* [CVE-2026-80921][IMPORTANT] KVM: s390: vsie: zero stale crypto bits
@ 2026-09-09 17:29 21% AL-KERNEL
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From: AL-KERNEL @ 2026-09-09 17:29 UTC (permalink / raw)
To: kernel-cve
CVE: CVE-2026-80921
Priority: IMPORTANT
AL-KERNEL base severity: IMPORTANT
KPANIC flag: YES
Patch: KVM: s390: vsie: zero stale crypto bits
Commit: d110b3297f11ef227098b8a82ade2d5f123b7d2f
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=d110b3297f11ef227098b8a82ade2d5f123b7d2f
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-80921
Analysis date: Wed, 09 Sep 2026 13:29:37 -0400
ActionableScore: 7
ActionableScore lower bound: 5
Actionable bucket: Strong Important candidate
Manual review required: YES
Summary:
No one-sentence report phrase was found.
======================================================================
ABOUT THIS REPORT
======================================================================
The original Linux kernel CVE announcement for CVE-2026-80921 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-80921
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: IMPORTANT
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-2026-80921 IMPORTANT https://www.kernelcve.org/list/?q=CVE-2026-80921 CHECK WITH IMPACT FROM ORIG NN MODERATE Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:H/PR:L/UI:N/S:C/C:H/I:H/A:H';CWE-665;CWE-863;CWE-284;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:C/C:L/I:L/A:L';BEST CVSS score: '7.8';DESCR 'KVM s390 vsie could leave stale crypto access bits in the shadow APCB when translating a format0 crypto control block. A nested guest may then gain access to a crypto device or queue that is no longer available to it, which is a virtualization access control failure rather than a simple crash. For the CVSS the PR:L is used because the attacker needs control over a guest or nested guest context, but does not need full host administrator privileges. The issue is not directly network reachable and requires a local virtualization path on an affected s390 KVM host with AP crypto and nested virtualization in use. Impact may include unauthorized crypto device access, possible confidentiality or integrity loss for crypto operations, and availability impact on the exposed device. YES REQUIRES MANUAL CHECK';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate (with actual score 6) YES VIRT SIMPLEFIX HARDWARE LINUS KPANIC INCREASED_TO_HIGH_BASED_ON_GUESSCVSS NO NO checked
======================================================================
ACTIONABLESCORE ANALYSIS
======================================================================
ActionableScore=7
ActionableScoreLower=5
PrimitiveClass=CLASS_A
RealUAF=NO
HostAdminRequired=NO
NamespaceScopedPrivilege=NO
ImportantCandidate=YES
ManualReview=REQUIRED
AutoDowngradeAllowed=NO
1. ActionableScore
* Raw conservative score: 5
* Conservative score: 5. No floor or cap changed the score.
* Raw paranoid score: 7
* Paranoid score: 7. No floor or cap changed the score.
* Final recommended bucket: **Strong Important candidate**::
2. Signal breakdown
Conservative signals:
* +2 Concrete authorization or security-boundary bypass. The patch states that stale crypto access bits can give a nested guest access to a device that is no longer available to it.
* +1 Cross-boundary virtualization policy impact. The affected state is KVM s390 vsie shadow crypto access control, so the bug crosses a guest or nested-guest resource-isolation boundary.
* +1 Confidentiality plausible. Unauthorized access to AP crypto resources can plausibly expose or enable use of crypto-related protected material or operations.
* +1 Integrity plausible. Unauthorized crypto device or queue access can plausibly allow operations that should have been denied by the current access mask.
Paranoid additional interpretation:
* +2 Protected crypto device or queue access treated as sensitive resource access. In the higher score, the stale APCB bits are treated as a practical protected-resource exposure, not only as abstract policy drift.
Non-awarded signals:
* RealUAF not awarded. This is stale authorization state in a shadow APCB, not a post-free object use.
* Memory corruption not awarded. The patch zeroes stale bits and does not show heap corruption, OOB write, arbitrary write, or controlled reclaim.
* Remote reachability not awarded. The issue is not directly network-triggerable.
* Strong arbitrary read or write not awarded. Unauthorized device access is established, but arbitrary host memory access is not.
* KVM guest-to-host memory corruption leverage not awarded. The evidence supports unauthorized nested guest crypto resource access, not a demonstrated guest-to-host memory corruption primitive.
