* [CVE-2026-89922][MODERATE 7.0] KVM: s390: Take srcu when importing watchpoint data
@ 2026-09-16 12:46 21% AL-KERNEL
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From: AL-KERNEL @ 2026-09-16 12:46 UTC (permalink / raw)
To: kernel-cve
CVE: CVE-2026-89922
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: KVM: s390: Take srcu when importing watchpoint data
Commit: 6830fbc3724bf49c142aae69a4694f115fa9cedd
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=6830fbc3724bf49c142aae69a4694f115fa9cedd
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89922
Analysis date: Wed, 16 Sep 2026 08:46:43 -0400
ActionableScore: 5
ActionableScore lower bound: unknown
Actionable bucket: Actionable Moderate / Strong Important candidate requiring manual review
Manual review required: YES
Summary:
KVM s390 guest debug watchpoint import can use KVM memslot data without SRCU protection, allowing a local VMM process with `/dev/kvm` access to race memslot updates and potentially trigger a host kernel use-after-free and crash.
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ABOUT THIS REPORT
======================================================================
The original Linux kernel CVE announcement for CVE-2026-89922 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89922
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-2026-89922 MODERATE https://www.kernelcve.org/list/?q=CVE-2026-89922 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:U/C:H/I:H/A:H';*CWE-416;CWE-362;CWE-667;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 guest debug watchpoint import can access guest memory through kvm_read_guest without holding kvm srcu. A concurrent memslot update can free the memslots array after the SRCU grace period while the import path still resolves or uses the old memslot data, creating a race based use-after-free candidate in KVM memory slot handling. For the CVSS the PR:L is used because a local user or service with access to /dev/kvm and control of the VM debug ioctls can reach the affected path without needing full host root in many deployments. The issue is not network reachable and requires local KVM access plus a timing race with memslot updates. Impact is at least a local denial of service via kernel crash and in worst case may allow confidentiality or integrity impact due to stale memslot use after free, so manual review is recommended.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate / Strong Important candidate requiring manual review (with actual score 5) YES VIRT SIMPLEFIX HARDWARE LINUS KPANIC INCREASED_TO_MODERATE7_BASED_ON_GUESSCVSS NO NO checked
======================================================================
ACTIONABLESCORE ANALYSIS
======================================================================
ActionableScore=5
PrimitiveClass=CLASS_B
RealUAF=YES
HostAdminRequired=NO
NamespaceScopedPrivilege=NO
ImportantCandidate=YES
ManualReview=REQUIRED
AutoDowngradeAllowed=NO
## 1. ActionableScore
* Raw conservative score: 5
* Conservative score: 5
* Raw paranoid score: 5
* Paranoid score: 5
* Floor/cap changes: none
* Final recommended bucket: **Actionable Moderate / Strong Important candidate requiring manual review**::
## 2. Signal breakdown
Positive signals:
* +2 Real UAF / lifetime violation: `kvm_read_guest()` resolves memslots via `__kvm_memslots()` without holding `kvm->srcu` or `slots_lock`, and the commit explicitly says a concurrent memslot update can free the memslots array under the caller after the SRCU grace period.
* +1 Reliable kernel crash / strong DoS: use of a freed memslots array in host KVM state can plausibly cause a host kernel fault or crash.
* +1 Structured stale-object exploitation surface: the stale object is not a passive buffer. It is active KVM memslot metadata used to resolve guest memory accesses.
* +1 Attacker-influenced active-object lifetime manipulation: under the local VMM attacker model, the same userspace process with `/dev/kvm` access can drive guest debug ioctl activity and memslot updates, making the lifetime transition repeatable and relevant to the stale use.
Negative signals:
* No race penalty applied: although this is a race, the relevant operations can plausibly be driven repeatedly by the VMM process, and the supplied evidence does not prove a narrow one-shot uncontrollable race.
* No host-admin penalty applied: host root is not positively required. `/dev/kvm` access is required, but that can be delegated below host-admin in normal deployments.
* No rare-hardware penalty applied: the bug is s390-specific, but the affected environment is KVM on s390, and no additional rare physical device requirement is established.
Important non-awarded signals:
* Controlled reclaim is not demonstrated.
