* [CVE-2026-89920][MODERATE 7.0] KVM: s390: Fix memory corruption by not reinjecting CK machine checks
@ 2026-09-16 16:38 21% AL-KERNEL
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From: AL-KERNEL @ 2026-09-16 16:38 UTC (permalink / raw)
To: kernel-cve
CVE: CVE-2026-89920
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: KVM: s390: Fix memory corruption by not reinjecting CK machine checks
Commit: 6ee52d3af2a1bf70349cf3f860933c8d221e229a
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=6ee52d3af2a1bf70349cf3f860933c8d221e229a
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89920
Analysis date: Wed, 16 Sep 2026 12:38:16 -0400
ActionableScore: 5
ActionableScore lower bound: 4
Actionable bucket: Actionable Moderate / Strong Important candidate requiring manual review
Manual review required: YES
Summary:
KVM/s390 CK machine-check reinjection could pass stack allocated interrupt state into a path that may call kfree on it, causing host kernel memory corruption or crash and requiring manual review for possible impact beyond DoS.
======================================================================
ABOUT THIS REPORT
======================================================================
The original Linux kernel CVE announcement for CVE-2026-89920 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89920
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-2026-89920 MODERATE https://www.kernelcve.org/list/?q=CVE-2026-89920 CHECK WITH IMPACT FROM ORIG NN LOW Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:H';CWE-590;CWE-763;*CWE-664;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:H';BEST CVSS score: '7';DESCR 'KVM/s390 machine check reinjection can lead to host kernel memory corruption because CK channel-subsystem machine checks were incorrectly treated as guest deliverable events. The vulnerable path could pass a stack allocated kvm_s390_interrupt_info object into the VM wide injection path and later code may attempt kfree() on that non heap object, which can corrupt kernel memory or crash the host. For the CVSS the PR:L because an attacker would typically need the ability to run or control a KVM guest or VMM context on the host, but not full host root in common delegated virtualization setups. The issue is not network reachable by packet traffic and requires a s390 KVM environment plus a relevant machine check condition, so AC:H is used. Impact is at least denial of service via host crash and in worst case may allow confidentiality or integrity impact due to kernel memory corruption.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate / Strong Important candidate requiring manual review (with actual score 5) MAYBE DANGER SIMPLEFIX DMAorINTERRUPT HARDWARE LINUS KPANIC INCREASED_TO_MODERATE7_FROM_LOW_BASED_ON_GUESSCVSS_AND_ACTIONABLESCOREHIGHER4 - - checked
======================================================================
ACTIONABLESCORE ANALYSIS
======================================================================
ActionableScore=5
ActionableScoreLower=4
PrimitiveClass=CLASS_B
RealUAF=NO
HostAdminRequired=UNKNOWN
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 memory corruption: the commit explicitly says the old CK reinjection path could cause `kfree` of a stack variable.
* +1 Reliable host crash / strong DoS: invalid freeing of stack memory in host kernel context can realistically crash or corrupt the host kernel.
* +1 KVM host security boundary relevance: the corruption occurs in host KVM/s390 machine-check reinjection logic while handling guest-related virtualization state.
Paranoid additional signal:
* +1 Weak exploitation concern: `kfree` on non-heap stack storage is a concrete allocator/lifetime misuse, and the post-corruption consequence is not fully resolved from the patch alone.
Non-awarded signals:
* RealUAF not awarded: this is invalid free / non-heap free, not a demonstrated use-after-free.
* Strong LPE / CLASS_A not awarded: no controlled reclaim, arbitrary write, callback control, refcount takeover, or demonstrated privilege escalation is shown.
* Guest-to-host strong exploitation leverage not awarded: the patch establishes host kernel memory corruption, but not a concrete guest-controlled host-memory primitive.
* Remote reachability not awarded: no packet/network path is involved.
* Extra availability not awarded: the same host crash/corruption consequence is already counted once.
* Confidentiality and integrity bonuses not awarded conservatively: C/I consequences remain unresolved and are not concretely demonstrated.
## 3. Reachability analysis
The affected code is KVM on s390 and is reached through machine-check backup and reinjection handling. The relevant CK machine checks are channel-subsystem damage events for the host channel subsystem, and the patch prevents them from being forwarded into the guest reinjection path.
