[CVE-2026-64559][MODERATE 7.0] s390/pkey: Check length in PKEY_VERIFYPROTK ioctl

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

CVE: CVE-2026-64559
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: s390/pkey: Check length in PKEY_VERIFYPROTK ioctl
Commit: 0a9e34ccbe772b8f321388cdbdf4f22b94e513e7
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=0a9e34ccbe772b8f321388cdbdf4f22b94e513e7
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64559
Analysis date: Wed, 29 Jul 2026 12:56:37 -0400
ActionableScore: 5
ActionableScore lower bound: 4
Actionable bucket: Actionable Moderate at minimum
Manual review required: YES

Summary:
A local caller can pass an oversized protected-key length to the s390 PKEY_VERIFYPROTK ioctl, causing downstream verification logic to operate beyond the fixed kernel buffer and creating a local DoS and possible kernel data exposure risk.

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ABOUT THIS REPORT
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The original Linux kernel CVE announcement for CVE-2026-64559 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64559

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.

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AL-KERNEL CLASSIFICATION RESULT
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CVE-2026-64559	MODERATE	CHECK WITH IMPACT FROM ORIG NN IMPORTANT	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:N/A:H';CWE-125;*CWE-20;CWE-130;Other CVSS 'AV:L/AC:L/PR:L/UI:N/S:U/C:L/I:N/A:H';BEST CVSS score: '7.1';DESCR 'PKEY_VERIFYPROTK in s390/pkey can trust a user supplied protkey length that is larger than the fixed protkey buffer copied into the kernel request structure. A local caller with access to the pkey ioctl can therefore cause later verification logic to read beyond the embedded buffer, which is an out-of-bounds read candidate in kernel memory. For the CVSS the PR:L is used because triggering requires a local process with access to the pkey device interface, but not full administrative control in the paranoid practical risk model. The issue is not network reachable and applies to s390 systems with the pkey driver and relevant crypto support available. Impact is at least local denial of service via kernel fault or invalid memory access, with possible confidentiality impact if adjacent kernel data can influence verification behavior or be exposed through a handler dependent side effect.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate at minimum (with actual score 5)	YES	SIMPLEFIX IMPROVEONLY LINUS  KPANIC DECREASED_TO_MODERATE70_BASED_ON_ACTIONABLESCORELOWERTHAN7	YES	NO	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=5
ActionableScoreLower=4

## 1. ActionableScore

* Conservative score: 4
* Paranoid score: 5
* Final recommended bucket: **Actionable Moderate at minimum**::

## 2. Signal breakdown

Triggered signals:

* Local unprivileged trigger: +1
  The affected path is a local ioctl, and the attacker needs a local process with access to the s390 pkey interface, not full host root in the practical model.

* Weak read-side memory access concern: +1
  The user-controlled `protkey.len` can exceed the fixed kernel buffer size. Later verification logic may consume this length and read past `kvp.protkey.protkey`. This is an OOB read candidate, not an OOB write.

* Reliable kernel crash / strong DoS: +1
  An oversized length can plausibly lead to invalid kernel memory access in downstream protected-key verification handling.

* Confidentiality impact plausible: +1
  Because the bug is an OOB read candidate in kernel memory near a copied request structure, limited confidentiality impact is plausible, although no direct disclosure primitive is demonstrated in the patch.

* Rare / architecture-specific subsystem: -1
  Exposure is limited to s390 systems with the pkey driver and relevant protected-key crypto support.

Paranoid-only adjustment:

* Handler-dependent confidentiality risk: +1
  If a downstream pkey handler uses the oversized length in a way that creates an observable oracle, error-dependent behavior, or reads adjacent sensitive kernel data, the practical impact could exceed pure DoS.

Not awarded:

* No strong LPE plausibility.
  The patch does not show attacker-controlled reclaim, write-after-free, type confusion, arbitrary write, callback control, or refcount takeover.

* No integrity impact.
  The primitive indicated by the patch is read-side bounds failure, not a write primitive.

* No remote reachability.
  This is a local ioctl path.

## 3. Reachability analysis

The bug is triggerable by a local process that can open and issue `PKEY_VERIFYPROTK` ioctl requests to the s390 pkey interface. It is not network-triggerable. Namespaces do not obviously make this remote, but device-node permissions or container device passthrough could affect whether a reduced-privilege process can reach the ioctl. The affected subsystem is architecture-specific and not broadly deployed across generic Linux systems.

Call-site confidence: medium. The patch shows the vulnerable ioctl handler and the missing length validation. Full downstream handler behavior is not included, so the exact disclosure or crash behavior depends on later pkey verification code.

## 4. Severity interpretation

This behaves like an actionable Moderate issue rather than a clear Important-class vulnerability. The realistic impact is local DoS plus possible limited confidentiality impact from an OOB read candidate. Theoretical privilege escalation is not supported by the patch because there is no write, UAF reclaim, type confusion, or object replacement primitive. Manual review is still justified because kernel OOB reads in ioctl paths can be underestimated if downstream handlers expose observable behavior.

## 5. One-sentence report phrase

A local caller can pass an oversized protected-key length to the s390 PKEY_VERIFYPROTK ioctl, causing downstream verification logic to operate beyond the fixed kernel buffer and creating a local DoS and possible kernel data exposure risk.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: this is a local kernel OOB read candidate in a crypto ioctl path, with possible confidentiality impact depending on downstream handler behavior.

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UPSTREAM PATCH SUMMARY
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Patch: s390/pkey: Check length in PKEY_VERIFYPROTK ioctl
Commit: 0a9e34ccbe772b8f321388cdbdf4f22b94e513e7
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=0a9e34ccbe772b8f321388cdbdf4f22b94e513e7

Commit description:

Explicitly check the buffer length request structure provided by
user-space and fail, if it exceeds the buffer size.

Cc: stable@vger.kernel.org
Fixes: 8fcc231 ("s390/pkey: Introduce pkey base with handler registry and handler modules")
Reported-by: Christian Borntraeger <borntraeger@linux.ibm.com>
Reviewed-by: Harald Freudenberger <freude@linux.ibm.com>
Reviewed-by: Ingo Franzki <ifranzki@linux.ibm.com>
Signed-off-by: Holger Dengler <dengler@linux.ibm.com>
Signed-off-by: Alexander Gordeev <agordeev@linux.ibm.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  drivers/s390/crypto/pkey_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=0a9e34ccbe772b8f321388cdbdf4f22b94e513e7

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

The original Linux kernel CVE announcement for CVE-2026-64559 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64559

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:L/PR:L/UI:N/S:U/C:L/I:N/A:H
  The Best / paranoid CVSS vector: AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:N/A:H
  The Best / paranoid CVSS score: 7.1

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
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).