[CVE-2026-80550][MODERATE 7.0] s390/vfio_ccw: Fix out of bounds check on CCW array

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

CVE: CVE-2026-80550
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: s390/vfio_ccw: Fix out of bounds check on CCW array
Commit: 0282fb1c4b638eecfe2cc558092c460911d8f7e2
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=0282fb1c4b638eecfe2cc558092c460911d8f7e2
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-80550
Analysis date: Wed, 26 Aug 2026 11:04:47 -0400
ActionableScore: 5
ActionableScore lower bound: 3
Actionable bucket: Actionable Moderate at minimum
Manual review required: YES

Summary:
An off-by-one bounds check in s390 vfio_ccw can make the host kernel inspect a 257th CCW outside the valid 256-entry channel program array, creating a guest-influenced OOB read with possible host DoS and limited confidentiality concern.

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ABOUT THIS REPORT
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The original Linux kernel CVE announcement for CVE-2026-80550 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-80550

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-2026-80550	MODERATE	CHECK WITH IMPACT FROM ORIG NN IMPORTANT	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:L/PR:L/UI:N/S:C/C:L/I:N/A:H';CWE-125;CWE-193;*CWE-787;Other CVSS 'AV:L/AC:L/PR:L/UI:N/S:C/C:N/I:N/A:L';BEST CVSS score: '7.3';DESCR 'An off by one bounds check in s390 vfio_ccw ccwchain_calc_length can make the host kernel examine a possible 257th CCW even though the valid CCW array range is limited to 256 entries. This is an out of bounds read on the vfio_ccw channel program parsing path and is influenced by a guest controlled channel program. For the CVSS the PR:L is used because practical triggering requires a local guest context or equivalent access to a vfio_ccw device rather than unauthenticated network access. The issue is not network reachable. The base impact is limited availability risk because the patch shows an out of bounds read rather than a write or direct information disclosure. For the paranoid score, limited confidentiality and high availability impact are kept because this is a host kernel memory safety issue across a virtualization boundary and should not be automatically dismissed.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate at minimum (with actual score 4)	YES	WRITE OOB SIMPLEFIX LINUS  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 guest or local vfio device trigger: +1. The CCW chain is influenced by a guest channel program or an equivalent local user with access to the vfio_ccw device.
* Weak memory safety primitive: +1. The patch fixes an off-by-one array bounds issue where the host kernel may inspect a 257th CCW outside the valid 0..255 range.
* Privileged kernel/device-management path: +1. The bug is in s390 vfio_ccw channel-program parsing, a trusted host kernel path processing guest/device-facing input.
* Rare subsystem/configuration: -1. This is specific to s390 vfio_ccw virtualization and assigned channel I/O devices, not a broad default Linux path.
* Availability concern: +1. An OOB read in kernel space can plausibly cause a fault, KASAN report, or host-side crash depending on allocation layout, though the commit does not demonstrate a reliable panic.

Paranoid additional signals:

* Confidentiality impact plausible: +1. The primitive is read-only OOB and no direct disclosure path is shown, but it crosses a guest-to-host boundary and may let adjacent kernel memory influence validation behavior.
* Stronger DoS concern: +1. If the 257th access lands on invalid or protected memory, the host kernel can plausibly crash.

Not awarded:

* No remote/network reachability.
* No strong LPE plausibility. The patch does not show attacker-controlled reclaim, overwrite, callback control, type confusion, or refcount takeover.
* No strong corruption primitive. This is an OOB read, not an OOB write or UAF.
* No Dirty-Pipe-like/page-cache/shared-page ownership violation.

## 3. Reachability analysis

The likely attacker is a guest or local process able to submit a crafted s390 channel program through vfio_ccw. Host-side setup of vfio_ccw and device assignment is privileged, but once the device is exposed to a guest, the triggering input is guest-controlled. This is not network reachable and does not apply to ordinary systems outside s390 vfio_ccw virtualization. Namespaces do not meaningfully lower the host setup requirement, but the practical security boundary is guest-to-host, so this should not be treated as a purely host-root-only correctness bug.

Call-site confidence: medium. The changed function and surrounding parsing context are visible, but deeper callers and exact allocation layout of cp->guest_cp are not included.

## 4. Severity interpretation

This is stronger than an ordinary low-priority cleanup because it is a host kernel OOB read on a guest-influenced vfio path. Realistically, the demonstrated primitive is limited: the kernel reads one CCW beyond the allowed array and then returns an error for oversized chains. There is no evidence of an attacker-controlled write, object replacement, or direct information leak.

Conservative handling is Borderline Moderate. Paranoid handling is Actionable Moderate because guest-to-host memory safety bugs deserve manual review even when the visible primitive is only an OOB read.

## 5. One-sentence report phrase

An off-by-one bounds check in s390 vfio_ccw can make the host kernel inspect a 257th CCW outside the valid 256-entry channel program array, creating a guest-influenced OOB read with possible host DoS and limited confidentiality concern.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: this is a guest-controlled host kernel memory safety issue in a virtualization/device path. The current evidence supports OOB read and DoS risk, not LPE or arbitrary corruption, but the guest-to-host boundary makes auto-closure unsafe.

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

Patch: s390/vfio_ccw: Fix out of bounds check on CCW array
Commit: 0282fb1c4b638eecfe2cc558092c460911d8f7e2
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=0282fb1c4b638eecfe2cc558092c460911d8f7e2

Commit description:

The routine ccwchain_calc_length() counts the number of channel
command words (CCWs) that are chained together in a single channel
program, and rejects anything larger than CCWCHAIN_LEN_MAX (256) CCWs.

The loop itself is "do..while (count < 257)", and while the logic in
is_cpa_within_range() correctly adjusts between the 0-index array of
CCWs and the count of CCWs starting at 1, this means it would look
at a possible 257th CCW before ending the loop and (correctly)
returning an error.

Fix this by restructuring the loop to break as soon as 256 CCWs
(thus indexes 0-255) are examined, without looking at memory
outside the range.

Fixes: 0a19e61 ("vfio: ccw: introduce channel program interfaces")
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

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=0282fb1c4b638eecfe2cc558092c460911d8f7e2

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

The original Linux kernel CVE announcement for CVE-2026-80550 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-80550

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:N/I:N/A:L
  The Best / paranoid CVSS vector: AV:L/AC:L/PR:L/UI:N/S:C/C:L/I:N/A:H
  The Best / paranoid CVSS score: 7.3

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: 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).