[CVE-2026-89459][MODERATE REGULAR] s390/percpu: Fix MVIY_PERCPU() with older binutils

[CVE-2026-89459][MODERATE REGULAR] s390/percpu: Fix MVIY_PERCPU() with older binutils

[AL-KERNEL]

CVE: CVE-2026-89459
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: s390/percpu: Fix MVIY_PERCPU() with older binutils
Commit: 91770b08a120967077ae78600612f18bc5ee3caf
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=91770b08a120967077ae78600612f18bc5ee3caf
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89459
Analysis date: Sat, 12 Sep 2026 04:53:38 -0400
ActionableScore: 4
ActionableScore lower bound: 3
Actionable bucket: Borderline / manual review recommended
Manual review required: YES

Summary:
No one-sentence report phrase was found.

======================================================================
ABOUT THIS REPORT
======================================================================

The original Linux kernel CVE announcement for CVE-2026-89459 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89459

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: YES

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

======================================================================
AL-KERNEL CLASSIFICATION RESULT
======================================================================

CVE-2026-89459	MODERATE	https://www.kernelcve.org/list/?q=CVE-2026-89459 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:L/I:L/A:H';CWE-758;CWE-670;CWE-682;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:L/A:H';BEST CVSS score: '5.8';DESCR 'MVIY_PERCPU() on s390 can be misparsed by GNU as versions before binutils 2.39 because the generated assembler macro invocation uses whitespace separated quoted arguments. The assembler may exit successfully without emitting the mviy instruction, so an interrupted per-CPU sequence is not marked in lowcore and the exception return path cannot repair the per-CPU address register after task migration. For the CVSS the PR:L is used because a local user or process is needed to create workload on an already affected kernel, while the vulnerable condition itself depends on how the kernel was built. The issue is not network reachable. Impact is at least local denial of service or kernel state inconsistency, with limited confidentiality or integrity impact kept only for the paranoid score due to possible incorrect per-CPU state access.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline / manual review recommended (with actual score 4)  SKIP CVE-2026-89459 UNKNOWN SKIP No affected files built, so skip this CVE NO  - - unknown 	MAYBE	OOB HARDWARE LINUS  INCREASED_FROM_LOW_BASED_ON_REQUIREMANUALCHECK DECREASED_TO_MODERATEREG_BASED_ON_FALSEPOSCHECKOFKP	-	-	checked

======================================================================
ACTIONABLESCORE ANALYSIS
======================================================================

ActionableScore=4
ActionableScoreLower=3
PrimitiveClass=CLASS_C
RealUAF=NO
HostAdminRequired=NO
NamespaceScopedPrivilege=NO
ImportantCandidate=NO
ManualReview=RECOMMENDED
AutoDowngradeAllowed=YES

1. ActionableScore

* Raw conservative score: 3
* Conservative score: 3. No floor or cap applied.
* Raw paranoid score: 4
* Paranoid score: 4. No floor or cap applied.
* Final recommended bucket: **Borderline / manual review recommended**::

2. Signal breakdown

Conservative signals:

* +1 Local unprivileged trigger. On an already affected s390 kernel, ordinary local workload can plausibly participate in task execution, interruption, and migration paths that expose the broken per-CPU repair behavior.
* +1 Weak / indirect corruption candidate. The patch establishes that the missing `mviy` marker can leave an interrupted per-CPU sequence unmarked in lowcore, so exception return cannot repair the per-CPU address register after migration.
* +1 Reliable kernel malfunction / DoS concern. A wrong per-CPU address register after migration can cause incorrect per-CPU state access and kernel instability, even though the supplied evidence does not show a specific exploit primitive.
* +1 Broad affected runtime path within affected architecture. `this_cpu` / per-CPU operations are fundamental kernel mechanisms on affected s390 builds, not a narrow optional feature once the kernel is built this way.
* -1 Special timing requirement. The failure requires an interrupted per-CPU sequence combined with migration and exception return behavior, which is a concrete timing/state requirement.

Paranoid additional signal:

* +1 Integrity concern from wrong per-CPU state access. In the higher but still grounded interpretation, a wrong per-CPU base can lead to unintended kernel state reads or writes in the wrong per-CPU area. This is not scored as controlled corruption.

Important non-awarded signals:

* RealUAF not awarded. No object lifetime expiration followed by stale object use is established.
* Controlled reclaim not demonstrated.
* Arbitrary or directed kernel write not demonstrated.
* Confidentiality not awarded. There is no concrete disclosure path to userspace, network, or another observer.
* Strong LPE leverage not awarded. The patch does not establish attacker-controlled replacement, callback control, credential corruption, or a demonstrated privilege escalation chain.
* Remote reachability not awarded. This is not network-triggered.
* Extra Availability not awarded. The same kernel malfunction / crash concern is not double-counted.
* Toolchain condition noted but not separately penalized in the score. The vulnerability only exists in affected s390 kernels built with older GNU as behavior, but for a deployed affected kernel this is an applicability condition rather than an attacker-side prerequisite.

