[CVE-2026-89959][MODERATE 7.0] s390/vfio-ap: Fix control domain removal in vfio_ap_mdev_cfg_remove

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

CVE: CVE-2026-89959
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: s390/vfio-ap: Fix control domain removal in vfio_ap_mdev_cfg_remove
Commit: 12871a4269f139221c9fde9ea3eb69c9228f59d4
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=12871a4269f139221c9fde9ea3eb69c9228f59d4
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89959
Analysis date: Wed, 16 Sep 2026 14:25:04 -0400
ActionableScore: 5
ActionableScore lower bound: 4
Actionable bucket: Actionable Moderate / Strong Important candidate requiring manual review
Manual review required: YES

Summary:
No one-sentence report phrase was found.

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

The original Linux kernel CVE announcement for CVE-2026-89959 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89959

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.

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

CVE-2026-89959	MODERATE	https://www.kernelcve.org/list/?q=CVE-2026-89959 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:L/I:L/A:L';CWE-284;CWE-863;CWE-266;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:C/C:L/I:L/A:N';BEST CVSS score: '5.3';DESCR 's390/vfio-ap can fail to remove an explicitly unplugged AP control domain from a KVM guest because vfio_ap_mdev_cfg_remove computes the control domain removal mask with bitmap_andnot instead of the intersection of requested removals and assigned domains. A guest that previously had access to an AP control domain may retain that control domain after the host side configuration attempted to remove it, resulting in an authorization revocation failure. For the CVSS the PR:L is used for the paranoid score because the practical attacker is a local guest or tenant context with prior access to the assigned vfio-ap device, not necessarily a fully privileged host administrator. The issue is not network reachable and requires an s390 KVM deployment using vfio-ap and AP crypto resources. Impact is mainly confidentiality and integrity policy bypass for AP control domain isolation, with limited availability impact in the paranoid interpretation.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate / Strong Important candidate requiring manual review (with actual score 5)	YES	SIMPLEFIX LINUS  KPANIC INCREASED_TO_MODERATE70_BASED_ON_ACTIONABLESCOREHIGHEREQTHAN5	NO	NO	checked

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

ActionableScore=5
ActionableScoreLower=4
PrimitiveClass=CLASS_A
RealUAF=NO
HostAdminRequired=UNKNOWN
NamespaceScopedPrivilege=NO
ImportantCandidate=YES
ManualReview=REQUIRED
AutoDowngradeAllowed=NO

1. ActionableScore

* Raw conservative score: 4
* Conservative score: 4
* Raw paranoid score: 5
* Paranoid score: 5
* Final recommended bucket: **Actionable Moderate / Strong Important candidate requiring manual review**::

No floor or cap changed the score.

2. Signal breakdown

Positive signals:

* +2 Concrete security-boundary / authorization revocation failure. The patch establishes that explicitly unplugged AP control domains may remain assigned to the KVM guest because the removal mask is computed incorrectly.
* +1 Confidentiality impact plausible. A guest may retain visibility or use of AP crypto control-domain resources after the host intended to remove that assignment.
* +1 Integrity impact plausible. Continued control-domain assignment may allow continued control or influence over AP crypto resources contrary to the host-side authorization state.
* Paranoid only: +1 Practical exploit chain strongly implied. The chain is host attempts control-domain removal -> wrong bitmap operation computes the wrong removal set -> guest retains access after revocation.

Negative / non-awarded signals:

* RealUAF not awarded. No stale pointer, post-free use, refcount misuse, or lifetime corruption is established.
* Memory corruption not awarded. The bug is a logic / authorization-state error, not an OOB, UAF, double free, or arbitrary write.
* Strong LPE not awarded. The evidence supports retained device-domain access, not host privilege escalation.
* Remote reachability not awarded. The issue is not reachable through network packets.
* Resource exhaustion not awarded. No leak or exhaustion primitive is shown.
* Rare hardware penalty not applied in arithmetic. The affected environment is specific to s390 KVM with vfio-ap and AP crypto, but this is the target deployment for the feature rather than a reason by itself to auto-downgrade the security boundary issue.
* Extra availability not awarded conservatively. Availability impact is not the main demonstrated consequence.

