* [CVE-2026-89921][MODERATE 7.0] KVM: s390: Zero initialize data structures for inject_pfault_token [ Upstream
@ 2026-09-16 14:58 21% AL-KERNEL
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From: AL-KERNEL @ 2026-09-16 14:58 UTC (permalink / raw)
To: kernel-cve
CVE: CVE-2026-89921
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: KVM: s390: Zero initialize data structures for inject_pfault_token [ Upstream
Commit: 9b046de62b8098af6e2ba820b125ec9dc5f162c8
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=9b046de62b8098af6e2ba820b125ec9dc5f162c8
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89921
Analysis date: Wed, 16 Sep 2026 10:58:06 -0400
ActionableScore: 5
ActionableScore lower bound: 4
Actionable bucket: Actionable Moderate / Strong Important candidate requiring manual review
Manual review required: YES
Summary:
No one-sentence report phrase was found.
======================================================================
ABOUT THIS REPORT
======================================================================
The original Linux kernel CVE announcement for CVE-2026-89921 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89921
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.
======================================================================
AL-KERNEL CLASSIFICATION RESULT
======================================================================
CVE-2026-89921 MODERATE https://www.kernelcve.org/list/?q=CVE-2026-89921 CHECK WITH IMPACT FROM ORIG NN LOW Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:N/A:N';*CWE-457;*CWE-200;CWE-665;Other CVSS 'AV:L/AC:L/PR:L/UI:N/S:U/C:L/I:N/A:N';BEST CVSS score: '5.5';DESCR 'KVM s390 pfault token injection used on stack interrupt structures without full zero initialization. Only selected fields were set, while the larger ext state copied for injection could retain stale kernel stack values in ext_params or padding. Interrupt delivery to the guest uses ext_params2, so the direct guest delivery path is not the leak path. A local host user with access to KVM migration ioctls can retrieve the affected state and may observe stale kernel stack data. For the CVSS the PR:L is used because triggering and reading the data requires a local host user or VM management process with access to KVM migration ioctls, not full host root privileges. The issue is not network reachable by itself. Impact is primarily confidentiality loss. The base score uses C:L due to the limited exposed data, while the paranoid score uses C:H because kernel stack disclosure can expose sensitive values or pointers and should be reviewed manually.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate / Strong Important candidate requiring manual review (with actual score 5) YES VIRT SIMPLEFIX LEAK DMAorINTERRUPT HARDWARE LINUS KPANIC INCREASED_FROM_LOW_BASED_ON_REQUIREMANUALCHECK NO NO checked
======================================================================
ACTIONABLESCORE ANALYSIS
======================================================================
ActionableScore=5
ActionableScoreLower=4
PrimitiveClass=CLASS_A
RealUAF=NO
HostAdminRequired=NO
NamespaceScopedPrivilege=NO
ImportantCandidate=YES
ManualReview=REQUIRED
AutoDowngradeAllowed=NO
1. ActionableScore
* Raw conservative score: 4
* Conservative score: 4
* Raw paranoid score: 5
* Paranoid score: 5
* Final recommended bucket: **Actionable Moderate / Strong Important candidate requiring manual review**::
No floor or cap was applied.
2. Signal breakdown
Conservative signals:
* +2 Concrete kernel stack disclosure primitive. The patch establishes that uninitialized fields from on-stack `struct kvm_s390_irq` and `struct kvm_s390_interrupt` can be copied into KVM state and later retrieved through migration ioctls.
* +1 Confidentiality plausible. The disclosed data are stale kernel stack values, which may include sensitive kernel data or pointers depending on stack contents.
* +1 KVM host userspace exposure path. The leak is not only internal kernel consumption. A host user with access to KVM migration ioctls can read the affected state.
Paranoid additional signal:
* +1 Higher confidentiality concern. Kernel stack disclosure can be security-relevant beyond the exact small fields because stale stack values may expose pointers or other values useful for bypassing hardening or chaining with other bugs.
Non-awarded signals:
* RealUAF not awarded. No use-after-free or stale lifetime object use is established.
