* [CVE-2026-89956][MODERATE 7.0] s390/vfio-ap: Fix missing lock required to access list of ap_matrix_mdev objects
@ 2026-09-16 14:03 21% AL-KERNEL
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From: AL-KERNEL @ 2026-09-16 14:03 UTC (permalink / raw)
To: kernel-cve
CVE: CVE-2026-89956
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: s390/vfio-ap: Fix missing lock required to access list of ap_matrix_mdev objects
Commit: 647916988272fe5ecb4bb30cd02b51af9960618a
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=647916988272fe5ecb4bb30cd02b51af9960618a
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89956
Analysis date: Wed, 16 Sep 2026 10:03:06 -0400
ActionableScore: 5
ActionableScore lower bound: 4
Actionable bucket: Actionable Moderate / Strong Important candidate requiring manual review
Manual review required: YES
Summary:
A missing `guests_lock` in s390 vfio-ap allows concurrent modification and traversal of `matrix_dev->mdev_list`, creating a race that can corrupt kernel list state or leave stale `ap_matrix_mdev` references and should be manually reviewed for UAF impact.
======================================================================
ABOUT THIS REPORT
======================================================================
The original Linux kernel CVE announcement for CVE-2026-89956 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89956
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-2026-89956 MODERATE https://www.kernelcve.org/list/?q=CVE-2026-89956 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-362;CWE-667;*CWE-416;Other CVSS 'AV:L/AC:H/PR:H/UI:N/S:U/C:L/I:L/A:H';BEST CVSS score: '7';DESCR 'A race in s390/vfio-ap can occur because matrix_dev mdev_list is modified or traversed without holding the required guests_lock. Concurrent mdev creation removal or status access can corrupt the list state or leave a stale ap_matrix_mdev reference, which makes this a use after free or kernel memory corruption candidate rather than a simple warning. For the CVSS the PR:L is used for the paranoid score because a delegated vfio management user, container root, or service account may be able to exercise mediated device control without full host administration. The issue is local and is not network reachable. Impact is at least local denial of service via kernel crash and in worst case may allow confidentiality or integrity impact due to kernel memory corruption, so it 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 LOCK DANGER UAF HARDWARE LINUS TEST KPANIC INCREASED_TO_MODERATE7_BASED_ON_GUESSCVSS NO NO checked
======================================================================
ACTIONABLESCORE ANALYSIS
======================================================================
ActionableScore=5
ActionableScoreLower=4
PrimitiveClass=CLASS_B
RealUAF=YES
HostAdminRequired=UNKNOWN
NamespaceScopedPrivilege=NO
ImportantCandidate=YES
ManualReview=REQUIRED
AutoDowngradeAllowed=NO
## 1. ActionableScore
* Raw conservative score: 3
* Conservative score: 4
* Raw paranoid score: 5
* Paranoid score: 5
* Final recommended bucket: **Actionable Moderate / Strong Important candidate requiring manual review**::
Conservative arithmetic: +2 real UAF / lifetime violation, +1 realistic kernel crash or list corruption DoS. Applied Real UAF FLOOR 4 because this is not merely passive metadata and host-admin-equivalent reachability is not positively established from the supplied evidence.
Paranoid arithmetic: conservative signals plus +1 attacker-influenced active-object lifetime manipulation, plus +1 structured stale-object leverage from concurrent list modification/traversal of active `ap_matrix_mdev` objects. No additional floor or cap applied.
## 2. Signal breakdown
Positive signals:
* +2 Real UAF / active lifetime violation: the commit explicitly states that unlocked access to `matrix_dev->mdev_list` can lead to kernel memory corruption or use-after-free when mdev objects are created or removed concurrently.
* +1 Reliable kernel crash / strong DoS: list corruption or stale `ap_matrix_mdev` traversal can realistically crash the kernel.
* +1 paranoid attacker-influenced active-object lifetime manipulation: an actor able to create/remove vfio-ap mediated devices can repeatedly affect the lifetime and membership of objects in `mdev_list`.
