[CVE-2026-80551][MODERATE 7.0] s390/vfio_ccw: Ensure first IDAW remains constant

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

CVE: CVE-2026-80551
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: s390/vfio_ccw: Ensure first IDAW remains constant
Commit: 0d46c2565f173bcddcdcaf44a4f69789a20535e2
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=0d46c2565f173bcddcdcaf44a4f69789a20535e2
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-80551
Analysis date: Wed, 26 Aug 2026 12:05:57 -0400
ActionableScore: 5
ActionableScore lower bound: 3
Actionable bucket: Actionable Moderate at minimum
Manual review required: YES

Summary:
A TOCTOU race in s390 vfio_ccw IDAL handling can allow a malicious guest to change the first IDAW between validation reads, causing host-side processing of an inconsistent DMA address list and requiring manual review for possible DoS or limited guest-to-host boundary impact.

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ABOUT THIS REPORT
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The original Linux kernel CVE announcement for CVE-2026-80551 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-80551

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-2026-80551	MODERATE	CHECK WITH IMPACT FROM ORIG NN IMPORTANT	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:H';CWE-367;CWE-362;*CWE-20;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:C/C:N/I:N/A:H';BEST CVSS score: '7';DESCR 'A TOCTOU race in s390 vfio_ccw IDAL handling can occur because the first IDAW is read once to calculate the required IDAL size and then the full IDAW list is read later without verifying that the first entry is unchanged. A malicious or compromised guest can modify the guest supplied IDAW list between these reads, causing the host vfio_ccw code to process an inconsistent DMA address list. For the CVSS the PR:L value is used because the attacker needs the ability to run or control code in a guest or VM context with access to a vfio_ccw device, but does not need host root privileges. The issue is not network reachable and is triggered through the local virtualization device path. Impact is at least denial of service against the host or assigned device path, and the paranoid score keeps limited confidentiality and integrity impact because this is a race on DMA translation metadata across a guest to host boundary.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate at minimum (with actual score 4)	YES	OOB SIMPLEFIX LINUS MEMORY  DECREASED_TO_MODERATE70_BASED_ON_ACTIONABLESCORELOWERTHAN6 DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN5	NO	NO	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=5
ActionableScoreLower=3

## 1. ActionableScore

* Conservative score: 3
* Paranoid score: 5
* Final recommended bucket: **Actionable Moderate at minimum**::

## 2. Signal breakdown

Conservative signals:

* Local unprivileged or guest-context trigger: +1
  A malicious or compromised guest with access to a passed-through vfio_ccw device can influence guest-supplied IDAW/IDAL contents without host-root privileges.
* Weak/indirect corruption candidate: +1
  The bug is a TOCTOU consistency issue in guest DMA address-list processing. It does not show arbitrary write or UAF, but it can make host vfio_ccw code process an inconsistent IDAL.
* Privileged kernel/device-management DMA-adjacent path: +1
  The affected code is in s390 vfio_ccw channel-program translation and DMA-adjacent device handling.
* Hard or timing-dependent race: -1
  The first IDAW must change between the earlier count/read phase and the later full IDAL read.
* Rare architecture/device configuration: -1
  This requires s390 plus vfio_ccw device assignment, which is not a broad default Linux deployment.
* Availability impact plausible: +1
  Inconsistent channel-program or IDAL handling can plausibly break the assigned device path or cause host-side failure, even though a concrete panic path is not shown.

Conservative total: 3

Paranoid additional signals:

* Cross-boundary virtualization concern: +1
  The inconsistency is across a guest-to-host trust boundary in a device pass-through path.
* Limited confidentiality/integrity concern from DMA metadata inconsistency: +1
  The patch does not prove unauthorized host memory access, but malformed or changing guest DMA metadata deserves manual review for possible unintended mapping or translation effects.

Paranoid total: 5

## 3. Reachability analysis

The bug is not network reachable. It is reachable through the local virtualization device path when a guest controls or can influence a vfio_ccw channel program and its IDAW list.

The attacker model is not ordinary host-local unprivileged code. It is more likely a malicious guest, compromised guest kernel, or guest-side actor with sufficient ability to drive the assigned ccw device. Host root is normally required to configure vfio_ccw passthrough, but after assignment the guest-to-host boundary is meaningful, so this should not be treated as a fully trusted host-root-only bug.

Namespaces and containers do not materially lower reachability on the host. The relevant boundary is VM guest to host, not a normal Linux user namespace path.

The path is not default on most systems because it requires s390 and vfio_ccw device assignment. However, in affected IBM Z or s390 virtualization environments, the code is security relevant.

Call-site confidence: medium. The patch shows both the earlier IDAW counting path and the later IDAL read path, but it does not include all downstream channel-program execution consequences.

## 4. Severity interpretation

This behaves more like an actionable Moderate than an ordinary Moderate.

Realistically, the demonstrated issue is a TOCTOU race that can make the host vfio_ccw code process inconsistent guest-supplied DMA address metadata. The patch does not demonstrate UAF, double free, arbitrary write, attacker-controlled reclaim, or a concrete host memory corruption primitive.

Theoretical risk is higher than a simple validation bug because the affected data controls DMA-address-list interpretation across a guest-to-host virtualization boundary. That supports manual review and a paranoid ActionableScore of 5, but not a strong Important classification without additional evidence of host memory corruption, unauthorized DMA mapping, or privilege escalation.

## 5. One-sentence report phrase

A TOCTOU race in s390 vfio_ccw IDAL handling can allow a malicious guest to change the first IDAW between validation reads, causing host-side processing of an inconsistent DMA address list and requiring manual review for possible DoS or limited guest-to-host boundary impact.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: the bug is a race in a virtualization device-assignment path involving guest-controlled DMA metadata. The patch does not prove a strong corruption primitive, but the guest-to-host boundary and IDAL consistency issue make auto-closure unsafe.

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

Patch: s390/vfio_ccw: Ensure first IDAW remains constant
Commit: 0d46c2565f173bcddcdcaf44a4f69789a20535e2
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=0d46c2565f173bcddcdcaf44a4f69789a20535e2

Commit description:

The first IDAW in a list does not need to be on a 2K/4K boundary
like all others, and so is read separately to accurately calculate
the size of the buffer needed to read the full IDAL.

Verify that the address found in the first IDAW is unchanged between
reads, to ensure a consistent set of IDAWs being worked with.

Fixes: 01aa26c ("s390/cio: Combine direct and indirect CCW paths")
Cc: stable@vger.kernel.org
Reviewed-by: Matthew Rosato <mjrosato@linux.ibm.com>
Signed-off-by: Eric Farman <farman@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/cio/vfio_ccw_cp.c
  drivers/s390/cio/vfio_ccw_cp.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=0d46c2565f173bcddcdcaf44a4f69789a20535e2

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

The original Linux kernel CVE announcement for CVE-2026-80551 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-80551

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:N/I:N/A:H
  The Best / paranoid CVSS vector: AV:L/AC:H/PR:L/UI:N/S:C/C:L/I:L/A:H
  The Best / paranoid CVSS score: 7

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: 3
  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: NO
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).