* Controlled reclaim not demonstrated.
* Arbitrary read or arbitrary write not demonstrated.
* Integrity impact not awarded. The patch shows disclosure of stale stack data, not unauthorized modification.
* Availability impact not awarded. No crash or DoS is described.
* Guest-to-host exploitation leverage not awarded. The commit explicitly says interrupt delivery uses `ext_params2`, so the direct guest interrupt path does not leak to the guest.
* Remote reachability not awarded. The issue is reached through local host KVM migration ioctls, not network traffic.
* Host-admin penalty not applied. The supplied evidence says a host user with migration ioctl access can retrieve the data, and full host administrator privilege is not positively established as required.
3. Reachability analysis
The evaluated attacker is a local host user or VMM process with access to KVM migration ioctls for an s390 KVM VM.
A malicious guest alone is not established as able to retrieve the leaked data. The commit states that interrupt delivery only uses `ext_params2`, so the guest delivery path is not the disclosure path.
A local VMM or host process with the relevant KVM migration ioctl access can read the affected saved interrupt state. This is below full host-admin in the supplied evidence, so HostAdminRequired=NO.
The issue is not namespace scoped. No user namespace or network namespace capability chain is involved.
No network trigger is established. No special remote packet path is involved.
There is an architecture and subsystem condition: affected systems must use KVM on s390 with the relevant pfault token injection and migration state handling. This limits exposure, but it is not scored as a negative because the patch context does not establish rare physical access or a hard-to-reach configuration beyond normal s390 KVM use.
4. Primitive and severity interpretation
The exact primitive is an uninitialized kernel stack information disclosure into KVM migration-visible state.
This is not an active structured UAF, not fixed stale storage, and not a repeated release or refcount mutation. There is no first lifetime operation that may free an object and no second lifetime mutation through a dangling reference.
PrimitiveClass=CLASS_A is used because a direct kernel stack disclosure to userspace is established. It is not memory corruption, but it is a concrete security primitive.
The demonstrated consequence is confidentiality loss. The practical base impact is likely limited disclosure of stale stack fields. The paranoid impact is higher because kernel stack values may include pointers or sensitive transient data.
ImportantCandidate=YES because kernel stack disclosure through KVM migration ioctls can be security relevant and should not be automatically dismissed as ordinary low-risk cleanup.
AutoDowngradeAllowed=NO because the established disclosure primitive would be lost by an automatic downgrade, and manual review is needed to determine exposed bytes, affected ioctl paths, and whether kernel pointers or other sensitive values are reachable.
Operational severity is Actionable Moderate, with Strong Important review potential if the migration state can expose substantial kernel stack contents or useful kernel pointers in realistic deployments.
5. One-sentence report phrase
KVM s390 pfault token injection failed to zero initialize stack based interrupt structures, allowing a local host user with KVM migration ioctl access to retrieve stale kernel stack values from migration visible state.
6. Manual review recommendation
MANUAL CHECK REQUIRED
Reason: this is a concrete KVM kernel stack information leak to host userspace via migration ioctls, and the sensitivity depends on the exact exposed fields, ioctl accessibility, and whether leaked stack values include kernel pointers or other useful secrets.
7. Call-site confidence
Call-site confidence: high
======================================================================
UPSTREAM PATCH SUMMARY
======================================================================
Patch: KVM: s390: Zero initialize data structures for inject_pfault_token [ Upstream
Commit: 9b046de62b8098af6e2ba820b125ec9dc5f162c8
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=9b046de62b8098af6e2ba820b125ec9dc5f162c8
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=9b046de62b8098af6e2ba820b125ec9dc5f162c8
======================================================================
DETAILED REPORT METHODOLOGY
======================================================================
The original Linux kernel CVE announcement for CVE-2026-89921 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89921
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:L/I:N/A:N
The Best / paranoid CVSS vector: AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:N/A:N
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: 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: YES
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 [email protected] (and both
send reply to CVE record itself too and see "reply" button below for howto reply).
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