[CVE-2026-64424][MODERATE 7.0] netpoll: fix a use-after-free on shutdown path

[CVE-2026-64424][MODERATE 7.0] netpoll: fix a use-after-free on shutdown path

[AL-KERNEL]

CVE: CVE-2026-64424
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: netpoll: fix a use-after-free on shutdown path
Commit: 95ecc5b58042f6b6743b589e6588f1cd7ba336aa
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=95ecc5b58042f6b6743b589e6588f1cd7ba336aa
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64424
Analysis date: Sat, 25 Jul 2026 14:04:46 -0400
ActionableScore: 5
ActionableScore lower bound: 3
Actionable bucket: Strong Important candidate / Actionable Moderate at minimum
Manual review required: YES

Summary:
A race in the netpoll shutdown path can leave delayed TX work running after `netpoll_info` is freed, causing a KASAN-confirmed use-after-free and at least a local kernel crash, with weak but review-worthy privilege-escalation concern due to asynchronous freed-object access.

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ABOUT THIS REPORT
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The original Linux kernel CVE announcement for CVE-2026-64424 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64424

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-2026-64424	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:H/I:H/A:H';*CWE-416;CWE-362;CWE-667;Other CVSS 'AV:L/AC:H/PR:H/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '7';DESCR 'A use-after-free can occur in the netpoll shutdown path when delayed TX work keeps running while netpoll_info is being released through RCU cleanup. The worker reaches the freed object through its embedded work item and can touch the freed TX queue after kfree, which is a kernel memory corruption condition and at least a crashable DoS. For the CVSS the PR:L is used for the paranoid score because a reduced capability local actor with delegated network or device administration may be able to trigger cleanup paths in some deployments, while the base case is closer to administrator controlled netpoll or netconsole teardown. The issue is not directly network reachable because the vulnerable window is in shutdown cleanup rather than packet receive processing. Impact is at least local denial of service via kernel crash and in the worst defensible case may include confidentiality or integrity impact due to UAF based memory corruption.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate / Actionable Moderate at minimum (with actual score 4)	YES	REMOTE READ DANGER UAF SKB LINUS  INCREASED_TO_MODERATE7_BASED_ON_GUESSCVSS DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN5	NO	NO	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=5
ActionableScoreLower=3

## 1. ActionableScore

* Conservative score: 3
* Paranoid score: 5
* Final recommended bucket: **Strong Important candidate / Actionable Moderate at minimum**::

## 2. Signal breakdown

Conservative signals:

* Memory corruption, weak primitive: +1
  The patch fixes a real KASAN-detected slab-use-after-free, but it is race-dependent and no attacker-controlled reclaim or replacement object is shown.

* Real lifetime corruption: +1
  `netpoll_info` can be freed via RCU while delayed TX work still reaches it through the embedded work item.

* Reliable kernel crash / strong DoS: +1
  KASAN shows a concrete UAF in `queue_process()` via `skb_dequeue()`, making kernel crash realistic.

* Hard or unreliable race / special timing required: -1
  Triggering requires overlap between netpoll cleanup and pending TX work.

* Requires admin/root/CAP-style control in typical deployments: -1
  Netpoll/netconsole setup and shutdown are usually administrator-controlled, but reduced-capability service or delegated network-management contexts may exist, so I do not apply the full -2 in the practical triage score.

* Privileged kernel/device-management lifetime path: +1
  The bug is in a shutdown/cleanup path tied to network device emergency logging and delayed work lifetime.

Paranoid additional interpretation:

* Weak LPE concern: +1
  The worker accesses a freed object asynchronously and `skb_dequeue()` can interact with queue metadata after free. No reclaim primitive is demonstrated, so this is weak concern only, not strong LPE plausibility.

Paranoid cap applied:

* This is a race-dependent UAF without demonstrated attacker-controlled reclaim, callback control, type confusion, or arbitrary write. Per the race-UAF downgrade rule, the conservative score should not be treated as Important solely from bug class, and the paranoid score is capped at 5.

## 3. Reachability analysis

The issue is not directly network-triggerable. It occurs in the netpoll shutdown path when `__netpoll_cleanup()` clears `dev->npinfo`, schedules RCU cleanup, and pending delayed TX work can still run afterward.

