANT	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-416;*CWE-664;CWE-362;*CWE-672;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:C/C:L/I:L/A:H';BEST CVSS score: '7.8';DESCR 'KVM x86 MMU page fault handling checked for a stale or obsolete root before making shadow MMU pages available. Reclaim in make_mmu_pages_available can zap an in use root and mark it invalid, after which the same fault path could still populate mappings below that invalid root. This can create child shadow pages that inherit an invalid role while being placed on the active MMU page list, violating KVM MMU lifetime invariants. For the CVSS the PR:L is used because a local user or guest controlled workload with access to KVM can potentially drive guest page faults and MMU pressure, while full host administrator privileges are not required in the paranoid practical risk model. The issue is not directly network reachable and is primarily a guest to host or local virtualization attack surface. Impact is at least denial of service on the host and in the paranoid interpretation may include confidentiality or integrity impact because stale KVM MMU pages and rmap related state can become a UAF candidate. YES REQUIRES MANUAL CHECK';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate / Actionable Moderate at minimum (with actual score 7)	YES	VIRT SIMPLEFIX HARDWARE  KPANIC INCREASED_TO_HIGH_BASED_ON_GUESSCVSS	NO	NO	guess

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

ActionableScore=7
ActionableScoreLower=5

## 1. ActionableScore

* Conservative score: 5
* Paranoid score: 7
* Final recommended bucket: **Strong Important candidate / Actionable Moderate at minimum**::

## 2. Signal breakdown

Triggered positive signals:

* Local unprivileged trigger: +1
  A local user with access to `/dev/kvm`, or a guest workload controlled by such a user, can plausibly drive guest page faults and MMU pressure.

* Memory corruption, weak to medium MMU lifetime primitive: +1
  The bug allows KVM to continue mapping below a root that may have been zapped and marked invalid, creating invalid child shadow pages on the active MMU page list.

* Real lifetime corruption: +1
  The flaw is tied to stale root lifetime and shadow page invalidation after `make_mmu_pages_available()`.

* Reliable kernel crash / strong DoS: +1
  Violating the invariant that invalid shadow pages must not be on the active list is likely to lead to host-side KVM MMU failure, BUG, WARN, or crash-class behavior.

* Privileged kernel memory-management state corruption path: +1
  The affected objects are KVM shadow MMU pages and roots, which are high-value virtualization memory-management structures.

* Broad virtualization exposure: +1
  KVM is a common Linux virtualization subsystem, especially on servers and cloud hosts.

Paranoid-only signals:

* Weak LPE concern: +1
  Stale KVM MMU pages and possible rmap traversal make this a UAF-candidate class, although no concrete controlled reclaim or write primitive is shown.

* Confidentiality impact plausible: +1
  In the paranoid interpretation, corrupted guest-to-host MMU state could potentially affect isolation.

* Integrity impact plausible: +1
  In the paranoid interpretation, stale or invalid shadow MMU state could potentially corrupt mappings or MMU metadata beyond a pure crash.

Negative / limiting signals:

* Hard timing or state condition: -1
  Triggering depends on a stale page fault plus MMU page reclaim or zap timing.

* No demonstrated arbitrary write or controlled reclaim: -1 in paranoid reasoning
  The patch does not show attacker-controlled replacement, function pointer control, arbitrary write, or a demonstrated LPE primitive.

## 3. Reachability analysis

The bug is locally reachable through KVM operation, not directly network reachable. A realistic attacker would need the ability to run or influence a VM workload and generate guest page faults while KVM is under shadow MMU page pressure. This is typically PR:L rather than PR:H in CVSS-like reasoning when `/dev/kvm` is available to non-root users through the `kvm` group or a virtualization service account.

Namespaces and containers can matter. A container root or service account with delegated KVM access should not be treated as full host root, because the bug crosses a guest or virtualization boundary into host kernel MMU state. The path is not a normal packet or syscall-only remote path, but KVM is widely deployed on virtualization hosts.

Call-site confidence: high. The patch shows both affected page fault paths, `direct_page_fault()` and shadow paging `FNAME(page_fault)()`, and the ordering around `make_mmu_pages_available()`, `is_page_fault_stale()`, `direct_map()`, and `fetch()` is explicit.

## 4. Severity interpretation

This is not an ordinary Moderate cleanup. The conservative interpretation is an actionable Moderate issue because it corrupts KVM MMU lifetime invariants and can plausibly crash the host from a guest-driven/local KVM path.

The paranoid interpretation is a Strong Important candidate because stale or invalid shadow MMU pages on the active list are close to UAF/rmap-style failure modes in a guest-to-host attack surface. However, the patch does not demonstrate controlled reclaim, arbitrary write, or a reliable privilege-escalation primitive, so the realistic evidence remains weaker than a proven LPE.

## 5. One-sentence report phrase

A KVM x86 MMU ordering bug can allow a stale page fault to populate mappings below an invalid root after shadow page reclaim, corrupting KVM MMU lifetime invariants and potentially causing host DoS with guest-to-host memory-corruption concerns.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: KVM MMU root invalidation, shadow page lifetime, active-list invariants, and possible rmap/UAF-style consequences make this unsuitable for auto-closure even though a concrete LPE primitive is not demonstrated in the patch.

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

Patch: KVM: x86: Check for invalid/obsolete root *after* making MMU pages available
Commit: 35e77467610c4a37cb0ff54ee56b85f73b1f5700
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=35e77467610c4a37cb0ff54ee56b85f73b1f5700

Commit description:

Check for a "stale" page fault, i.e. for an invalid and/or obsolete root,
after making MMU pages available for the shadow MMU.  If reclaiming shadow
pages zaps an in-use root, i.e. marks it invalid, then KVM will attempt to
map memory into an invalid root.  On its own, populating an invalid root is
"fine", but because child shadow pages inherit their parent's role, any
children created during the map/fetch will be created as invalid pages,
thus violating KVM's invariant that invalid pages are never on the list of
active MMU pages.

Note, the underlying flaw has existed since KVM first started tracking
invalid roots in 2008 (commit 2e53d63 , "KVM: MMU: ignore zapped root
pagetables"), but the true badness only came along in 2020 (Linux 5.9)
with the invariant that invalid shadow pages can't be on the list of
active pages.

Note #2, inheriting role.invalid when creating child shadow pages is also
far from ideal; that flaw will be addressed separately.

Reported-by: Hyunwoo Kim <imv4bel@gmail.com>
Fixes: f95eec9 ("KVM: x86/mmu: Don't put invalid SPs back on the list of active pages")
Cc: stable@vger.kernel.org
Signed-off-by: Sean Christopherson <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/mmu/mmu.c
  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=35e77467610c4a37cb0ff54ee56b85f73b1f5700

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

The original Linux kernel CVE announcement for CVE-2026-64561 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64561

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:C/C:L/I:L/A:H
  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: 7

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).