* +1 paranoid structured stale-object leverage: the stale object is an active structured object, and later traversal consumes list links and `matrix_mdev` fields such as AP matrix bitmaps rather than only touching a fixed stale buffer.
Non-awarded signals:
* Controlled reclaim not demonstrated.
* Arbitrary kernel read/write not demonstrated.
* Callback/function-pointer hijack not demonstrated.
* Confidentiality and integrity bonuses not awarded separately because no concrete disclosure or unauthorized modification primitive is established beyond the unresolved memory corruption candidate.
* Strong LPE +2 not awarded because the patch does not establish controlled replacement, refcount takeover, credential manipulation, or demonstrated privilege escalation.
* Remote reachability not established.
* Extra Availability not awarded because the same crash/list-corruption DoS is already counted.
* Concrete repeated release/refcount signal not awarded. The patch shows missing locking around list add/traverse, not a first put followed by a second put on the exact same dangling reference.
Repeated-release/refcount checklist:
* First release can end lifetime: not established in the visible patch.
* Exact same reference survives to a second release: not established.
* Second operation is a real lifetime/refcount mutation: not established.
* +1 Concrete repeated release/refcount awarded: no.
* Paranoid repeated-refcount leverage awarded: no.
* FLOOR 5 based on repeated release: no.
Negative signals:
* No host-admin penalty applied because host-admin-equivalent privilege is plausible for mdev management but not positively established by the supplied diff and text.
* No race penalty applied. The bug is a race, but the evidence does not establish a narrow, one-shot, externally uncontrollable timing window.
* No rare-hardware numeric penalty applied in the score. The issue is s390/vfio-ap specific and asset relevance is limited, but the primitive itself is concrete kernel memory corruption/UAF in the applicable environment.
## 3. Reachability analysis
The vulnerable operations are in s390 `vfio-ap` mediated-device management and AP queue status handling. The race involves adding/removing or traversing `matrix_dev->mdev_list` without holding the required `guests_lock`.
The most plausible attacker model is a local actor with some ability to exercise vfio-ap mediated-device lifecycle operations, or a management service/account performing those operations while status reads occur. Creating or removing mdev objects is commonly administrator-controlled, but the supplied evidence does not prove the exact complete privilege boundary. Therefore `HostAdminRequired=UNKNOWN`, not `YES`.
`NamespaceScopedPrivilege=NO` because this is not shown to be a user/network namespace scoped capability path. It is tied to s390 AP/vfio mediated-device infrastructure.
Prior setup likely includes s390 AP hardware or virtualized AP support and vfio-ap configuration. That limits exposed systems, but does not eliminate the need for manual review where the subsystem is deployed.
Race repeatability is plausible if the same actor can loop mdev create/remove operations and status reads, but exact exploitability beyond crash remains unresolved.
## 4. Primitive and severity interpretation
The exact primitive is a missing-lock race on `matrix_dev->mdev_list`. The list contains active structured `ap_matrix_mdev` objects. Concurrent list mutation and traversal can leave a stale object reference or corrupt list state.
This is not fixed stale storage. It is an active structured-object lifetime/list consistency issue. The vulnerable stale use is not merely a destructor-only cleanup dereference; one path iterates the list to locate the mdev associated with an AP queue and consumes fields inside the object.
Primitive class is `CLASS_B`: a real kernel lifetime/list corruption primitive exists, but there is no demonstrated controlled reclaim, arbitrary write, callback control, or LPE chain.
Demonstrated consequence is kernel memory corruption or use-after-free with likely crash/DoS. Speculative consequence is possible confidentiality/integrity impact if the stale object can be meaningfully reclaimed or corrupted, but this is not established by the supplied patch.
`ImportantCandidate=YES` because the patch explicitly describes memory corruption/UAF in an active kernel object list and the exploitation leverage is unresolved. `AutoDowngradeAllowed=NO` because automatic Moderate closure would discard a concrete kernel corruption primitive that requires manual Important-vs-Moderate review.
