[CVE-2026-72011][LOW] s390/diag: Add missing array_index_nospec() call to memtop_get_page_count()

[CVE-2026-72011][LOW] s390/diag: Add missing array_index_nospec() call to memtop_get_page_count()

[AL-KERNEL]

CVE: CVE-2026-72011
Priority: LOW
AL-KERNEL base severity: LOW
KPANIC flag: NO
Patch: s390/diag: Add missing array_index_nospec() call to memtop_get_page_count()
Commit: c6b4d454865a81ceea1422243aaaedc363b6f713
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=c6b4d454865a81ceea1422243aaaedc363b6f713
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-72011
Analysis date: Sat, 15 Aug 2026 09:05:34 -0400
ActionableScore: 2
ActionableScore lower bound: 1
Actionable bucket: Ordinary Moderate / Low-like
Manual review required: YES

Summary:
A user controlled `level` value in s390 diag310 memory topology handling was bounds checked architecturally but used without `array_index_nospec()`, allowing a local Spectre v1 style side channel risk with possible confidentiality exposure only.

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

The original Linux kernel CVE announcement for CVE-2026-72011 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-72011

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: LOW
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-2026-72011	LOW	CHECK WITH IMPACT FROM ORIG NN MODERATE	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-208;*CWE-200;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:N/A:N';BEST CVSS score: '4.7';DESCR 'A user controlled level value in s390 diag310 memory topology handling is bounds checked architecturally but was still used as an array index without array_index_nospec. This can allow speculative out of bounds reads from memtop_pages and creates a Spectre v1 style local side channel risk. For the CVSS the PR:L is used because a local user or local process is needed to reach the interface and provide the level value. The issue is not network reachable and does not provide a direct write primitive. Impact is limited to possible confidentiality exposure through speculative execution, with no evidence of integrity impact, kernel crash, or privilege escalation.';NO MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate / Low-like (with actual score 2)	YES	READ SIMPLEFIX HARDWARE LINUS  DECREASED_TO_LOW_FROM_MODERATE_BASED_ON_ACTIONABLESCORELESSTHAN3	YES	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 / Low-like**

## 2. Signal breakdown

* Local unprivileged trigger: **+1**. The `level` value is user controlled and can be supplied by a local userspace caller reaching the s390 diag310 memory topology path.
* Confidentiality impact plausible: **+1**. Missing `array_index_nospec()` can permit Spectre v1 style speculative out of bounds reads, so limited side channel disclosure is plausible.
* Rare and architecture specific path: **-1** in the conservative score. The bug is s390 specific and limited to diag310 memory topology handling, not a broad default cross architecture kernel path.
* No generic memory corruption: **+0**. The architectural bounds check remains present. The issue is speculative execution hardening, not an architectural OOB read/write, UAF, double free, or type confusion.
* No privilege escalation signal: **+0**. The patch does not show writable stale state, object replacement, arbitrary write, callback control, refcount misuse, or another LPE primitive.
* No strong DoS: **+0**. No crash, panic, WARN based DoS, or availability failure is described.

Paranoid score keeps the local trigger and confidentiality signal but does not apply the architecture rarity subtraction, resulting in **2**.

## 3. Reachability analysis

A local userspace process that can call the affected s390 diag310 memory topology interface can influence `level`. Namespaces or containers do not materially improve the score unless that interface is exposed inside the container. The path is not network reachable. Exploitation requires s390 hardware or virtualization support and a microarchitectural side channel setup, so realistic exploitation is specialized and high complexity.

Call-site confidence: **medium**. The patch directly shows the user controlled index and the missing nospec barrier, but broader userspace entry details are not included.

## 4. Severity interpretation

This behaves like a low end Moderate speculative information disclosure hardening issue, not an Important kernel vulnerability. The theoretical impact is confidentiality exposure via Spectre v1 style speculative access. Realistic evidence for privilege escalation, integrity impact, or memory corruption is absent.

## 5. One-sentence report phrase

A user controlled `level` value in s390 diag310 memory topology handling was bounds checked architecturally but used without `array_index_nospec()`, allowing a local Spectre v1 style side channel risk with possible confidentiality exposure only.

## 6. Manual review recommendation

**NO MANUAL CHECK NEEDED**

NO MANUAL CHECK REQUIRED COULD BE AUTOCLOSED. This is an architecture specific speculative information leak hardening fix with no memory corruption primitive, no network reachability, no integrity impact, and no demonstrated privilege escalation path.

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

Patch: s390/diag: Add missing array_index_nospec() call to memtop_get_page_count()
Commit: c6b4d454865a81ceea1422243aaaedc363b6f713
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=c6b4d454865a81ceea1422243aaaedc363b6f713

Commit description:

'level' is user space controlled and used to read from an array. Add the
missing array_index_nospec() call to prevent speculative execution.

Cc: stable@vger.kernel.org
Fixes: 0d30871 ("s390/diag: Add memory topology information via diag310")
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>
Reviewed-by: Mete Durlu <meted@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/kernel/diag/diag310.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=c6b4d454865a81ceea1422243aaaedc363b6f713

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

The original Linux kernel CVE announcement for CVE-2026-72011 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-72011

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: LOW
KPANIC detected for this report: NO
Published priority for this report (same as in Subject): LOW

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).