[CVE-2026-89923][LOW] KVM: s390: Free guest debug data on vcpu destroy

[CVE-2026-89923][LOW] KVM: s390: Free guest debug data on vcpu destroy

[AL-KERNEL]

CVE: CVE-2026-89923
Priority: LOW
AL-KERNEL base severity: LOW
KPANIC flag: NO
Patch: KVM: s390: Free guest debug data on vcpu destroy
Commit: 053f2068cf9b0654b651fed5ad7649d3deb982bf
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=053f2068cf9b0654b651fed5ad7649d3deb982bf
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89923
Analysis date: Wed, 16 Sep 2026 12:02:10 -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-2026-89923 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89923

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-2026-89923	LOW	https://www.kernelcve.org/list/?q=CVE-2026-89923 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 data can be leaked when a vCPU is destroyed while hardware breakpoints or watchpoints are still armed. The leak covers hw_bp_info, hw_wp_info, and old_data buffers, and is bounded per vCPU but unbounded across repeated VM lifetimes. For the CVSS the PR:L is used because an attacker needs local ability to run a VMM with /dev/kvm access and enable guest debugging, but does not need full host root in common KVM deployments. The issue is not network reachable by itself. Impact is denial of service through kernel memory or memcg charge exhaustion, with no evidence of UAF, OOB access, information disclosure, or privilege escalation.';NO MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate (with actual score 2)	YES	VIRT INIT SIMPLEFIX LEAK 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

Conservative signals:

* +1 Concrete persistent kernel resource/reference leak. Guest debug allocations for hw_bp_info, hw_wp_info, and old_data buffers are not released when the vCPU is destroyed.
* +1 Persistent accumulation / memcg persistence. The leak is bounded per vCPU but unbounded across VM lifetimes, and GFP_KERNEL_ACCOUNT charges can outlive the exiting VMM process and pin dying memcgs.
* -1 Reduced/delegated local privilege. Triggering requires a local VMM process with /dev/kvm access and use of KVM guest debugging, but host root is not positively required in common KVM deployments.

Paranoid additional signal:

* +1 Sustained resource exhaustion concern. Repeated VM/vCPU lifetimes can realistically accumulate the leak enough to cause memory or memcg pressure, although this remains resource exhaustion rather than corruption.

Non-awarded signals:

* No UAF. The patch fixes missing cleanup of allocated debug data, not stale post-free use.
* No controlled reclaim, arbitrary write, OOB access, type confusion, callback control, or information disclosure is established.
* No KVM guest-to-host exploitation primitive is established. This is local VMM-triggered resource leakage, not a guest-only host corruption path.
* No reliable immediate kernel crash is directly established. DoS is via accumulated resource exhaustion.
* No extra availability point for the same leak beyond the paranoid sustained-exhaustion interpretation.
* No confidentiality or integrity impact is awarded.

3. Reachability analysis

The evaluated attacker is a local VMM or local host process with access to /dev/kvm and the ability to enable guest debugging for KVM s390 vCPUs. The trigger is not guest-only, because the relevant operation is host-side KVM guest debug setup and vCPU destruction. Host administrator privilege is not positively required, since /dev/kvm is commonly delegated to non-root virtualization users, but ordinary unprivileged reachability is not established either.

Namespace-scoped privilege is not involved. No network trigger is present. No special race is required. The issue is architecture-specific to s390 KVM and requires KVM guest debugging use, but the supplied evidence does not justify a rare-hardware penalty beyond the already low practical score.

4. Primitive and severity interpretation

The exact primitive is a missing release of KVM s390 guest debug allocations during vCPU destruction. This is a persistent resource leak, not memory corruption. The stale resources are not used after free, and there is no second release/refcount mutation through a dangling object.

PrimitiveClass is NONE because no meaningful corruption, authorization bypass, ownership violation, or UAF primitive is established. The demonstrated consequence is resource exhaustion over repeated VM lifetimes. The speculative consequence is broader memory or memcg pressure if an attacker can repeat the lifecycle enough times.

ImportantCandidate is NO because the issue lacks a corruption or boundary-bypass primitive. AutoDowngradeAllowed is YES because the bug is a resource leak with no unresolved exploitation leverage beyond DoS by accumulation. Likely operational severity is Low to ordinary Moderate.

5. One-sentence report phrase

KVM s390 guest debug data may leak when a vCPU is destroyed with hardware breakpoints or watchpoints still armed, allowing a local VMM with /dev/kvm access to cause bounded per vCPU but accumulated kernel memory and memcg resource exhaustion over repeated VM lifetimes.

6. Manual review recommendation

NO MANUAL CHECK NEEDED

Reason: this is a concrete persistent resource leak, but there is no UAF, memory corruption, authorization bypass, information disclosure, privilege escalation, or network exposure.

7. Call-site confidence

Call-site confidence: high

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

Patch: KVM: s390: Free guest debug data on vcpu destroy
Commit: 053f2068cf9b0654b651fed5ad7649d3deb982bf
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=053f2068cf9b0654b651fed5ad7649d3deb982bf

Commit description:

kvm_s390_clear_bp_data() is only called from
kvm_arch_vcpu_ioctl_set_guest_debug(), i.e. when user space changes or
disables debugging. A vCPU that is destroyed while hardware breakpoints
are still armed - the normal case when the VMM just exits or crashes -
leaks hw_bp_info, hw_wp_info and all old_data buffers, since generic KVM
frees the vCPU right after kvm_arch_vcpu_destroy().

That is bounded by MAX_BP_COUNT entries, so roughly 8 KiB per vCPU, but
it is unbounded over VM lifetimes. The allocations are
GFP_KERNEL_ACCOUNT, so the charge also outlives the exiting process and
pins dying memcgs.

Fix by clearing the debug data on vCPU destruction. Calling it
unconditionally is fine: struct kvm_vcpu is zero allocated, so for a vCPU
that never enabled debugging the counters are 0 and the pointers NULL.

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: <20260805110455.7200-8-borntraeger@linux.ibm.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  arch/s390/kvm/kvm-s390.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=053f2068cf9b0654b651fed5ad7649d3deb982bf

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

The original Linux kernel CVE announcement for CVE-2026-89923 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89923

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).