3. Reachability analysis

The vulnerable operation is in s390 vfio-ap mediated device configuration removal. The affected scenario requires an s390 KVM deployment using vfio-ap and AP crypto resources, with a guest that had an AP control domain assigned.

The directly changed operation appears to be host-side device assignment / removal logic. That configuration path is normally controlled by a host management component or administrator. However, the security consequence benefits a guest or tenant context that previously had access and may retain access after revocation. Because the supplied evidence does not fully resolve whether the complete trigger can be initiated below host-admin privilege, HostAdminRequired is UNKNOWN rather than YES or NO.

NamespaceScopedPrivilege is NO. This is not a namespace-scoped Linux capability issue. It is a virtualization and mediated-device assignment issue.

There is no network trigger. There is no race requirement. There is a prior setup requirement: vfio-ap resources must have been assigned to the guest, and a later host-side removal must occur.

4. Primitive and severity interpretation

The exact primitive is an authorization / revocation failure for AP control-domain assignment. The old assigned domain state may remain effective in the guest after the host-side configuration attempted to remove it.

This is not active structured UAF and not fixed stale storage. There is no concrete repeated release or refcount mutation. There is no evidence of controlled reclaim, arbitrary read/write, callback control, or host memory corruption.

PrimitiveClass is CLASS_A because the patch directly establishes a meaningful security-boundary failure: a control domain explicitly removed by the host may not actually be removed from the guest. The demonstrated consequence is retained access to a previously assigned AP control domain. The speculative consequence is broader impact depending on what crypto resources and operational policies are tied to that control domain.

ImportantCandidate is YES because virtualization device-assignment revocation failures can cross a meaningful guest/host administrative security boundary and should not be treated as ordinary low-risk logic cleanup. AutoDowngradeAllowed is NO because automatically closing this would risk missing a real isolation and authorization consequence.

Likely operational severity is Actionable Moderate, with Strong Important review warranted if the retained AP control domain exposes sensitive crypto operations, keys, or cross-tenant resources in the evaluated environment.

5. One-sentence report phrase

s390/vfio-ap may fail to remove an explicitly unplugged AP control domain from a KVM guest because the removal bitmap is computed incorrectly, allowing a guest with prior access to retain AP crypto control-domain authorization after host-side revocation.

6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: this is not memory corruption, but it is a concrete virtualization device-assignment revocation failure with possible AP crypto security-boundary impact.

7. Call-site confidence

Call-site confidence: medium

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

Patch: s390/vfio-ap: Fix control domain removal in vfio_ap_mdev_cfg_remove
Commit: 12871a4269f139221c9fde9ea3eb69c9228f59d4
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=12871a4269f139221c9fde9ea3eb69c9228f59d4

Commit description:

The vfio_ap_config_remove function uses the bitmap_andnot function to clear
bits from the matrix_mdev->matrix.adm bitmap (specifies the control domains
assigned to the mdev). This prevents the explicitly unplugged control
domains from being removed the KVM guest. The bitmap_and function is used
instead.

Fixes: eeb386a ("s390/vfio-ap: handle config changed and scan complete notification")
Cc: stable@vger.kernel.org
Signed-off-by: Anthony Krowiak <akrowiak@linux.ibm.com>
Reviewed-by: Matthew Rosato <mjrosato@linux.ibm.com>
Signed-off-by: Christian Borntraeger <borntraeger@linux.ibm.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  drivers/s390/crypto/vfio_ap_ops.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=12871a4269f139221c9fde9ea3eb69c9228f59d4

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

The original Linux kernel CVE announcement for CVE-2026-89959 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89959

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:N
  The Best / paranoid CVSS vector: AV:L/AC:H/PR:L/UI:N/S:C/C:L/I:L/A:L
  The Best / paranoid CVSS score: 5.3

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).