[CVE-2026-74515][LOW] KVM: s390: pci: Reject adapter interrupt forwarding if already enabled

[CVE-2026-74515][LOW] KVM: s390: pci: Reject adapter interrupt forwarding if already enabled

[AL-KERNEL]

CVE: CVE-2026-74515
Priority: LOW
AL-KERNEL base severity: LOW
KPANIC flag: NO
Patch: KVM: s390: pci: Reject adapter interrupt forwarding if already enabled
Commit: 78d9648e7e960546d5b72504a0b0358cd8bb1e9d
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=78d9648e7e960546d5b72504a0b0358cd8bb1e9d
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74515
Analysis date: Sat, 15 Aug 2026 18:50:26 -0400
ActionableScore: 2
ActionableScore lower bound: 1
Actionable bucket: Ordinary Moderate / Low-like
Manual review required: NO

Summary:
KVM s390 zPCI adapter interrupt forwarding could be enabled multiple times for the same device, overwriting existing forwarding state and leaking host resources, with local KVM device-management access required and denial of service as the expected impact.

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

The original Linux kernel CVE announcement for CVE-2026-74515 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74515

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: LOW
Manual review required: NO

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-2026-74515	LOW	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:U/C:N/I:N/A:H';*CWE-772;**CWE-400;CWE-404;Other CVSS 'AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:L';BEST CVSS score: '5.5';DESCR 'KVM s390 zPCI adapter interrupt forwarding could be enabled more than once for the same device even though the underlying MPCIFC flow requires unregister before another register operation. Repeated enable requests can overwrite the stored forwarding state and leak host resources associated with the previous registration. For the CVSS the PR:L is used because a local user or service with access to KVM device management can trigger the ioctl path, while full host root is not always a required practical assumption in KVM deployments. The issue is not network reachable and is triggered through the local KVM control plane rather than guest packet or storage traffic. Impact is denial of service through resource leakage, with no concrete evidence of memory corruption, information disclosure, or privilege escalation.';NO MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate / Low-like (with actual score 2)	YES	VIRT SIMPLEFIX LEAK HARDWARE LINUS  DECREASED_TO_LOW_FROM_MODERATE_BASED_ON_ACTIONABLESCORELESSTHAN3	NO	NO	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=2
ActionableScoreLower=1

## 1. ActionableScore

* Conservative score: 1
* Paranoid score: 2
* Final recommended bucket: **Ordinary Moderate / Low-like**

## 2. Signal breakdown

Triggered signals:

* Local privileged or delegated KVM trigger: +1
  The issue is reached through a KVM s390 zPCI control path, likely via an ioctl or VM device-management operation.
* Availability impact realistic: +1 in paranoid scoring
  Repeated enable requests can overwrite existing AIF state and leak host resources, so sustained abuse may cause resource exhaustion.
* Rare and configuration-dependent subsystem: -1 in conservative scoring
  This requires s390 KVM with zPCI adapter interrupt forwarding, which is not a broad default Linux exposure.

Not triggered:

* Remote reachable: +0
  No network or guest packet path is shown.
* Firmware-mediated external influence: +0
  The trigger is a local KVM control-plane request, not firmware-fed input.
* Generic memory corruption: +0
  The patch describes overwritten registration state and leaked resources, not UAF, OOB write, double free, type confusion, or arbitrary write.
* Privilege escalation plausible: +0
  No concrete LPE primitive is supported by the patch.
* Strong DoS: +0 conservative, weakly +1 only in paranoid availability interpretation
  Resource exhaustion is plausible through repeated calls, but no immediate crash or panic path is demonstrated.

Call-site confidence: medium. The patch shows the guarded function and the state check, but not the full ioctl call path or exact privilege checks.

## 3. Reachability analysis

The likely attacker is a local user, VM management process, or service account with access to KVM s390 zPCI device-management operations. In typical deployments this is not exposed to arbitrary remote users and is not reachable through packet traffic.

Namespaces or containers may matter if KVM device access is delegated to a reduced-privilege service or container root. In that case PR:L is more practical than PR:H, but the attacker still needs access to a specialized KVM device-management path.

The affected configuration is not common across all Linux systems. It requires s390 KVM and zPCI adapter interrupt forwarding. Realistic exploitation is mainly repeated ioctl abuse causing host resource leakage, not memory corruption or privilege escalation.

## 4. Severity interpretation

This behaves like an ordinary Moderate or Low-like resource-leak issue rather than an Important-class kernel vulnerability. The main impact is local denial of service through leaked adapter interrupt forwarding resources.

Theoretical concern is limited to state overwrite and resource leakage. Realistic exploitation evidence does not show UAF, stale callback, type confusion, attacker-controlled reclaim, arbitrary write, or cross-boundary data corruption.

## 5. One-sentence report phrase

KVM s390 zPCI adapter interrupt forwarding could be enabled multiple times for the same device, overwriting existing forwarding state and leaking host resources, with local KVM device-management access required and denial of service as the expected impact.

## 6. Manual review recommendation

NO MANUAL CHECK NEEDED

This can be auto-closed for default triage because it is a local, configuration-specific KVM resource leak without a demonstrated memory corruption or privilege-escalation primitive.

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

Patch: KVM: s390: pci: Reject adapter interrupt forwarding if already enabled
Commit: 78d9648e7e960546d5b72504a0b0358cd8bb1e9d
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=78d9648e7e960546d5b72504a0b0358cd8bb1e9d

Commit description:

The MPCIFC instruction doesn't allow registering adapter interrupts without
first unregistering. So reject any request to enable interrupt forwarding
if its already enabled for the zPCI device. This also fixes overwriting and
thus leaking resources when the ioctl is called multiple times for the same
device.

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=78d9648e7e960546d5b72504a0b0358cd8bb1e9d

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

The original Linux kernel CVE announcement for CVE-2026-74515 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74515

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:L
  The Best / paranoid CVSS vector: AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS score: 5.5

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: 1
  Paranoid ActionableScore: 2

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: LOW
KPANIC detected for this report: NO
Published priority for this report (same as in Subject): LOW

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).