3. Reachability analysis

The vulnerable condition is created at build time when s390 kernel code using `MVIY_PERCPU()` is assembled with GNU as versions before binutils 2.39 that misparse whitespace-separated quoted macro arguments. The attacker does not need to control the build process if the deployed kernel is already affected.

Runtime triggering is local. A local user or local process can plausibly create workload that exercises kernel paths using per-CPU operations and can be scheduled, interrupted, and migrated. Host administrator privilege is not positively required for the runtime part, so `HostAdminRequired=NO`.

This is not namespace-scoped. No namespace capability, network namespace, container capability, or delegated namespace operation is part of the complete trigger.

There is no direct network vector. There is no firmware-mediated vector. Special hardware access is not required beyond running on the affected s390 architecture. The main applicability prerequisite is the build/toolchain combination: s390 plus older assembler behavior.

Race or timing repeatability is uncertain. The issue depends on interruption and migration of an affected per-CPU sequence, so exploitation reliability is not established and a timing penalty is applied.

4. Primitive and severity interpretation

The exact primitive is a runtime kernel state consistency failure caused by a build-time assembler macro parsing problem. The intended `mviy` instruction is omitted, so interrupted per-CPU sequences are not marked in lowcore. After migration, the exception return path may fail to repair the per-CPU address register.

This is not a UAF, double free, refcount bug, OOB access, authorization bypass, or controlled write primitive. It is best classified as `CLASS_C`: demonstrated behavior is primarily invalid kernel state with DoS or weak corruption concern, without an independently useful exploitation primitive.

The affected state is active kernel execution state rather than passive metadata, but the supplied evidence does not show controlled target selection, controlled payload, controlled memory corruption, or a practical LPE chain.

Demonstrated consequence: kernel instability or local DoS due to incorrect per-CPU address-register repair after migration.

Speculative consequence: limited unintended per-CPU state access or corruption. This supports the paranoid score increase, but not Class A or Important-grade treatment.

`ImportantCandidate=NO` because the patch does not establish a strong corruption primitive, real UAF, repeated lifetime mutation, authorization bypass, shared-page ownership violation, or demonstrated LPE.

`AutoDowngradeAllowed=YES` because the issue remains a constrained architecture/toolchain/runtime-state bug with no established strong exploitation leverage. Manual review is still recommended to confirm product build-toolchain exposure.

Likely operational severity: Borderline Moderate. It should not be treated as a pure cosmetic build warning if affected binaries exist, but it also does not meet Important-class evidence from the supplied data.

5. One-sentence report phrase

On affected s390 kernels built with older GNU as behavior, MVIY_PERCPU() may fail to emit the required mviy marker, leaving interrupted per-CPU sequences unrepaired after migration and causing local kernel instability or DoS without a demonstrated privilege escalation primitive.

6. Manual review recommendation

MANUAL CHECK RECOMMENDED

Reason: confirm whether the evaluated product actually built affected s390 kernels with a vulnerable assembler version and whether any observed per-CPU state corruption goes beyond crash or instability.

7. Call-site confidence

Call-site confidence: medium

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

Patch: s390/percpu: Fix MVIY_PERCPU() with older binutils
Commit: 91770b08a120967077ae78600612f18bc5ee3caf
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=91770b08a120967077ae78600612f18bc5ee3caf

Commit description:

Commit a737737 ("s390/percpu: Infrastructure for more efficient
this_cpu operations") introduced MVIY_PERCPU(), which stringifies
arguments that are already C string literals. This generates an
assembler macro invocation with whitespace-separated quoted arguments:

  GEN_MVIY "459712" "%r3"

GNU as versions prior to binutils 2.39 drop the separating whitespace
between quoted macro arguments during input scrubbing. They
consequently parse the invocation as a single argument and emit
repeated warnings:

  Warning: missing closing `"'

The .ifc in GEN_MVIY never matches and GNU as exits successfully
without emitting the mviy instruction. As a result, the interrupted
per-CPU sequence is not marked in lowcore and the exception return
path cannot repair the per-CPU address register after migration.

All MVIY_PERCPU() callers pass C string literals. Use them directly
and separate the assembler macro arguments with an explicit comma. The
resulting invocation is:

  GEN_MVIY 459712, %r3

This form is unambiguous for GNU as and LLVM's integrated assembler.
This behavior was fixed in GNU as from binutils 2.39, but Linux
supports binutils 2.30.

Fixes: a737737 ("s390/percpu: Infrastructure for more efficient this_cpu operations")
Cc: stable@vger.kernel.org
Assisted-by: Codex:gpt-5.6-sol
Signed-off-by: Karl Mehltretter <kmehltretter@gmail.com>
Reviewed-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  arch/s390/include/asm/percpu.h

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=91770b08a120967077ae78600612f18bc5ee3caf

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

The original Linux kernel CVE announcement for CVE-2026-89459 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89459

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:N/I:L/A:H
  The Best / paranoid CVSS vector: AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:H
  The Best / paranoid CVSS score: 5.8

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: 3
  Paranoid ActionableScore: 4

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).