Attacker model is unresolved. A malicious guest alone is not clearly shown to be able to create the required host CK machine-check condition. A local VMM or process with KVM access may participate in the relevant KVM context, but the supplied evidence does not prove that ordinary unprivileged users can trigger the complete sequence. Host-admin-equivalent privilege is also not positively established as required, so `HostAdminRequired=UNKNOWN`.
Namespace-scoped privilege is not applicable from the supplied evidence. No network namespace, user namespace, or namespace-scoped capability path is established. There is no direct network trigger. The issue is architecture and KVM specific, but I did not subtract a rare-hardware penalty because the supplied evidence is for a real supported KVM/s390 host path rather than an optional peripheral quirk.
Race repeatability is unclear. The trigger depends on relevant machine-check conditions, so exploitation is likely constrained, but the patch does not establish a narrow attacker-controlled race window.
## 4. Primitive and severity interpretation
The exact primitive is an invalid free of stack memory: the previous CK path selected `inti.mchk`, where `inti` is a stack object, and passed it through VM-wide injection handling that could later free it as if it were heap-backed interrupt state.
This is not an ordinary stale cleanup dereference and not a classic UAF. It is a concrete kernel memory corruption / lifetime ownership bug involving non-heap memory being released through a heap-style lifetime path.
Primitive class is `CLASS_B`: a real kernel corruption primitive exists and needs exploitation-leverage review, but the patch does not establish controlled corruption, arbitrary read/write, controlled reclaim, or demonstrated guest-to-host escape.
Demonstrated consequence is host kernel corruption and likely host crash / DoS. Speculative consequence is possible stronger exploitation due to allocator corruption, but that is not proven. `ImportantCandidate=YES` because concrete host kernel memory corruption in KVM should not be automatically downgraded from a small patch alone. `AutoDowngradeAllowed=NO` because unresolved kernel corruption leverage would be lost by automatic closure.
Likely operational severity is actionable Moderate at minimum, with manual review needed to decide whether it reaches Important in the evaluated product and threat model.
## 5. One-sentence report phrase
KVM/s390 CK machine-check reinjection could pass stack allocated interrupt state into a path that may call kfree on it, causing host kernel memory corruption or crash and requiring manual review for possible impact beyond DoS.
## 6. Manual review recommendation
MANUAL CHECK REQUIRED
Reason: the patch explicitly fixes host kernel memory corruption via `kfree` of a stack variable in KVM/s390 code, and the exploitability beyond crash is unresolved.
## 7. Call-site confidence
Call-site confidence: medium
======================================================================
UPSTREAM PATCH SUMMARY
======================================================================
Patch: KVM: s390: Fix memory corruption by not reinjecting CK machine checks
Commit: 6ee52d3af2a1bf70349cf3f860933c8d221e229a
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=6ee52d3af2a1bf70349cf3f860933c8d221e229a
Commit description:
Channel-subsystem damage machine checks are for the host channel
subsystem. The guest channel subsystem is emulated in the userspace VMM.
There is no point in forwarding such machine checks into the guest.
This also simplifies the machine check reinjection and avoids kfree of a
stack variable as reported by sashiko. There might be still machine
checks that have the ck bit set with another bit (like instruction
damage), mask out the CK bit in s390_backup_mcck_info(), like the CP and
ED bits already are.
Fixes: 4d62fcc ("KVM: s390: Inject machine check into the guest")
Cc: [email protected]
Signed-off-by: Christian Borntraeger <[email protected]>
Acked-by: Heiko Carstens <[email protected]>
Acked-by: Claudio Imbrenda <[email protected]>
Signed-off-by: Claudio Imbrenda <[email protected]>
Message-ID: <[email protected]>
Signed-off-by: Greg Kroah-Hartman <[email protected]>
Changed files:
arch/s390/include/asm/nmi.h
arch/s390/kernel/nmi.c
arch/s390/kvm/interrupt.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=6ee52d3af2a1bf70349cf3f860933c8d221e229a
======================================================================
DETAILED REPORT METHODOLOGY
======================================================================
The original Linux kernel CVE announcement for CVE-2026-89920 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89920
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:H/PR:L/UI:N/S:U/C:L/I:L/A:H
The Best / paranoid CVSS vector: AV:L/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:H
The Best / paranoid CVSS score: 7
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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