[CVE-2026-98476][MODERATE REGULAR] s390/cio: Fix NULL pointer dereference in ccw_device_get_util_str()

[AL-KERNEL <alkernel@kernelcve.org>]

CVE: CVE-2026-98476
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: s390/cio: Fix NULL pointer dereference in ccw_device_get_util_str()
Commit: 9d8b21820caed9128945fffcedf03e2a4c21eee2
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=9d8b21820caed9128945fffcedf03e2a4c21eee2
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98476
Analysis date: Mon, 19 Oct 2026 06:22:01 -0400
ActionableScore: 0
ActionableScore lower bound: unknown
Actionable bucket: Ordinary Moderate
Manual review required: NO

Summary:
A stale CHPID reference in an s390 subchannel PMCW can make chpid_to_chp return NULL and cause ccw_device_get_util_str to trigger a kernel Oops or panic when the missing channel path is accessed.

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ABOUT THIS REPORT
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The original Linux kernel CVE announcement for CVE-2026-98476 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98476

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 REGULAR
Manual review required: NO

A detailed explanation of the methodology and priority rules is included
at the end of this message.

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AL-KERNEL CLASSIFICATION RESULT
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CVE-2026-98476	MODERATE	https://www.kernelcve.org/list/?q=CVE-2026-98476 CHECK WITH IMPACT FROM ORIG NN MODERATE | AEGIS IMPACT MODERATE_KPANIC AEGIS_AS=0 LEGACY=MODERATE RESULT=DIFF (final Legacy and Aegis differ; FINAL: MODERATE, Aegis: MODERATE_KPANIC)	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H';**CWE-476;CWE-362;CWE-754;Other CVSS 'AV:L/AC:H/PR:H/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '4.7';DESCR 'A race in the s390 channel path lifecycle can leave a subchannel PMCW referring to a CHPID whose channel path registry entry has already been removed. In that state chpid_to_chp() returns NULL and ccw_device_get_util_str() dereferences the result while preparing the utility string, which can cause a kernel Oops or panic. For the CVSS the PR:L value is used for the paranoid score because a local user or service may be able to invoke a utility string query after the registry entry was removed independently by an administrator or a hardware event. Reliable creation of the stale state may still require privileged channel path management, which is reflected by PR:H in the base score. The race requires a specific timing and device state, so AC:H is appropriate. The issue is not network reachable based on the available call path. Impact is denial of service only because the returned pointer is NULL and the patch provides no evidence of UAF, OOB access, information disclosure, or a write primitive. Automated impact decision path. The original SimpleNet classifier selected MODERATE_KPANIC. The primary LLM estimated paranoid CVSS 4.7 and the production ActionableScore returned 0 with recommendation Ordinary Moderate. The production rules then evaluated CVSS thresholds and ActionableScore thresholds. Recorded rule transitions were CVSS and initial triage stage MODERATE_KPANIC to MODERATE because the CVSS score, vector or bug class threshold rule was applied. The legacy KP specialist has no review artifact for this run. The legacy FD or DH downgrade specialist has no review artifact for this run. Exact triggered rule markers were DECREASED_TO_MODERATE_REGULAR_FROM_MODERATE7_BASED_ON_GUESSCVSS_AND_CIANNH and V41_INHERITED_KPANIC_PROMOTION_BLOCKED and MANUAL_REVIEW_REQUIRED. The independent weaker Aegis shadow evaluated CVSS 2.5 and ActionableScore 0 with primitive class CLASS_C and manual review RECOMMENDED. Its independently reconciled impact was MODERATE_KPANIC. The models disagreed but the aggregation rules retained the production result. The final reported impact is MODERATE.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate (with actual score 0)	MAYBE	NULLPTR SIMPLEFIX HARDWARE LINUS  DECREASED_TO_MODERATE_REGULAR_FROM_MODERATE7_BASED_ON_GUESSCVSS_AND_CIANNH V41_INHERITED_KPANIC_PROMOTION_BLOCKED MANUAL_REVIEW_REQUIRED	-	-	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=0
PrimitiveClass=CLASS_C
RealUAF=NO
HostAdminRequired=UNKNOWN
NamespaceScopedPrivilege=NO
ImportantCandidate=NO
ManualReview=RECOMMENDED
AutoDowngradeAllowed=YES
RequiresHostUID0=UNKNOWN
AuthorizationDecisionBypass=NO
ProtectedResourceImpact=NONE
B4PendingReview=NO
B4PendingConfirmed=NO

1. ActionableScore

* Raw conservative score: -1
* Conservative score: 0
* Raw paranoid score: -1
* Paranoid score: 0
* Applied score minimum: -1 -> 0
* Final recommended bucket: **Ordinary Moderate**::

