[CVE-2026-64123][MODERATE REGULAR] net: hsr: defer node table free until after RCU readers [ Upstream

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

CVE: CVE-2026-64123
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: net: hsr: defer node table free until after RCU readers [ Upstream
Commit: 0ea70fb46940620848c08d9d399455c9e82fecdb
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=0ea70fb46940620848c08d9d399455c9e82fecdb
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64123
Analysis date: Sun, 19 Jul 2026 14:41:27 -0400
ActionableScore: 4
ActionableScore lower bound: 3
Actionable bucket: Borderline manual review / Actionable Moderate candidate
Manual review required: YES

Summary:
HSR generic netlink readers can race with HSR device teardown and dereference a freed `hsr_node` object because node deletion used immediate `list_del()` and free instead of RCU-delayed lifetime handling.

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

The original Linux kernel CVE announcement for CVE-2026-64123 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64123

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-2026-64123	MODERATE	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:L/I:L/A:H';*CWE-416;CWE-362;*CWE-664;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:N/A:H';BEST CVSS score: '5.8';DESCR 'HSR generic netlink node operations can race with RTM_DELLINK teardown because readers walk the HSR node database under rcu_read_lock while hsr_del_nodes removed entries with list_del and freed them immediately. A local attacker who can operate on HSR devices could race node status or node list queries against link deletion and make the reader dereference a freed hsr_node object when copying node macaddress_A. For the CVSS the PR:L is used for the paranoid score because delegated CAP_NET_ADMIN or network namespace control can be sufficient in practical deployments, even if full host administration is not required. The issue is not directly network reachable and requires local control plane activity rather than packet traffic. Impact is at least local denial of service via kernel crash and may include limited confidentiality impact from stale object reads. Limited integrity impact is kept only in the paranoid score because the patch demonstrates a use after free race but not an arbitrary write primitive.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline manual review / Actionable Moderate candidate (with actual score 4)	YES	INIT UAF NETWORK LINUS  DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN5	YES	NO	checked

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

ActionableScore=4
ActionableScoreLower=3

## 1. ActionableScore

* Conservative score: **3**
* Paranoid score: **4**
* Final recommended bucket: **Borderline manual review / Actionable Moderate candidate**

## 2. Signal breakdown

Conservative signals:

* **Memory corruption, weak primitive: +1**
  The patch fixes a demonstrated slab use after free, but the shown access is a stale read of `hsr_node` data during generic netlink reporting.
* **Real lifetime corruption: +1**
  This is an RCU lifetime bug: readers run under `rcu_read_lock()` while teardown freed list nodes immediately.
* **Reliable kernel crash / strong DoS: +1**
  KASAN reproduces a slab use after free, and non KASAN kernels may crash or behave unpredictably.
* **Hard or unreliable race / special timing required: -1**
  The issue requires racing generic netlink node queries with `RTM_DELLINK` teardown.
* **Requires CAP_NET_ADMIN in typical deployments: -1**
  Creating or deleting HSR devices is normally privileged, but namespace delegation can reduce this from full host root.

Paranoid adjustment:

* **Local trigger via delegated net namespace or CAP_NET_ADMIN: +1**
  In environments where untrusted users can create user and network namespaces or have delegated network administration, practical reachability is better than full host root.

Not counted:

* No remote reachability bonus. This is control plane activity, not packet-triggered.
* No strong LPE bonus. The patch shows stale pointer dereference/read, not attacker-controlled reclaim, overwrite, callback control, type confusion, or refcount takeover.
* No confidentiality/integrity impact in the conservative score. The demonstrated primitive is not a clear info leak or write primitive.

## 3. Reachability analysis

The bug is triggered locally by racing HSR generic netlink node-list or node-status operations with link teardown through `RTM_DELLINK`. In normal host deployments, this usually requires `CAP_NET_ADMIN` to create or delete the HSR interface. In systems with unprivileged user namespaces or delegated network namespaces, a less privileged local user may gain enough network administration capability inside a namespace to exercise the path. The issue is not directly network reachable and does not appear triggerable by ordinary HSR packet traffic alone. HSR is not a default mainstream interface type on most systems, but it is reachable through standard Linux networking control APIs when enabled.

## 4. Severity interpretation

This behaves more like **borderline Actionable Moderate** than Important. The bug is a real RCU use-after-free and therefore deserves manual review, but the demonstrated access is a stale read during netlink reporting, not a controlled write or reclaim-based exploitation primitive. Realistic impact is mainly local DoS through crash or memory-safety failure. A privilege escalation path is not strongly supported by the patch context, but the lifetime bug class and namespace-sensitive reachability make auto-closing risky.

## 5. One-sentence report phrase

HSR generic netlink readers can race with HSR device teardown and dereference a freed `hsr_node` object because node deletion used immediate `list_del()` and free instead of RCU-delayed lifetime handling.

## 6. Manual review recommendation

**MANUAL CHECK REQUIRED**

Reason: this is a demonstrated RCU race with use-after-free in kernel networking code, and namespace or delegated `CAP_NET_ADMIN` deployments may make the local trigger more reachable than full host-root-only scenarios.

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

Patch: net: hsr: defer node table free until after RCU readers [ Upstream
Commit: 0ea70fb46940620848c08d9d399455c9e82fecdb
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=0ea70fb46940620848c08d9d399455c9e82fecdb

Changed files:
  net/hsr/hsr_framereg.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=0ea70fb46940620848c08d9d399455c9e82fecdb

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

The original Linux kernel CVE announcement for CVE-2026-64123 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64123

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:H
  The Best / paranoid CVSS vector: AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:H
  The Best / paranoid CVSS score: 5.8

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: 3
  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).