* Arbitrary host kernel read/write is not demonstrated.
* Strong guest-to-host exploitation leverage is not awarded, because guest influence plus host KVM UAF is not enough without a concrete boundary-crossing primitive.
* Confidentiality and integrity bonuses are not awarded, because stale memslot use creates unresolved corruption risk but does not itself prove disclosure or unauthorized modification.
* Extra availability is not awarded, because the likely crash is already counted once.
* Concrete repeated release/refcount mutation is not present. There is no first put/free followed by a second lifetime-mutating operation on the same exact reference.
## 3. Reachability analysis
The evaluated attacker is primarily a **local VMM / `/dev/kvm` process**, not a pure malicious guest.
A malicious guest alone is not shown to be able to perform the complete trigger sequence, because guest code normally cannot issue host-side `KVM_SET_GUEST_DEBUG` or update KVM memslots. A local VMM process with access to `/dev/kvm` can plausibly call the guest debug ioctl path and also perform concurrent memslot updates. That is below host-admin in deployments where `/dev/kvm` is delegated to a user or service account, so `HostAdminRequired=NO`.
The trigger is not network reachable. It is not namespace-scoped. No evidence shows that an attacker-created user or network namespace is sufficient for the complete operation.
Prior setup is the existence of a KVM VM on s390 with guest debugging / hardware watchpoint support used. This is configuration-dependent but not enough to apply a scoring penalty. The race requires concurrency between watchpoint import and memslot update, but the VMM can plausibly repeat both operations.
## 4. Primitive and severity interpretation
The exact primitive is a host KVM memslot lifetime violation: `kvm_s390_import_bp_data()` can call into guest memory reading code without SRCU protection, while a concurrent memslot update can free the memslots array.
This is an **active structured object UAF**, not fixed stale storage. The stale object participates in guest memory translation / lookup decisions. The demonstrated consequence is at least host kernel DoS. The speculative consequence is stronger host-state corruption if stale memslot metadata is consumed after reuse, but controlled reclaim or arbitrary R/W is not established.
Primitive class is **CLASS_B**. It is not CLASS_A because there is no demonstrated controlled reclaim, directed write, callback control, credential manipulation, or confirmed guest-to-host exploit chain.
`ImportantCandidate=YES` because this is a real KVM host-state UAF in active memslot metadata, reachable by a local VMM actor below host-admin, with unresolved exploitation leverage beyond an ordinary fixed stale pointer crash.
`AutoDowngradeAllowed=NO` because automatic closure would discard a concrete host-kernel lifetime primitive in KVM memslot handling. This does not prove Important severity, but it requires manual review.
## 5. One-sentence report phrase
KVM s390 guest debug watchpoint import can use KVM memslot data without SRCU protection, allowing a local VMM process with `/dev/kvm` access to race memslot updates and potentially trigger a host kernel use-after-free and crash.
## 6. Manual review recommendation
MANUAL CHECK REQUIRED
Reason: this is a real host KVM memslot UAF involving active memory-translation metadata, with plausible DoS and unresolved exploitation leverage beyond a simple NULL dereference.
## 7. Call-site confidence
Call-site confidence: high
======================================================================
UPSTREAM PATCH SUMMARY
======================================================================
Patch: KVM: s390: Take srcu when importing watchpoint data
Commit: 6830fbc3724bf49c142aae69a4694f115fa9cedd
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=6830fbc3724bf49c142aae69a4694f115fa9cedd
Commit description:
__import_wp_info() backs up the original guest memory contents of a
watchpoint with read_guest_abs(), which is kvm_read_guest() and therefore
resolves the memslot via __kvm_memslots(). That requires kvm->srcu (or
kvm->slots_lock) to be held, otherwise a concurrent memslot update can
free the memslots array under us once its SRCU grace period has elapsed.
As this is not fast path, following lock ordering (mutex first, then
srcu) take the big hammer and hold the srcu for the full import.
Fixes: 27291e2 ("KVM: s390: hardware support for guest debugging")
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/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=6830fbc3724bf49c142aae69a4694f115fa9cedd
======================================================================
DETAILED REPORT METHODOLOGY
======================================================================
The original Linux kernel CVE announcement for CVE-2026-89922 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89922
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: unknown
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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