2. Signal breakdown

* +1 Reliable kernel fault or DoS. `chpid_to_chp()` can return NULL, and the subsequent utility-string access dereferences that pointer, causing a kernel Oops and potentially a panic when `panic_on_oops` is enabled.
* -2 Rare hardware-only path. The vulnerable code is specific to the s390 channel I/O architecture and requires an affected CCW device and channel-path lifecycle state.
* +0 Weak corruption concern. The established operation is a NULL dereference, not an attacker-directed write or useful invalid-object operation.
* No race penalty was applied. The patch establishes a lifecycle inconsistency, but it does not show that exploitation requires a narrow or externally uncontrollable timing window.
* No UAF was awarded. `chpid_to_chp()` returns NULL rather than a pointer to an already freed channel-path object.
* No controlled reclaim, OOB access, arbitrary write, information disclosure, authorization bypass, or privilege escalation is demonstrated.
* No additional availability point was awarded because the possible Oops or panic is the same consequence already counted.
* B5-R does not apply because there is no heap OOB write, retry-accumulated offset, or later consumption of corrupted structured state.
* B3-PENDING and the V32 to V34 networking rules do not apply.
* B4-PENDING does not apply because the new condition is an object-existence check, not a mandatory authorization guard that changes DENY to ALLOW.
* V33 does not apply because no attacker-influenced corruption of active structured state is established.

3. Reachability analysis

The complete trigger chain is unresolved. The channel-path registry entry must disappear while the subchannel PMCW still contains the corresponding CHPID, after which a caller must invoke `ccw_device_get_util_str()` for that path.

The supplied patch does not show the callers or the user-facing interface that reaches this helper. It therefore does not establish whether an ordinary local user, a device-management service, or a host administrator can perform the complete trigger. `HostAdminRequired` remains UNKNOWN rather than being inferred as either YES or NO.

The CIO channel-path registry is global s390 hardware state rather than a user or network namespace resource, so `NamespaceScopedPrivilege=NO`. No remote packet path is present. The required platform and CCW device state are hardware-specific. It is also unclear whether registry removal is attacker-controlled, administrator-driven, or caused by an independent hardware event.

The patch describes a state-lifetime race or mismatch, but it does not establish a narrow timing window. The stale PMCW reference may remain observable after registry removal, so no hard-race penalty is justified.

4. Primitive and severity interpretation

The exact primitive is a NULL pointer dereference. A CHPID remains in the subchannel PMCW after the corresponding registry object has been removed. `chpid_to_chp()` then returns NULL, and the caller subsequently accesses the missing channel-path descriptor.

This is not a real UAF because no stale non-NULL object pointer is shown to survive object destruction. It is not a repeated release or refcount mutation. There is no second lifetime-changing operation, active-object replacement, corrupted descriptor consumption, or post-free structured processing.

`PrimitiveClass=CLASS_C` is appropriate because the demonstrated result is invalid-state consumption ending in an Oops or possible panic. The practical impact is denial of service. Stronger consequences such as memory corruption, disclosure, or privilege escalation are unsupported.

`ImportantCandidate=NO` because there is no independently useful corruption or boundary primitive. `AutoDowngradeAllowed=YES` because the established defect is a constrained architecture-specific NULL dereference, although caller reachability should still be checked before final operational closure.

5. One-sentence report phrase

A stale CHPID reference in an s390 subchannel PMCW can make chpid_to_chp return NULL and cause ccw_device_get_util_str to trigger a kernel Oops or panic when the missing channel path is accessed.

6. Manual review recommendation

MANUAL CHECK RECOMMENDED

Confirm the callers of `ccw_device_get_util_str()`, their access controls, whether an unprivileged actor can initiate the query, and whether channel-path registry removal can be attacker-influenced. The established primitive itself remains crash-only.

7. Call-site confidence

Call-site confidence: medium

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UPSTREAM PATCH SUMMARY
======================================================================

Patch: s390/cio: Fix NULL pointer dereference in ccw_device_get_util_str()
Commit: 9d8b21820caed9128945fffcedf03e2a4c21eee2
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=9d8b21820caed9128945fffcedf03e2a4c21eee2

Commit description:

The channel path registry entry associated with a CHPID may be removed
while the subchannel's PMCW still references that CHPID. In this case,
chpid_to_chp() can return NULL, leading to a NULL pointer dereference.

Add the missing NULL check before dereferencing the returned pointer.

Fixes: 1996523 ("s390/cio: add helper to query utility strings per given ccw device")
Cc: stable@vger.kernel.org
Signed-off-by: Vineeth Vijayan <vneethv@linux.ibm.com>
Reviewed-by: Peter Oberparleiter <oberpar@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  drivers/s390/cio/device_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=9d8b21820caed9128945fffcedf03e2a4c21eee2

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DETAILED REPORT METHODOLOGY
======================================================================

The original Linux kernel CVE announcement for CVE-2026-98476 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98476

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:N/I:N/A:H
  The Best / paranoid CVSS vector: AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS score: 4.7

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: 0

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: NO
Published priority for this report (same as in Subject): MODERATE REGULAR

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 alexanjelausa@gmail.com (and both
send reply to CVE record itself too and see "reply" button below for howto reply).