[CVE-2026-80891][MODERATE 7.0] KVM: s390: pci: Validate AIBV and AISB before pinning guest pages

[CVE-2026-80891][MODERATE 7.0] KVM: s390: pci: Validate AIBV and AISB before pinning guest pages

[AL-KERNEL]

CVE: CVE-2026-80891
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: KVM: s390: pci: Validate AIBV and AISB before pinning guest pages
Commit: 3a1c64220ede4ac9ef9a3e76f008ff60a34f4c48
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=3a1c64220ede4ac9ef9a3e76f008ff60a34f4c48
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-80891
Analysis date: Fri, 04 Sep 2026 18:33:41 -0400
ActionableScore: 7
ActionableScore lower bound: 4
Actionable bucket: Strong Important candidate / Actionable Moderate at minimum
Manual review required: YES

Summary:
KVM s390 PCI interrupt forwarding failed to validate that the guest supplied AIBV stayed within one page and that AISB was 8 byte aligned before pinning pages, allowing a malicious guest to create unsafe host handling of interrupt forwarding memory with possible host DoS and worst case memory safety impact.

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ABOUT THIS REPORT
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The original Linux kernel CVE announcement for CVE-2026-80891 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-80891

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.

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AL-KERNEL CLASSIFICATION RESULT
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CVE-2026-80891	MODERATE	https://www.kernelcve.org/list/?q=CVE-2026-80891 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:C/C:H/I:H/A:H';*CWE-20;CWE-119;*CWE-787;Other CVSS 'AV:L/AC:L/PR:L/UI:N/S:C/C:L/I:L/A:H';BEST CVSS score: '8.8';DESCR 'KVM s390 PCI interrupt forwarding accepted guest supplied AIBV and AISB parameters without fully validating the AIBV page boundary and AISB doubleword alignment. A malicious guest with access to the affected s390 KVM PCI configuration could provide an AIBV whose computed bit vector crosses a page boundary even though only one guest page is pinned, or provide a misaligned AISB address. This can create unsafe host handling of guest memory used for interrupt forwarding and may lead to host denial of service or memory safety impact. For the CVSS the PR:L is used because exploitation requires control of a guest or local KVM process rather than full host administrative privileges. The issue is not directly network reachable and is a guest to host virtualization boundary risk. Impact is at least host denial of service and in the paranoid case may include confidentiality and integrity impact due to possible memory corruption.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate / Actionable Moderate at minimum (with actual score 6)	YES	OOB VIRT SIMPLEFIX HARDWARE LINUS KPANIC MEMORY  KPANIC INCREASED_TO_HIGH_BASED_ON_GUESSCVSS DECREASED_TO_MODERATE7_BASED_ON_FALSEPOSCHECKOFKP	NO	NO	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=7
ActionableScoreLower=4

## 1. ActionableScore

* Conservative score: 4
* Paranoid score: 7
* Final recommended bucket: **Strong Important candidate / Actionable Moderate at minimum**::

## 2. Signal breakdown

Conservative signals:

* Local or guest-controlled trigger: +1. The attacker needs control of a guest or KVM process using s390 PCI interrupt forwarding, not full host root.
* Weak or indirect corruption candidate: +1. Guest supplied AIBV size and AISB alignment were not validated before page pinning, creating unsafe memory handling, but the patch does not show a concrete arbitrary write or UAF primitive.
* Cross-boundary isolation risk: +1. The affected path is KVM guest-to-host interrupt forwarding, so even a host DoS or unsafe host memory access crosses a virtualization boundary.
* Availability impact plausible: +1. Invalid AIBV/AISB state can plausibly cause host-side failure in interrupt forwarding paths.
* Rare configuration or hardware path: -1. This requires s390 KVM with zPCI interrupt forwarding, which is not a broad default deployment.

