[CVE-2026-89957][IMPORTANT] s390/vfio-ap: Fix hot-unplug skipped when last AP adapter or domain removed

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

CVE: CVE-2026-89957
Priority: IMPORTANT
AL-KERNEL base severity: IMPORTANT
KPANIC flag: YES
Patch: s390/vfio-ap: Fix hot-unplug skipped when last AP adapter or domain removed
Commit: f58e824de8577fe411f6199d659731a0f86a7275
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=f58e824de8577fe411f6199d659731a0f86a7275
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89957
Analysis date: Wed, 16 Sep 2026 14:17:40 -0400
ActionableScore: 6
ActionableScore lower bound: 5
Actionable bucket: Actionable Moderate / Strong Important candidate requiring manual review
Manual review required: YES

Summary:
No one-sentence report phrase was found.

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

The original Linux kernel CVE announcement for CVE-2026-89957 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89957

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.

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AL-KERNEL CLASSIFICATION RESULT
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CVE-2026-89957	IMPORTANT	https://www.kernelcve.org/list/?q=CVE-2026-89957 CHECK WITH IMPACT FROM ORIG NN LOW	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:N';CWE-253;CWE-284;*CWE-672;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:C/C:L/I:L/A:N';BEST CVSS score: '7.5';DESCR 's390 vfio_ap can skip the guest APCB update when the last AP adapter, domain, or control domain assigned to an mdev is removed from the host configuration. The shadow APCB bits are cleared, but the guest update is not sent because bitmap_andnot() returns false when the resulting bitmap is empty. A guest that previously had access to the AP resource may keep stale hardware access after the host intended to hot unplug it, creating a virtualization access control bypass rather than a kernel crash. For the CVSS the PR:L is used for the paranoid score because a guest or tenant context with prior access to the assigned vfio_ap device can benefit from the stale access, although reliable manifestation depends on a host side AP configuration removal event. The issue is not network reachable. Impact is mainly confidentiality and integrity risk for AP crypto resource isolation, with severity depending on how the AP adapter and domains are used in the environment.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate / Strong Important candidate requiring manual review (with actual score 6)	YES	HARDWARE LINUS  KPANIC INCREASED_TO_MODERATE7_FROM_LOW_BASED_ON_GUESSCVSS_AND_ACTIONABLESCOREHIGHER4 KPANIC INCREASED_TO_HIGH_BASED_ON_ACTIONABLESCOREHIGHERTHAN5	NO	NO	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=6
ActionableScoreLower=5
PrimitiveClass=CLASS_A
RealUAF=NO
HostAdminRequired=UNKNOWN
NamespaceScopedPrivilege=NO
ImportantCandidate=YES
ManualReview=REQUIRED
AutoDowngradeAllowed=NO

1. ActionableScore

* Raw conservative score: 5
* Conservative score: 5
* Raw paranoid score: 6
* Paranoid score: 6
* 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 / revocation bypass. The guest can retain stale APCB based access to AP crypto devices after the host configuration removed the last assigned adapter, domain, or control domain.
* +1 Active-state retirement / synchronization bypass. The intended guest APCB update is the active revocation mechanism, and the bug skips it exactly in the empty-result case.
* +1 Confidentiality plausible. Continued access to a revoked AP crypto resource can expose or allow use of crypto-domain material or operations depending on deployment.
* +1 Integrity plausible. Continued unauthorized use of AP crypto resources can violate the host intended isolation and resource assignment policy.
* Paranoid +1 Practical exploit chain strongly implied. The chain is directly described as host removal mask intersects assigned resource, shadow APCB cleared, guest APCB update skipped, guest keeps stale hardware access.

Non-awarded signals:

* RealUAF not awarded. This is stale authorization or device-access state, not post-free object use.
* Kernel memory corruption not awarded. No UAF, OOB, arbitrary write, refcount corruption, or controlled reclaim is established.
* Reliable kernel crash / DoS not awarded. The demonstrated consequence is stale guest hardware access, not a kernel panic.
* Remote reachability not awarded. This is not network-triggered.
* Local unprivileged trigger not awarded. A guest or tenant may benefit, but the complete trigger depends on host AP configuration removal.
* Host-admin penalty not applied. Host-admin-equivalent attacker privilege is not proven as a requirement for the attacker who benefits from stale access.
* Rare hardware penalty not applied. s390 vfio-ap is specialized, but the patch establishes a real virtualization isolation failure in the affected deployment.
* Sensitive credential or FD theft not awarded. No specific stolen key, credential, FD, or protected file is demonstrated.
* Strong LPE or guest-to-host memory corruption not awarded. The issue is an AP resource isolation bypass, not arbitrary host memory access.