Typical triggering requires local administrative control over netpoll or netconsole configuration, device teardown, module unload, or network device shutdown paths. In containerized or appliance environments, a reduced-capability root, service account, or delegated network-management component might influence these paths, which supports the paranoid PR:L-style interpretation but not a true remote or unprivileged default trigger.

Netpoll/netconsole is not a default common packet-processing path. Realistic exploitation depends on netpoll being configured and on winning a shutdown race with queued TX work.

Call-site confidence: high. The commit message gives the failing call trace, allocation/free path, and the exact delayed work versus RCU cleanup lifetime mismatch.

## 4. Severity interpretation

This is more than an ordinary Moderate correctness fix because it is a confirmed kernel UAF in asynchronous workqueue lifetime management. Realistic demonstrated impact is local DoS through kernel crash.

At the same time, the patch does not show attacker-controlled reclaim, object replacement, callback dispatch, or arbitrary write. Therefore, practical privilege escalation is not strongly demonstrated. The best handling is Actionable Moderate at minimum, with manual review recommended because UAF lifetime bugs are often under-scored during early triage.

## 5. One-sentence report phrase

A race in the netpoll shutdown path can leave delayed TX work running after `netpoll_info` is freed, causing a KASAN-confirmed use-after-free and at least a local kernel crash, with weak but review-worthy privilege-escalation concern due to asynchronous freed-object access.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: this is a real kernel UAF, not just a warning or resource leak. Even though the demonstrated impact is DoS and exploitation appears race-dependent and usually privileged, the asynchronous workqueue access to freed network state should not be auto-closed without human review.

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

Patch: netpoll: fix a use-after-free on shutdown path
Commit: 95ecc5b58042f6b6743b589e6588f1cd7ba336aa
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=95ecc5b58042f6b6743b589e6588f1cd7ba336aa

Commit description:

There is a use-after-free error on netpoll, which is clearly detected by
KASAN.

      BUG: KASAN: slab-use-after-free in _raw_spin_lock_irqsave+0x3b/0x80
      Read of size 1 at addr ... by task kworker/9:1
      Workqueue: events queue_process
      Call Trace:
       skb_dequeue+0x1e/0xb0
       queue_process+0x2c/0x600
       process_scheduled_works+0x4b6/0x850
       worker_thread+0x414/0x5a0
      Allocated by task 242:
       __netpoll_setup+0x201/0x4a0
       netpoll_setup+0x249/0x550
       enabled_store+0x32f/0x380
      Freed by task 0:
       kfree+0x1b7/0x540
       rcu_core+0x3f8/0x7a0

The problem happens when there is a pending TX worker running in
parallel with the cleanup path.

This is what happens on netpoll shutdown path:

1) __netpoll_cleanup() is called
2) set dev->npinfo to NULL
3) call_rcu() with rcu_cleanup_netpoll_info()
  3.1) rcu_cleanup_netpoll_info() tries to cancel all workers with
       cancel_delayed_work(), but doesn't wait for the worker to finish
4) and kfree(npinfo);

Because 3.1) doesn't really cancel the work, as the comment says "we
can't call cancel_delayed_work_sync here, as we are in softirq", the TX
worker can run after 4).

Tl;DR: queue_process() is not an RCU reader, it reaches npinfo through
the work item via container_of().

Use disable_delayed_work_sync() to ensure the worker is completely
stopped and prevent any future re-arming attempts. Once npinfo is set
to NULL, senders will bail out and not queue new work. The disable flag
ensures any in-flight re-arming attempts also fail silently.

In the future, we can do the cleanup inline here without needing the
npinfo->rcu rcu_head, but that is net-next material.

Cc: stable@vger.kernel.org
Fixes: 38e6bc1 ("netpoll: make __netpoll_cleanup non-block")
Reviewed-by: Pavan Chebbi <pavan.chebbi@broadcom.com>
Signed-off-by: Breno Leitao <leitao@debian.org>
Link: https://patch.msgid.link/20260625-netpoll_rcu_fix-v2-1-0748ffac1e98@debian.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  net/core/netpoll.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=95ecc5b58042f6b6743b589e6588f1cd7ba336aa

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

The original Linux kernel CVE announcement for CVE-2026-64424 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64424

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:H/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:H/I:H/A:H
  The Best / paranoid CVSS score: 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: 3
  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: NO
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).