[CVE-2026-89926][LOW] KVM: s390: Fix length check __import_wp_info()

[CVE-2026-89926][LOW] KVM: s390: Fix length check __import_wp_info()

[AL-KERNEL]

CVE: CVE-2026-89926
Priority: LOW
AL-KERNEL base severity: LOW
KPANIC flag: NO
Patch: KVM: s390: Fix length check __import_wp_info()
Commit: aef540194effd768bed0bd2e6a74fe135a3d7c3a
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=aef540194effd768bed0bd2e6a74fe135a3d7c3a
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89926
Analysis date: Wed, 16 Sep 2026 11:50:08 -0400
ActionableScore: 2
ActionableScore lower bound: 1
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-2026-89926 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89926

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-2026-89926	LOW	https://www.kernelcve.org/list/?q=CVE-2026-89926 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-681;CWE-789;**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 debugging accepts a user controlled 64 bit breakpoint length and stores a truncated value in an int field for bounds validation. The vulnerable path then uses the original untruncated length for kmalloc, so a local user with KVM access can request an oversized allocation larger than 4 GB and trigger a WARN or availability impact. For the CVSS the PR:L is used because triggering requires a local process that can access KVM ioctls, but it does not necessarily require full administrator privileges. The issue is not network reachable and does not provide a demonstrated memory corruption primitive. Impact is primarily local denial of service, with higher availability impact only when WARN conditions can panic the host or otherwise disrupt the system.';NO MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate (with actual score 2)	YES	OOB VIRT SIMPLEFIX HARDWARE LINUS  DECREASED_TO_LOW_FROM_MODERATE_BASED_ON_ACTIONABLESCORELESSTHAN3	NO	NO	checked

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

ActionableScore=2
ActionableScoreLower=1
PrimitiveClass=NONE
RealUAF=NO
HostAdminRequired=NO
NamespaceScopedPrivilege=NO
ImportantCandidate=NO
ManualReview=NO
AutoDowngradeAllowed=YES

1. ActionableScore

* Raw conservative score: 1
* Conservative score: 1
* 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 Memory layout invariant restoration. The patch makes allocation size use the same already validated value, wp_info->len, instead of the original user controlled 64 bit bp_data->len.
* +1 paranoid only for conditional availability risk. The oversized allocation is expected to fail and produce a WARN, but in configurations where WARN is fatal, this can become host DoS.

Non-awarded signals:

* No UAF, stale reference, double free, refcount mutation, OOB write, or arbitrary write is established.
* No controlled reclaim or attacker controlled kernel object replacement is demonstrated.
* No confidentiality or integrity impact is awarded. The evidence supports failed oversized allocation and WARN, not disclosure or corruption.
* No KVM guest to host exploitation primitive is awarded. The attacker model is local host userspace or VMM access to KVM ioctls, not a demonstrated malicious guest escape.
* No extra availability point is awarded for the same WARN condition beyond the paranoid conditional DoS interpretation.
* No manual review escalation is needed because this is validation and allocation size mismatch without a concrete corruption primitive.

3. Reachability analysis

The vulnerable value is controlled by userspace through KVM s390 guest debugging state. The realistic attacker is a local host userspace process or VMM process with access to the relevant KVM ioctls. This is below host administrator in common KVM deployments where /dev/kvm access is delegated, so HostAdminRequired=NO. It is not a pure guest only trigger because the supplied evidence points to host userspace providing struct kvm_hw_breakpoint data. It is not network reachable.

Namespace scoped privilege is not established and does not appear relevant to this s390 KVM ioctl path, so NamespaceScopedPrivilege=NO. No race is involved. No special timing requirement is involved. The architecture is s390 specific, but the issue is in KVM guest debugging and no rare physical hardware penalty is applied from the supplied evidence.

4. Primitive and severity interpretation

The exact primitive is an integer truncation and validation mismatch. A user controlled __u64 length is truncated into an int for bounds checking, but the original untruncated length is then used for kmalloc. This allows a request for an oversized allocation larger than 4 GB that fails and triggers a WARN due to MAX_PAGE_ORDER.

PrimitiveClass=NONE because the patch does not establish memory corruption, lifetime violation, UAF, authorization bypass, semantic reinterpretation, or ownership violation. The demonstrated consequence is WARN and possible local availability impact. The speculative consequence is stronger DoS only on systems where WARN is fatal or where such allocation warnings disrupt service. There is no demonstrated leverage beyond crash or warning behavior. ImportantCandidate=NO because no Important preserving primitive is established. AutoDowngradeAllowed=YES because this is a constrained validation/resource allocation issue without unresolved corruption or boundary bypass.

5. One-sentence report phrase

KVM/s390 guest debugging used a user controlled untruncated breakpoint length for kmalloc after validating only a truncated int value, allowing local KVM userspace to trigger oversized allocation failure and WARN based denial of service.

6. Manual review recommendation

NO MANUAL CHECK NEEDED

Reason: the patch supports an integer truncation and oversized allocation WARN only, with no UAF, OOB write, controlled corruption, authorization bypass, or demonstrated privilege escalation.

7. Call-site confidence

Call-site confidence: high

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

Patch: KVM: s390: Fix length check __import_wp_info()
Commit: aef540194effd768bed0bd2e6a74fe135a3d7c3a
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=aef540194effd768bed0bd2e6a74fe135a3d7c3a

Commit description:

struct kvm_hw_breakpoint::len is a __u64 that is fully controlled by user
space. This is then assigned to wp_info->len, which is an int. The bounds
check is done on the truncated value while the allocation uses the
untruncated one:

	wp_info->len = bp_data->len;
	[...]
	if (wp_info->len < 0 || wp_info->len > MAX_WP_SIZE)
		return -EINVAL;

	wp_info->old_data = kmalloc(bp_data->len, GFP_KERNEL_ACCOUNT);

Use the validated value for the allocation as intended. Without this
fix userspace can trigger >4GB allocations which will fail and result
in a WARN due to MAX_PAGE_ORDER.

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: Claudio Imbrenda <imbrenda@linux.ibm.com>
Signed-off-by: Claudio Imbrenda <imbrenda@linux.ibm.com>
Message-ID: <20260805110455.7200-9-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=aef540194effd768bed0bd2e6a74fe135a3d7c3a

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

The original Linux kernel CVE announcement for CVE-2026-89926 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89926

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).