3. Reachability analysis

The affected environment is s390 KVM with vfio-ap mediated devices assigned to a guest. The vulnerable condition occurs when the last AP adapter, AP domain, or control domain assigned to an mdev is removed from the host AP configuration.

The attacker model is not a pure host-admin attacker. A guest or tenant that previously had legitimate AP access may continue using stale hardware access after the host intended to revoke it. However, the supplied evidence does not prove that the guest alone can cause the host-side AP configuration removal event. Therefore HostAdminRequired is UNKNOWN rather than YES or NO.

NamespaceScopedPrivilege is NO because this is not a user or network namespace scoped capability path. It is a mediated device and KVM AP assignment path.

There is prior host setup because vfio-ap assignment requires host configuration. That does not erase the boundary-bypass consequence, because the security issue is failure to revoke access after the host changes policy.

No packet or network path is involved. No race timing is required by the patch. The logic bug is deterministic for the last-resource-removal case because bitmap_andnot() returns false when the destination bitmap becomes empty.

4. Primitive and severity interpretation

The exact primitive is a stale guest hardware authorization state. The host shadow APCB is updated, but the guest APCB is not updated when the removal leaves the bitmap empty. This leaves the guest with stale access to AP devices that the host intended to unplug.

This is not fixed stale storage and not an active kernel UAF. It is an active authorization / device-access state mismatch across a virtualization boundary.

PrimitiveClass is CLASS_A because the patch directly establishes a meaningful security-boundary and revocation bypass. The guest retains access to a hardware crypto resource after host policy removed that access.

The demonstrated consequence is unauthorized continued AP device access. The speculative consequence is higher confidentiality or integrity impact depending on whether the AP resource is shared, tenant-separated, key-bearing, or security-sensitive in that deployment.

ImportantCandidate is YES because this is a virtualization isolation and crypto-resource access-control issue. AutoDowngradeAllowed is NO because automatically treating it as ordinary Moderate would discard a concrete security-boundary bypass.

Likely operational severity is Actionable Moderate at minimum, with Strong Important review justified for multi-tenant or sensitive crypto AP deployments.

5. One-sentence report phrase

s390 vfio-ap can skip the guest APCB update when the last assigned AP resource is removed, leaving a KVM guest with stale access to a revoked crypto device and creating a virtualization access-control bypass that requires manual review.

6. Manual review recommendation

MANUAL CHECK REQUIRED

Manual review is required because the issue is not a crash-only bug but a stale hardware access and virtualization isolation bypass involving cryptographic AP resources.

7. Call-site confidence

Call-site confidence: high

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

Patch: s390/vfio-ap: Fix hot-unplug skipped when last AP adapter or domain removed
Commit: f58e824de8577fe411f6199d659731a0f86a7275
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=f58e824de8577fe411f6199d659731a0f86a7275

Commit description:

The vfio_ap_mdev_hot_unplug_cfg() function uses the return value of
bitmap_andnot() to determine whether the guest APCB needs to be updated.
However, bitmap_andnot() returns false when the resulting destination
bitmap is empty. This means that if the only adapter, domain or control
domain assigned to an mdev is removed from the host's AP configuration,
the bit is correctly cleared from the shadow APCB, but bitmap_andnot()
returns false because the result is an empty bitmap. Consequently,
do_hotplug remains 0 and vfio_ap_mdev_update_guest_apcb() is never called,
leaving the KVM guest with stale hardware access to the unplugged AP
devices.

Fix this by replacing the bitmap_andnot() return value check with
bitmap_intersects() to determine whether the shadow APCB actually
overlaps with the removal mask. If there is an intersection, call
bitmap_andnot() solely for its side effect of clearing the bits, then
unconditionally set do_hotplug to trigger the guest APCB update.

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=f58e824de8577fe411f6199d659731a0f86a7275

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

The original Linux kernel CVE announcement for CVE-2026-89957 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89957

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:H/I:H/A:N
  The Best / paranoid CVSS score: 7.5

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: 5
  Paranoid ActionableScore: 6

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).