Likely operational severity: actionable Moderate at minimum for affected s390/vfio-ap deployments, with Strong Important candidacy depending on real-world privilege boundary and exploitability of the list/UAF corruption.
## 5. One-sentence report phrase
A missing `guests_lock` in s390 vfio-ap allows concurrent modification and traversal of `matrix_dev->mdev_list`, creating a race that can corrupt kernel list state or leave stale `ap_matrix_mdev` references and should be manually reviewed for UAF impact.
## 6. Manual review recommendation
MANUAL CHECK REQUIRED
Reason: the patch explicitly establishes possible kernel memory corruption or use-after-free in an active structured object list, and the privilege boundary and post-lifetime exploitation leverage are not fully resolved.
## 7. Call-site confidence
Call-site confidence: high
======================================================================
UPSTREAM PATCH SUMMARY
======================================================================
Patch: s390/vfio-ap: Fix missing lock required to access list of ap_matrix_mdev objects
Commit: 647916988272fe5ecb4bb30cd02b51af9960618a
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=647916988272fe5ecb4bb30cd02b51af9960618a
Commit description:
In order to traverse or add/remove ap_matrix_mdev objects in the
matrix_dev->mdev_list, the matrix_dev->guests_lock mutex must be held.
There are two functions that access the list without holding the mutex:
vfio_ap_mdev_probe function
~~~~~~~~~~~~~~~~~~~~~~~~~~~
The vfio_ap_mdev_probe function uses the matrix_dev->mdevs_lock
mutex to guard the add of a newly created ap_matrix_mdev object to the
matrix_dev->mdev_list. This mutex does not protect list access; its purpose
is to guard against concurrent access to fields contained in an
ap_matrix_mdev object. This could lead to kernel memory corruption or
use-after-free if another mdev is created or removed concurrently.
The adding of an ap_matrix_mdev object to matrix_dev->mdev_list
is now guarded by the matrix_dev->guests_lock which is the correct
way to protect against concurrent mdev_list access.
Also removed the following two lines of code because the matrix_mdev is
allocated via vfio_alloc_device macro which uses kzalloc, so req_trigger
and cfg_chg_trigger are already zero-initialised when the struct is
allocated before the call to vfio_register_emulated_iommu_dev. This
prevents a window whereby these triggers are set to NULL after
the device is exposed to userspace.
matrix_mdev->req_trigger = NULL;
matrix_mdev->cfg_chg_trigger = NULL;
vfio_ap_mdev_for_queue function
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
The status_show function that supports display of the status attribute of
the devices in /sys/bus/ap/devices calls the vfio_ap_mdev_for_queue
function which iterates the matrix_dev->mdev_list to find the object
representing the queue device whose status is to be displayed. In order to
traverse this list, the matrix_dev->guests_lock mutex must be held.
To fix this, the guests_lock mutex is taken prior to taking the
matrix_dev->mdevs_lock mutex in the status_show function. It is taken
there rather than the vfio_ap_mdev_for_queue function - where it is
needed - because it must be taken prior to the mdevs_lock mutex in order to
adhere to the proper locking order and prevent a lockdep splat; also
because the mdevs_lock is needed there to access fields within
the matrix_mdev object in that function.
See the vfio-ap-locking.rst in the linux kernel tree.
Fixes: 2c1ee89 ("s390/vfio-ap: prepare for dynamic update of guest's APCB on queue probe/remove")
Cc: [email protected]
Signed-off-by: Anthony Krowiak <[email protected]>
Reviewed-by: Matthew Rosato <[email protected]>
Signed-off-by: Christian Borntraeger <[email protected]>
Signed-off-by: Greg Kroah-Hartman <[email protected]>
Changed files:
drivers/s390/crypto/vfio_ap_ops.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=647916988272fe5ecb4bb30cd02b51af9960618a
======================================================================
DETAILED REPORT METHODOLOGY
======================================================================
The original Linux kernel CVE announcement for CVE-2026-89956 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89956
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:H/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
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