[CVE-2026-64576][MODERATE REGULAR] nexthop: initialize extack in nh_res_bucket_migrate() [ Upstream

[CVE-2026-64576][MODERATE REGULAR] nexthop: initialize extack in nh_res_bucket_migrate() [ Upstream

[AL-KERNEL]

CVE: CVE-2026-64576
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: nexthop: initialize extack in nh_res_bucket_migrate() [ Upstream
Commit: c0936c131a71657afc635d0db2ab096d15d473e1
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=c0936c131a71657afc635d0db2ab096d15d473e1
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64576
Analysis date: Wed, 05 Aug 2026 04:58:12 -0400
ActionableScore: 4
ActionableScore lower bound: 2
Actionable bucket: Borderline manual-review Moderate
Manual review required: NO

Summary:
An uninitialized `netlink_ext_ack` in `nh_res_bucket_migrate()` can leave `extack._msg` as a stale stack pointer, causing printk to dereference invalid kernel memory and panic the system when resilient nexthop notifier setup fails.

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

The original Linux kernel CVE announcement for CVE-2026-64576 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64576

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: NO

A detailed explanation of the methodology and priority rules is included
at the end of this message.

======================================================================
AL-KERNEL CLASSIFICATION RESULT
======================================================================

CVE-2026-64576	MODERATE	CHECK WITH IMPACT FROM ORIG NN LOW	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-457;CWE-824;CWE-755;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 uninitialized netlink_ext_ack in nh_res_bucket_migrate can leave extack._msg as a stale stack pointer when call_nexthop_res_bucket_notifiers fails before any notifier sets an error message. The error path then passes extack._msg to printk as a string, which can dereference uninitialized stack memory and cause a general protection fault or kernel panic. For the CVSS the PR:L is used because triggering the path requires a local process with enough network administration rights to create or modify resilient nexthop state through rtnetlink, but it does not require full host root in delegated CAP_NET_ADMIN or container style deployments. The issue is not remotely reachable by packet traffic and is exposed through the local rtnetlink control plane. Impact is primarily denial of service. For the paranoid score, limited confidentiality impact is kept as a review sensitive interpretation because the primitive is an uninitialized pointer read passed to printk, but the patch does not show an arbitrary write or stable privilege escalation primitive.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline manual-review Moderate (with actual score 3)	YES	REMOTE KASAN OOB ERRORPATH NETWORK  DECREASED_TO_MODERATE_REGULAR_FROM_MODERATE7_BASED_ON_GUESSCVSS_AND_CIANNH	YES	NO	checked

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

ActionableScore=4
ActionableScoreLower=2

## 1. ActionableScore

* Conservative score: 2
* Paranoid score: 4
* Final recommended bucket: **Borderline manual-review Moderate**::

## 2. Signal breakdown

Conservative signals:

* Weak or indirect corruption candidate: +1. `extack._msg` is an uninitialized stack pointer later treated as a string pointer by `printk`.
* Reliable kernel crash / strong DoS: +1. The report shows GPF, KASAN wild-memory-access, and kernel panic.
* Availability impact realistic: +1. The failure is host-wide when the bad error path is reached.
* Requires CAP_NET_ADMIN or equivalent in typical deployments: -1. Triggering uses rtnetlink nexthop control plane, but delegated netns or container `CAP_NET_ADMIN` makes this less than full host-root in some environments.
* Special condition / reliability constraint: -1. The demonstrated path needs an early notifier allocation failure such as `kzalloc` returning `-ENOMEM`, and impact is clearer with `CONFIG_INIT_STACK_NONE`.

Paranoid-only signals:

* Local unprivileged trigger concern: +1. On systems with unprivileged user namespaces and network namespaces, a local user may obtain effective `CAP_NET_ADMIN` in a namespace and reach rtnetlink nexthop paths.
* Confidentiality impact plausible: +1. A stale pointer passed to `%s` can cause unintended kernel memory reads into printk before faulting, although this is not a demonstrated stable info leak.

No added signals:

* No strong memory corruption primitive. The patch shows an uninitialized pointer dereference, not UAF, OOB write, double-free, refcount takeover, or attacker-controlled overwrite.
* No privilege escalation bonus. There is no demonstrated reclaim, write primitive, callback control, or type confusion.
* No remote/network-triggerable bonus. This is rtnetlink control plane, not packet-triggered data plane.

## 3. Reachability analysis

The bug is triggered through local rtnetlink operations involving resilient nexthop groups, likely via `rtm_new_nexthop` and `nh_res_table_upkeep`. In normal host deployments this requires `CAP_NET_ADMIN`, so it is not a plain remote or fully unprivileged local issue. Namespace behavior matters because some systems allow unprivileged creation of user and network namespaces, which can make effective `CAP_NET_ADMIN` reachable by a local user inside a namespace. The path is not triggered by ordinary packet traffic. Realistic exploitation also depends on forcing or encountering the allocation failure path before any notifier initializes `extack._msg`.

Call-site confidence: high, because the stack trace and caller path are included.

## 4. Severity interpretation

This behaves more like a borderline Moderate requiring manual review than an Important-class vulnerability. The realistic impact is kernel crash / DoS through an error path. Theoretical confidentiality impact exists because a stale stack pointer is used as a string pointer, but the patch does not support arbitrary write, reliable memory disclosure, or LPE. The paranoid score is higher to avoid auto-closing an uninitialized kernel pointer dereference in networking control-plane code.

## 5. One-sentence report phrase

An uninitialized `netlink_ext_ack` in `nh_res_bucket_migrate()` can leave `extack._msg` as a stale stack pointer, causing printk to dereference invalid kernel memory and panic the system when resilient nexthop notifier setup fails.

## 6. Manual review recommendation

MANUAL CHECK RECOMMENDED

Reason: the primary impact is DoS and the trigger is constrained, but the issue involves an uninitialized stack pointer dereference in networking control-plane code with a plausible limited information-disclosure angle under some configurations.

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

Patch: nexthop: initialize extack in nh_res_bucket_migrate() [ Upstream
Commit: c0936c131a71657afc635d0db2ab096d15d473e1
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=c0936c131a71657afc635d0db2ab096d15d473e1

Changed files:
  lib/vsprintf.c
  kernel/printk/printk.c
  net/ipv4/nexthop.c
  net/core/rtnetlink.c
  net/netlink/af_netlink.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=c0936c131a71657afc635d0db2ab096d15d473e1

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

The original Linux kernel CVE announcement for CVE-2026-64576 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64576

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: 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).