Paranoid additional signals:

* Stronger memory safety concern: +1. AIBV can cross a page boundary while only one page is pinned, so later host code may operate on memory outside the validated pinned range.
* Confidentiality impact plausible: +1. In the worst defensible case, unsafe guest memory interpretation near a pinned boundary could expose unintended memory state.
* Integrity impact plausible: +1. In the worst defensible case, interrupt forwarding updates could affect memory outside the intended guest supplied object.
* Privileged kernel or device-management memory path: +1. The bug is in KVM s390 PCI interrupt forwarding and page pinning logic, a trusted virtualization and device-management path.

Call-site confidence: medium. The patch shows the validation site and pinning calls, but not all later users of the pinned AIBV/AISB memory.

## 3. Reachability analysis

The issue is not network reachable. It is reachable through KVM s390 PCI interrupt forwarding when a guest or KVM userspace can supply malformed AIBV or AISB values. For the CVSS the PR:L interpretation is reasonable because the attacker needs control over a guest or local KVM context, but not full host administrative privileges. Namespace effects are not the main factor here. The realistic deployment scope is limited because it requires s390 hardware or s390 virtualization with zPCI interrupt forwarding enabled.

## 4. Severity interpretation

This is not an ordinary low-risk validation cleanup because the rejected input affects guest page pinning and interrupt-forwarding memory used by KVM. Conservative handling is borderline manual review because the patch does not demonstrate a direct arbitrary write, UAF, or working LPE primitive. Paranoid handling is Strong Important candidate because a guest-controlled page-boundary violation before pinning may create host-side memory safety impact across the virtualization boundary. The realistic demonstrated impact is closer to host DoS or unsafe memory handling, while full confidentiality or integrity compromise remains worst-case and requires manual source review.

## 5. One-sentence report phrase

KVM s390 PCI interrupt forwarding failed to validate that the guest supplied AIBV stayed within one page and that AISB was 8 byte aligned before pinning pages, allowing a malicious guest to create unsafe host handling of interrupt forwarding memory with possible host DoS and worst case memory safety impact.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: this is a KVM guest-to-host boundary issue involving guest-controlled memory layout and page pinning. The patch does not prove a full exploit primitive, but the affected path is security-sensitive enough that auto-closing would risk missing a host memory safety issue.

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UPSTREAM PATCH SUMMARY
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Patch: KVM: s390: pci: Validate AIBV and AISB before pinning guest pages
Commit: 3a1c64220ede4ac9ef9a3e76f008ff60a34f4c48
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=3a1c64220ede4ac9ef9a3e76f008ff60a34f4c48

Commit description:

The AIBV holds one bit per MSI-X vector for a given function. The size of
the bit vector is derived from the NOI and the AIBVO. If the size of the
AIBV exceeds a single page boundary, then reject the request as we cannot
safely pin the guest AIBV.

Similarly reject the request if the AISB address is not 8-byte aligned as
the architecture requires doubleword alignment for the summary bit address.
Since the AISBO can address up to 64 bits, the size of the AISB can only be
8 bytes for the function. This also ensures the AISB doesn't exceed a
single page boundary.

Fixes: 3c5a1b6 ("KVM: s390: pci: provide routines for enabling/disabling interrupt forwarding")
Cc: stable@vger.kernel.org
Reviewed-by: Christian Borntraeger <borntraeger@linux.ibm.com>
Reviewed-by: Matthew Rosato <mjrosato@linux.ibm.com>
Signed-off-by: Farhan Ali <alifm@linux.ibm.com>
Tested-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:
  arch/s390/kvm/pci.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=3a1c64220ede4ac9ef9a3e76f008ff60a34f4c48

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

The original Linux kernel CVE announcement for CVE-2026-80891 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-80891

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:C/C:L/I:L/A:H
  The Best / paranoid CVSS vector: AV:L/AC:L/PR:L/UI:N/S:C/C:H/I:H/A:H
  The Best / paranoid CVSS score: 8.8

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: 4
  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: 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).