[CVE-2026-53306][MODERATE REGULAR] tty: hvc_iucv: fix off-by-one in number of supported devices [ Upstream

[CVE-2026-53306][MODERATE REGULAR] tty: hvc_iucv: fix off-by-one in number of supported devices [ Upstream

[AL-KERNEL]

CVE: CVE-2026-53306
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: tty: hvc_iucv: fix off-by-one in number of supported devices [ Upstream
Commit: 3d3b89e6ab93bdd0efd45828bda6b0e61cc46dff
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=3d3b89e6ab93bdd0efd45828bda6b0e61cc46dff
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-53306
Analysis date: Fri, 26 Jun 2026 15:55:46 -0400
ActionableScore: 3
ActionableScore lower bound: 2
Actionable bucket: Borderline Ordinary Moderate, manual review recommended
Manual review required: YES

Summary:
An off-by-one bounds check in `hvc_iucv_get_private()` can allow one-past-end access to `hvc_iucv_table` on s390 z/VM hvc IUCV configurations, leading primarily to a local kernel crash risk.

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

The original Linux kernel CVE announcement for CVE-2026-53306 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-53306

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-2026-53306	MODERATE	CHECK	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:N/A:H';CWE-193;CWE-125;CWE-129;**CWE-476;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '5.3';DESCR 'An off by one bounds check in hvc_iucv_get_private can allow hvc_iucv_table to be indexed one element past the end when the device counter reaches the maximum supported value. This is an out of bounds read of a kernel pointer table and may later lead to an invalid pointer dereference, causing a kernel crash or oops. For the CVSS the PR:L is used because a local user or local process context is needed to reach the hvc IUCV console path, while configuring the maximum number of devices is normally an administrative deployment condition. The issue is not network reachable and is specific to s390 z/VM IUCV hvc console configurations. Impact is primarily local denial of service, with only limited confidentiality concern in the paranoid score because the primitive is an out of bounds read rather than an overwrite.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline Ordinary Moderate, manual review recommended (with actual score 3)	YES	PRINTF-TTY SIMPLEFIX HARDWARE LINUS 	NO	NO	checked

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

ActionableScoreLower=2
ActionableScore=3

## 1. ActionableScore

* Conservative score: 2
* Paranoid score: 3
* Final recommended bucket: **Borderline Ordinary Moderate, manual review recommended**

## 2. Signal breakdown

Triggered signals:

* Local unprivileged trigger: +1
  A local process may be able to reach the hvc IUCV console path if the device exists and permissions allow access.
* Memory corruption, weak/indirect corruption candidate: +1
  The bug is an off-by-one bounds check that permits `hvc_iucv_table[8]` when the valid range is `0..7`. This is an out-of-bounds read of a kernel pointer table, not an overwrite.
* Reliable kernel crash / strong DoS: +1
  If the one-past-end pointer is consumed as a `struct hvc_iucv_private *`, an invalid pointer dereference can plausibly cause an Oops or kernel crash.
* Rare AND difficult-to-reach subsystem/configuration: -1
  The issue is specific to s390 z/VM IUCV hvc console support and requires the maximum configured device count condition.

Paranoid-only signal:

* Confidentiality impact plausible: +1
  Limited confidentiality concern is defensible because this is an OOB read of a kernel pointer table, but no user-visible disclosure path is shown.

Not awarded:

* No strong memory corruption +2 because there is no OOB write, UAF, double free, type confusion, controlled overwrite, or reclaim primitive.
* No privilege escalation plausible bonus because the patch only shows one-past-end pointer access and possible invalid dereference.
* No network reachability bonus.
* No additional availability impact bonus because crash impact is already counted.

## 3. Reachability analysis

The bug is locally reachable only in systems using `hvc_iucv` on s390 z/VM with `hvc_iucv_devices` reaching the maximum supported count. The vulnerable condition occurs when `num == hvc_iucv_devices == 8`, where the old check `num > hvc_iucv_devices` allowed one-past-end access.

Namespaces and containers do not materially improve reachability unless the relevant hvc device node is exposed into the container. In typical deployments this is platform-specific and not a broadly default interface. The bug is not remotely network-triggerable.

## 4. Severity interpretation

This behaves more like an ordinary Moderate or Low-like kernel memory-safety issue than an Important vulnerability. The theoretical concern is kernel OOB pointer-table access, but the realistic evidence supports local DoS through invalid pointer dereference rather than privilege escalation.

The paranoid score is slightly higher because kernel OOB reads deserve manual review, but the patch does not support a strong corruption primitive or practical LPE chain.

## 5. One-sentence report phrase

An off-by-one bounds check in `hvc_iucv_get_private()` can allow one-past-end access to `hvc_iucv_table` on s390 z/VM hvc IUCV configurations, leading primarily to a local kernel crash risk.

## 6. Manual review recommendation

MANUAL CHECK RECOMMENDED

Reason: this is a real kernel out-of-bounds access, but it is read-only, platform-specific, not network reachable, and no write primitive or LPE path is shown by the patch.

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

Patch: tty: hvc_iucv: fix off-by-one in number of supported devices [ Upstream
Commit: 3d3b89e6ab93bdd0efd45828bda6b0e61cc46dff
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=3d3b89e6ab93bdd0efd45828bda6b0e61cc46dff

Changed files:
  drivers/tty/hvc/hvc_iucv.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=3d3b89e6ab93bdd0efd45828bda6b0e61cc46dff

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

The original Linux kernel CVE announcement for CVE-2026-53306 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-53306

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:N/A:H
  The Best / paranoid CVSS vector: AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:N/A:H
  The Best / paranoid CVSS score: 5.3

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: 3

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).