[CVE-2026-89925][LOW] KVM: s390: Fix memory leak in guest debug handling

[CVE-2026-89925][LOW] KVM: s390: Fix memory leak in guest debug handling

[AL-KERNEL]

CVE: CVE-2026-89925
Priority: LOW
AL-KERNEL base severity: LOW
KPANIC flag: NO
Patch: KVM: s390: Fix memory leak in guest debug handling
Commit: 39e7cebb889839b5762c9ad22e2d715b5883b288
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=39e7cebb889839b5762c9ad22e2d715b5883b288
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89925
Analysis date: Wed, 16 Sep 2026 14:37:55 -0400
ActionableScore: 2
ActionableScore lower bound: unknown
Actionable bucket: Ordinary Moderate
Manual review required: NO

Summary:
No one-sentence report phrase was found.

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

The original Linux kernel CVE announcement for CVE-2026-89925 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89925

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.

======================================================================
AL-KERNEL CLASSIFICATION RESULT
======================================================================

CVE-2026-89925	LOW	https://www.kernelcve.org/list/?q=CVE-2026-89925 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-401;*CWE-772;**CWE-400;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 guest debug handling leaks bp_data on every successful KVM_SET_GUEST_DEBUG call because the temporary buffer is only freed on the error path. A local user or process with access to /dev/kvm can repeatedly use the guest debug ioctl and gradually consume kernel memory, causing denial of service through resource exhaustion. For the CVSS the PR:L is used because reliable triggering requires local KVM ioctl access, but not necessarily full administrator privileges on systems where /dev/kvm is delegated to the kvm group or a virtualization service account. The issue is not network reachable by itself and requires local code execution or control of a local virtualization process. Impact is availability loss only. There is no evidence of memory corruption, use after free, information disclosure, or privilege escalation from this patch context.';NO MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate (with actual score 2) 	MAYBE	VIRT LEAK UAF HARDWARE LINUS  DECREASED_TO_LOW_FROM_MODERATE_BASED_ON_ACTIONABLESCORELESSTHAN3	-	-	checked

======================================================================
ACTIONABLESCORE ANALYSIS
======================================================================

ActionableScore=2
PrimitiveClass=CLASS_C
RealUAF=NO
HostAdminRequired=NO
NamespaceScopedPrivilege=NO
ImportantCandidate=NO
ManualReview=NO
AutoDowngradeAllowed=YES

1. ActionableScore

* Raw conservative score: 2
* Conservative score: 2
* Raw paranoid score: 2
* Paranoid score: 2
* Final recommended bucket: **Ordinary Moderate**::

No floor or cap changed the score.

2. Signal breakdown

Positive signals:

* +1 Concrete persistent kernel resource/reference leak. The patch directly adds `kfree(bp_data)` on the successful return path, and the commit states that every successful `KVM_SET_GUEST_DEBUG` leaks `bp_data`.
* +1 Meaningful sustained resource exhaustion potential. Repeated successful debug ioctls can accumulate leaked kernel memory and may eventually cause availability impact through memory pressure.

Non-awarded signals:

* Real UAF not awarded. The bug is a missing free on success, not post-free use.
* Concrete repeated release/refcount not awarded. There is no first release followed by a second lifetime mutation on the same dangling reference.
* Strong corruption not awarded. No overwrite, controlled reclaim, type confusion, arbitrary write, callback control, or refcount takeover is shown.
* KVM guest-to-host exploitation leverage not awarded. The trigger is a host-side VMM or local process ioctl path, not a demonstrated malicious guest to host corruption primitive.
* Confidentiality and integrity not awarded. The patch supports only a memory leak of kernel allocation accounting, not information disclosure or unauthorized modification.
* Reliable immediate kernel crash not awarded. DoS is possible through accumulation, not from a single demonstrated panic path.
* Remote reachability not awarded. This is not network-triggerable by itself.

3. Reachability analysis

The direct trigger is a local host process with access to KVM ioctls, specifically a VMM or process that can use `/dev/kvm`, create or control a vCPU, and invoke `KVM_SET_GUEST_DEBUG` on s390. This is below host-admin-equivalent in deployments where `/dev/kvm` is delegated to the `kvm` group or to a virtualization service account, so HostAdminRequired is NO. It is not ordinary network reachability and not guest-only reachability.

NamespaceScopedPrivilege is NO because this is not a namespace-scoped capability path. Access is governed by host device permissions and the KVM file descriptors rather than attacker-created user or network namespaces. The issue is architecture-specific to s390 KVM guest debug handling, but no separate rare-hardware penalty is applied for affected s390 virtualization hosts. There is no race requirement.

4. Primitive and severity interpretation

The exact primitive is a persistent kernel memory leak of `bp_data` on the successful `KVM_SET_GUEST_DEBUG` path. This is a resource-management defect, not active structured UAF and not fixed stale storage. There is no cleanup/dereference UAF and no broader post-lifetime object consumption.

PrimitiveClass is CLASS_C because the demonstrated behavior is resource exhaustion and DoS-like availability impact without an independently useful corruption primitive. The demonstrated consequence is gradual kernel memory consumption. The speculative consequence is system availability degradation or failure under sustained repeated calls. There is no supported privilege escalation, guest-to-host escape, arbitrary read/write, or security-boundary bypass.

ImportantCandidate is NO because this is a local resource leak with no memory corruption and no unresolved exploitation leverage beyond exhaustion. AutoDowngradeAllowed is YES because automatic Important-style preservation would not be justified by the supplied patch context.

5. One-sentence report phrase

KVM s390 guest debug handling leaks bp_data on every successful KVM_SET_GUEST_DEBUG call, allowing a local process with /dev/kvm access to consume kernel memory over repeated calls and cause a resource exhaustion DoS.

6. Manual review recommendation

NO MANUAL CHECK NEEDED

This can be handled as an ordinary local resource leak because there is no UAF, no repeated refcount mutation, no controlled corruption, no network reachability, and no privilege escalation evidence.

7. Call-site confidence

Call-site confidence: high

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

Patch: KVM: s390: Fix memory leak in guest debug handling
Commit: 39e7cebb889839b5762c9ad22e2d715b5883b288
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=39e7cebb889839b5762c9ad22e2d715b5883b288

Commit description:

bp_data is freed only for the error case by kfree(bp_data).
Every successful KVM_SET_GUEST_DEBUG will leak bp_data.

Fixes: 27291e2 ("KVM: s390: hardware support for guest debugging")
Cc: stable@vger.kernel.org
Signed-off-by: Christian Borntraeger <borntraeger@linux.ibm.com>
Reviewed-by: Matthew Rosato <mjrosato@linux.ibm.com>
Reviewed-by: Claudio Imbrenda <imbrenda@linux.ibm.com>
Signed-off-by: Claudio Imbrenda <imbrenda@linux.ibm.com>
Message-ID: <20260805110455.7200-5-borntraeger@linux.ibm.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  arch/s390/kvm/guestdbg.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=39e7cebb889839b5762c9ad22e2d715b5883b288

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

The original Linux kernel CVE announcement for CVE-2026-89925 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89925

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: unknown
  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).