[CVE-2026-64369][MODERATE REGULAR] s390: Revert support for DCACHE_WORD_ACCESS

[CVE-2026-64369][MODERATE REGULAR] s390: Revert support for DCACHE_WORD_ACCESS

[AL-KERNEL]

CVE: CVE-2026-64369
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: s390: Revert support for DCACHE_WORD_ACCESS
Commit: c9e0f1517631ac08987f8385817119bccf2f1f12
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=c9e0f1517631ac08987f8385817119bccf2f1f12
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64369
Analysis date: Sat, 25 Jul 2026 15:18:45 -0400
ActionableScore: 4
ActionableScore lower bound: 2
Actionable bucket: Borderline manual review / Actionable Moderate watchlist
Manual review required: YES

Summary:
s390 Secure Execution systems may enter an endless exception loop when `load_unaligned_zeropad()` crosses into Ultravisor donated memory, causing host denial of service without evidence of memory corruption or privilege escalation.

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

The original Linux kernel CVE announcement for CVE-2026-64369 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64369

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-2026-64369	MODERATE	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:C/C:N/I:N/A:H';CWE-755;*CWE-835;CWE-703;Other CVSS 'AV:L/AC:H/PR:H/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '5.6';DESCR 's390 DCACHE_WORD_ACCESS uses load_unaligned_zeropad() to read an unaligned word and handle page crossing faults by zero padding the inaccessible part. On Secure Execution systems, a cross page read into memory donated to the Ultravisor can hit secure storage access handling that does not resolve the exception correctly. This may lead to an endless exception loop and a host kernel availability failure. For the CVSS the PR:L is used for the paranoid score because a local user or guest controlled workload may be able to reach the affected path in a vulnerable Secure Execution configuration, while normal host setup of such environments is often more privileged. The issue is not directly network reachable. Impact is denial of service via an endless exception loop, with no supported evidence of information disclosure, data modification, UAF, OOB access, or privilege escalation.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline manual review / Actionable Moderate watchlist (with actual score 3)	YES	KASAN HARDWARE LINUS  INCREASED_FROM_LOW_BASED_ON_REQUIREMANUALCHECK DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN5	YES	NO	checked

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

ActionableScore=4
ActionableScoreLower=2

## 1. ActionableScore

* Conservative score: 2
* Paranoid score: 4
* Final recommended bucket: **Borderline manual review / Actionable Moderate watchlist**::

## 2. Signal breakdown

Conservative signals:

* +1 Firmware-mediated external influence: the failure depends on s390 Secure Execution donated pages and Ultravisor secure storage exception behavior.
* +1 Reliable kernel crash / strong DoS: the commit explicitly says the exception can result in an endless exception loop that will never be resolved.
* +1 Availability impact realistic: this can affect host kernel availability, not just produce a warning.
* -1 Hard or special triggering condition: it requires an unaligned word access crossing into a problematic Secure Execution donated page.
* -1 Rare / configuration-dependent exposure: s390 Secure Execution is not a broad default deployment path.

Paranoid additional signals:

* +1 Local or guest-controlled trigger concern: after a protected virtualization environment exists, a guest workload or local controlled workload may plausibly influence memory access patterns that reach the affected path.
* +1 Cross-boundary availability concern: the affected condition is tied to Secure Execution / Ultravisor donated memory and can plausibly affect host availability from a protected workload context.

Not counted:

* No generic memory corruption: no UAF, double-free, OOB write, type confusion, refcount takeover, or arbitrary write is shown.
* No LPE bonus: the patch supports DoS via exception-loop behavior, not privilege escalation.
* No confidentiality or integrity impact: no disclosure or data modification primitive is visible.

## 3. Reachability analysis

The issue is local or guest-context reachable, not network reachable. Triggering requires s390 with Secure Execution donated memory and a page-crossing unaligned load through `load_unaligned_zeropad()`. Host setup of Secure Execution is privileged, but once such an environment exists, the practical trigger may come from a guest or workload rather than full host-root. Namespaces are not the main factor here. Deployment exposure is narrow because it is s390-specific and Secure Execution-specific.

Call-site confidence: medium. The patch removes an architecture helper and exception-table handler, but the exact higher-level callers that produce the problematic unaligned access are not included in the input.

## 4. Severity interpretation

This behaves more like an Actionable Moderate / manual-review DoS than an Important-class memory corruption issue. The theoretical concern is host availability impact across a protected virtualization boundary. The realistic evidence is an endless exception loop, with no demonstrated memory corruption, data leak, write primitive, or LPE path. The paranoid score is higher because host DoS from Secure Execution context should not be auto-closed without product-specific review.

## 5. One-sentence report phrase

s390 Secure Execution systems may enter an endless exception loop when `load_unaligned_zeropad()` crosses into Ultravisor donated memory, causing host denial of service without evidence of memory corruption or privilege escalation.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

Manual review is required because the issue affects host availability in s390 Secure Execution configurations, and the practical triggerability depends on protected virtualization deployment details.

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

Patch: s390: Revert support for DCACHE_WORD_ACCESS
Commit: c9e0f1517631ac08987f8385817119bccf2f1f12
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=c9e0f1517631ac08987f8385817119bccf2f1f12

Commit description:

load_unaligned_zeropad() reads eight bytes from unaligned addresses and may
cross page boundaries. It handles exceptions which may happen if reading
from the second page results in an exception.

For pages which are donated to the Ultravisor for secure execution purposes
the do_secure_storage_access() exception handler however does not handle
such exceptions correctly. Such an exception may result in an endless
exception loop which will never be resolved.

An attempt to fix this [1] turned out to be not sufficient. For now revert
load_unaligned_zeropad() until this problem has been resolved in a proper
way.

Note that the implementation of load_unaligned_zeropad() itself is
correct. The revert is just a temporary workaround until there is complete
fix for secure storage access exceptions.

[1] commit b00be77 ("s390/mm: Add missing secure storage access fixups for donated memory")

Fixes: 802ba53 ("s390: add support for DCACHE_WORD_ACCESS")
Cc: stable@vger.kernel.org
Acked-by: Christian Borntraeger <borntraeger@linux.ibm.com>
Signed-off-by: Alexander Gordeev <agordeev@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>

Changed files:
  arch/s390/include/asm/asm-extable.h
  arch/s390/include/asm/word-at-a-time.h
  arch/s390/mm/extable.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=c9e0f1517631ac08987f8385817119bccf2f1f12

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

The original Linux kernel CVE announcement for CVE-2026-64369 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64369

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:C/C:N/I:N/A:H
  The Best / paranoid CVSS score: 5.6

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: 2
  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).