* Reliable kernel crash not awarded. The described impact is unauthorized device access, not panic or host crash.
* Race penalty not applied. The bug is stale initialization or stale state, not a demonstrated narrow timing race.
* Hardware or niche deployment penalty not applied. s390 AP crypto and nested virtualization limit exposure, but within affected deployments the bug is a concrete security-boundary issue.
3. Reachability analysis
The evaluated attacker is a nested guest or guest-controlled context operating through KVM s390 vsie. The commit explicitly says that the stale bits can give a nested guest potential access to a device no longer available, so host administrator privilege is not required for the security-relevant consequence once the affected virtualization configuration exists.
This is not namespace-scoped Linux privilege. It is a virtualization boundary issue, not a network namespace or user namespace issue.
Prior host setup is required in practice: the host must be s390, KVM must be used, nested virtualization via vsie must be relevant, and AP crypto access control must be configured. Those prerequisites limit exposure but do not erase the boundary bypass for affected systems.
The trigger is not remote packet processing. It is local to the virtualization stack. No timing race is required by the patch evidence.
4. Primitive and severity interpretation
The exact primitive is stale security-relevant crypto access bits in the shadow APCB used for nested virtualization. The old bits 64..255 can remain from previous vsie page contents when shadowing a format0 APCB, allowing access that the current host and guest masks should not grant.
This is not active structured UAF and not fixed stale storage. It is a security-state initialization and authorization-state propagation bug.
PrimitiveClass is CLASS_A because the patch directly establishes a virtualization access-control failure: an intended device access denial can become unintended access due to stale authorization bits.
Demonstrated consequence: nested guest may access a crypto device or queue that is no longer available to it.
Speculative consequence: broader confidentiality, integrity, or availability impact depends on exact AP crypto device configuration and tenant/resource assignment. Arbitrary host compromise is not demonstrated.
ImportantCandidate=YES because the bug crosses a KVM virtualization security boundary and involves unauthorized access to crypto resources.
AutoDowngradeAllowed=NO because automatically closing this as ordinary Moderate would lose a concrete authorization/resource-isolation bypass, even though memory corruption is not shown.
Likely operational severity: Important candidate for affected s390 KVM deployments using AP crypto and nested virtualization. Exposure is configuration-specific, but the primitive is security-boundary relevant.
5. One-sentence report phrase
KVM s390 vsie may preserve stale crypto access bits in a shadow APCB, allowing a nested guest to access an AP crypto device or queue that should no longer be available to it.
6. Manual review recommendation
MANUAL CHECK REQUIRED
Reason: the issue is a concrete KVM virtualization access-control bypass involving crypto device resources, and the real impact depends on AP crypto assignment, nested guest exposure, and tenant isolation policy.
7. Call-site confidence
Call-site confidence: medium
======================================================================
UPSTREAM PATCH SUMMARY
======================================================================
Patch: KVM: s390: vsie: zero stale crypto bits
Commit: d110b3297f11ef227098b8a82ade2d5f123b7d2f
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=d110b3297f11ef227098b8a82ade2d5f123b7d2f
Commit description:
When shadowing crypto access bits from a format0 apcb (crycb 0 or 1),
the bits 64..255 are unchanged from whatever is in the vsie page in the
crycb and thus in the apcb. This gives a nested guest potential access
to a device no longer available. Zero out the remaining bits.
Fixes: 6b79de4 ("KVM: s390: vsie: allow guest FORMAT-1 CRYCB on host FORMAT-2")
Cc: [email protected]
Signed-off-by: Christian Borntraeger <[email protected]>
Reviewed-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/kvm/vsie.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=d110b3297f11ef227098b8a82ade2d5f123b7d2f
======================================================================
DETAILED REPORT METHODOLOGY
======================================================================
The original Linux kernel CVE announcement for CVE-2026-80921 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-80921
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:C/C:L/I:L/A:L
The Best / paranoid CVSS vector: AV:L/AC:H/PR:L/UI:N/S:C/C:H/I:H/A:H
The Best / paranoid CVSS score: 7.8
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: 5
Paranoid ActionableScore: 7
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: IMPORTANT
KPANIC detected for this report: YES
Published priority for this report (same as in Subject): IMPORTANT
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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