[CVE-2026-80554][MODERATE 7.0] s390/vfio_ccw: Limit the number of channel program segments

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

CVE: CVE-2026-80554
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: s390/vfio_ccw: Limit the number of channel program segments
Commit: 15fb4559a7fdf0b8725e433a71cfd03a1313a48b
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=15fb4559a7fdf0b8725e433a71cfd03a1313a48b
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-80554
Analysis date: Wed, 26 Aug 2026 11:51:59 -0400
ActionableScore: 5
ActionableScore lower bound: 4
Actionable bucket: Actionable Moderate at minimum
Manual review required: YES

Summary:
s390 vfio_ccw failed to limit TIC-linked channel program segments, allowing a malicious guest-controlled channel program to force excessive recursive host-kernel processing and potentially cause host-side denial of service.

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ABOUT THIS REPORT
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The original Linux kernel CVE announcement for CVE-2026-80554 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-80554

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-2026-80554	MODERATE	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:U/C:L/I:L/A:H';CWE-674;**CWE-400;CWE-770;Other CVSS 'AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '5.8';DESCR 's390 vfio_ccw processes guest channel programs recursively and did not limit the number of channel program segments created by TIC CCWs. A malicious or malformed guest channel program could force excessive recursive processing and potentially exhaust host kernel stack or stall the vfio_ccw processing path, causing host side denial of service. For the CVSS the PR:L is used for the paranoid score because triggering requires control over a guest or process that can submit channel programs through vfio_ccw, but it does not require full host administrator privileges. The issue is not network reachable and is limited to s390 virtualization setups using vfio_ccw device passthrough. Impact is at least local denial of service via host kernel crash or hang and in worst case may allow limited confidentiality or integrity impact due to kernel stack exhaustion risk and should be reviewed manually.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate at minimum (with actual score 5)	YES	SIMPLEFIX HARDWARE LINUS  INCREASED_FROM_LOW_BASED_ON_REQUIREMANUALCHECK DECREASED_TO_MODERATEREG_BASED_ON_FALSEPOSCHECKOFKP	NO	NO	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=5
ActionableScoreLower=4

## 1. ActionableScore

* Conservative score: 4
* Paranoid score: 5
* Final recommended bucket: **Actionable Moderate at minimum**::

## 2. Signal breakdown

Conservative signals:

* Local or guest-controlled trigger: +1
  A malicious guest or process able to submit vfio_ccw channel programs can influence the number of TIC-linked segments.

* Reliable kernel crash / strong DoS: +1
  Recursive processing of too many channel program segments can exhaust stack or stall the vfio_ccw processing path.

* Cross-boundary isolation impact: +1
  The trigger originates from a guest-controlled channel program but affects host-side vfio_ccw kernel logic, so the impact crosses a virtualization boundary.

* Privileged kernel/device-management path: +1
  Guest-supplied channel program structure is parsed inside trusted host kernel s390 vfio_ccw device-management code.

* Availability impact realistic: +1
  The realistic consequence is host-side crash, hang, or serious vfio_ccw processing failure, not just a local process failure.

Negative signals:

* Rare and configuration-dependent subsystem: -1
  Exposure is limited to s390 systems using vfio_ccw device passthrough, not a broad default Linux deployment.

Paranoid-only signal:

* Weak indirect corruption candidate: +1
  Excessive recursion in host kernel context can create stack exhaustion risk, but the patch does not show a controlled overwrite, UAF reclaim, callback control, or arbitrary write primitive.

## 3. Reachability analysis

The bug is triggerable by a malicious or malformed guest channel program when vfio_ccw is used for s390 channel device passthrough. It is not network reachable. It does not require full host root privileges once the guest or delegated vfio_ccw user has the ability to submit channel programs, but initial device assignment normally requires host administrative setup.

Namespaces and containers do not appear to be the main exposure model. The more relevant boundary is guest-to-host isolation. The path is not default-enabled on ordinary systems and requires s390 virtualization with vfio_ccw, but in that environment the guest-controlled input reaches host kernel parsing logic.

Call-site confidence: medium. The patch shows the recursive channel-program segmentation logic and the added limit, but not the full external submission path.

## 4. Severity interpretation

This is stronger than an ordinary Moderate resource bug because guest-controlled input can drive recursive host-kernel processing across a virtualization boundary. The demonstrated realistic impact is denial of service, not proven privilege escalation.

Theoretical memory corruption potential is limited to stack exhaustion concerns. The patch does not support strong LPE scoring because it does not show UAF, controlled reclaim, arbitrary write, type confusion, callback dispatch, or attacker-controlled object replacement. Conservative handling is Borderline to Actionable Moderate. Paranoid handling reaches Actionable Moderate and should not be auto-closed.

## 5. One-sentence report phrase

s390 vfio_ccw failed to limit TIC-linked channel program segments, allowing a malicious guest-controlled channel program to force excessive recursive host-kernel processing and potentially cause host-side denial of service.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

Manual review is required because this is a guest-controlled input path into host kernel vfio_ccw logic and the realistic DoS is clear, while the paranoid stack-exhaustion impact needs human confirmation before deciding whether it remains DoS-only or deserves higher severity.

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

Patch: s390/vfio_ccw: Limit the number of channel program segments
Commit: 15fb4559a7fdf0b8725e433a71cfd03a1313a48b
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=15fb4559a7fdf0b8725e433a71cfd03a1313a48b

Commit description:

The processing of channel programs, and the CCWs within them, is done
recursively. As such, there is an arbitrary (but not architectural)
limit to the number of CCWs that can exist in a single channel program.

The vfio-ccw logic breaks these channel programs into segments whenever
it encounters a Transfer-In-Channel (TIC) CCW, and the combined number
of segments count towards the global limit. Impose an equivalent limit
to the number of segments until such logic can be made non-recursive.

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
  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=15fb4559a7fdf0b8725e433a71cfd03a1313a48b

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

The original Linux kernel CVE announcement for CVE-2026-80554 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-80554

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:U/C:N/I:N/A:H
  The Best / paranoid CVSS vector: AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:H
  The Best / paranoid CVSS score: 5.8

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: 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).