[CVE-2026-74466][MODERATE REGULAR] s390/zcrypt: Close speculative mem read possibility

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

CVE: CVE-2026-74466
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: s390/zcrypt: Close speculative mem read possibility
Commit: 82b62eda68abfb7a33ac40f24b8c4128c2891148
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=82b62eda68abfb7a33ac40f24b8c4128c2891148
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74466
Analysis date: Sat, 15 Aug 2026 14:23:30 -0400
ActionableScore: 2
ActionableScore lower bound: 1
Actionable bucket: Ordinary Moderate / not Actionable Moderate, manual review recommended for confidentiality side-channel
Manual review required: YES

Summary:
A local s390 zcrypt ioctl path used a user-controlled domain value as an index after bounds checking but without a speculation barrier, allowing a possible Spectre style confidentiality leak under custom device and administrative CPRB conditions.

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ABOUT THIS REPORT
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The original Linux kernel CVE announcement for CVE-2026-74466 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74466

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.

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AL-KERNEL CLASSIFICATION RESULT
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CVE-2026-74466	MODERATE	CHECK WITH IMPACT FROM ORIG NN IMPORTANT	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:N/A:N';CWE-203;CWE-125;CWE-129;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:N/A:N';BEST CVSS score: '4.7';DESCR 'An s390 zcrypt ioctl path uses a user controlled domain value as an index into the custom device admin permission bitmap after a bounds check but without a speculation barrier. Under branch misprediction, the CPU may transiently use an out of range domain and speculatively read permission bitmap or adjacent memory through test_bit_inv, creating a Spectre style information disclosure risk. For the CVSS the PR:L is used because reliable triggering requires local code with access to the zcrypt device node and the ability to submit the relevant CCA or EP11 administrative CPRB path, but full root privileges are not necessarily required when a custom device node is delegated. The issue is not network reachable and does not provide a write primitive or direct privilege escalation. Impact is confidentiality only, with base C:L and paranoid C:H due to possible speculative kernel memory disclosure under special s390 zcrypt configurations.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate / not Actionable Moderate, manual review recommended for confidentiality side-channel (with actual score 2)	YES	OOB SIMPLEFIX SPECTRE HARDWARE LINUS  LOWERED_FROM_HIGH_BASED_ON_GUESSCVSS  DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN5	NO	NO	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=2
ActionableScoreLower=1

## 1. ActionableScore

* Conservative score: 1
* Paranoid score: 2
* Final recommended bucket: **Ordinary Moderate / not Actionable Moderate, manual review recommended for confidentiality side-channel**

## 2. Signal breakdown

Triggered signals:

* Local unprivileged trigger: +1
  Applies in the paranoid interpretation because a local process with access to the relevant zcrypt custom device node may submit the CCA or EP11 ioctl path. This does not require full host root if the device node is delegated.

* Confidentiality impact plausible: +1
  The missing `array_index_nospec()` creates a Spectre V1 style speculative read possibility from `perms->adm` or adjacent memory after a bounds check on a user-controlled `domain` value.

Subtracting / limiting factors:

* Rare AND difficult-to-reach subsystem/configuration: -1 in conservative score
  The issue is limited to s390 zcrypt, custom device nodes, administrative CPRB handling, and speculative execution conditions. This is not a broad default Linux attack surface.

Not awarded:

* Remote reachable: +0
  No network path is shown.

* Generic memory corruption: +0
  This is not UAF, OOB write, double free, type confusion, or refcount corruption.

* Privilege escalation plausible: +0
  No write primitive, object corruption, callback control, or reclaim primitive is present.

* Reliable kernel crash / strong DoS: +0
  The patch prevents speculative information disclosure, not a crash path.

## 3. Reachability analysis

The bug is reachable through local ioctl handling in `drivers/s390/crypto/zcrypt_api.c`. The attacker needs access to a zcrypt device node and the ability to submit the relevant CCA or EP11 CPRB path. In typical deployments this is restricted and architecture-specific, but custom device nodes may delegate access to non-root users or service accounts.

Namespaces and containers do not automatically make this reachable unless the zcrypt device node is passed through or exposed. The affected path is not network reachable. Realistic exploitation also depends on s390 hardware, zcrypt configuration, administrative CPRB filtering, and whether a usable speculative side channel exists in practice.

## 4. Severity interpretation

This behaves like an ordinary Moderate speculative information disclosure issue, not an Important-class kernel memory corruption vulnerability. The theoretical worst case is local confidentiality impact through transient execution, possibly higher than a permission bitmap leak if adjacent sensitive kernel memory can be inferred. Realistic exploitation evidence is limited by the specialized subsystem, local device access requirement, and side-channel complexity.

It should not be auto-classified as Low because the affected code is crypto permission handling and the patch explicitly adds a Spectre mitigation for a user-controlled index. It also should not be treated as Important because there is no supported privilege escalation, write primitive, crash primitive, or broad remote exposure.

## 5. One-sentence report phrase

A local s390 zcrypt ioctl path used a user-controlled domain value as an index after bounds checking but without a speculation barrier, allowing a possible Spectre style confidentiality leak under custom device and administrative CPRB conditions.

## 6. Manual review recommendation

MANUAL CHECK RECOMMENDED

Manual review is recommended because this is an architecture-specific speculative execution information leak in crypto permission handling. However, the practical score remains low because there is no memory corruption, no DoS, no network reachability, and no demonstrated privilege escalation path.

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

Patch: s390/zcrypt: Close speculative mem read possibility
Commit: 82b62eda68abfb7a33ac40f24b8c4128c2891148
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=82b62eda68abfb7a33ac40f24b8c4128c2891148

Commit description:

The domain value is extracted from a given CCA or EP11 ioctl struct
when a CPRB is about to be sent. Thus this is a user controlled value.
Under some special conditions (custom device node used, administrative
load) this value is used as an array index after bounds checking, but
without speculation barrier.

Add the missing array_index_nospec() call to prevent speculative
execution where this domain value is used.

Fixes: cfd68b3 ("s390/zcrypt: Filter admin CPRBs on custom devices")
Cc: stable@vger.kernel.org
Reported-by: Christian Borntraeger <borntraeger@linux.ibm.com>
Reviewed-by: Finn Callies <fcallies@linux.ibm.com>
Signed-off-by: Harald Freudenberger <freude@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  drivers/s390/crypto/zcrypt_api.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=82b62eda68abfb7a33ac40f24b8c4128c2891148

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

The original Linux kernel CVE announcement for CVE-2026-74466 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74466

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:N/A:N
  The Best / paranoid CVSS vector: AV:L/AC:H/PR:L/UI:N/S:U/C:H/I:N/A:N
  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: 1
  Paranoid ActionableScore: 2

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).