[CVE-2026-64027][MODERATE 7.0] net: shaper: rework the VALID marking (again) [ Upstream

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

CVE: CVE-2026-64027
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: net: shaper: rework the VALID marking (again) [ Upstream
Commit: 2417df5e7bb4184b9d3a2988036bf2c46e594545
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=2417df5e7bb4184b9d3a2988036bf2c46e594545
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64027
Analysis date: Sun, 19 Jul 2026 21:44:59 -0400
ActionableScore: 5
ActionableScore lower bound: 4
Actionable bucket: Strong Important candidate / Actionable Moderate at minimum
Manual review required: YES

Summary:
A race in net shaper VALID state handling can expose lockless readers to a stale or freed `net_shaper` object after XArray entry replacement, creating a UAF risk with at least local kernel DoS impact and requiring manual review for possible escalation.

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

The original Linux kernel CVE announcement for CVE-2026-64027 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64027

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
======================================================================

CVE-2026-64027	MODERATE	CHECK WITH IMPACT FROM ORIG NN IMPORTANT	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:L/UI:N/S:U/C:L/I:L/A:H';BEST CVSS score: '7';DESCR 'A race in net shaper can let a reader observe a VALID state that belonged to an old XArray slot and then load a different or freed net_shaper object after an entry replacement. This creates a use after free risk because rollback could free entries without RCU protection while lockless readers may still reach them. For the CVSS the PR:L is used because triggering requires a local process with access to net shaper control operations, typically CAP_NET_ADMIN or delegated network administration rights rather than full host root. The issue is not directly network reachable and is triggered through control plane configuration changes rather than packet traffic. Impact is at least a local denial of service via kernel crash and in the worst defensible case may allow confidentiality or integrity impact due to use after free on kernel objects.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate / Actionable Moderate at minimum (with actual score 5)	YES	REMOTE RACE UAF HARDWARE LINUS KPANIC  KPANIC DECREASED_TO_MODERATE70_BASED_ON_ACTIONABLESCORELOWERTHAN7	YES	NO	checked

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

ActionableScore=5
ActionableScoreLower=4

## 1. ActionableScore

* Conservative score: 4
* Paranoid score: 5
* Final recommended bucket: **Strong Important candidate / Actionable Moderate at minimum**::

## 2. Signal breakdown

Positive signals:

* Memory corruption, weak primitive: +1
  The commit explicitly describes a possible UAF caused by a race between XArray VALID marks and entry replacement.

* Real lifetime corruption: +1
  The fix changes `kfree()` to `kfree_rcu()`, which strongly indicates an RCU lifetime bug where lockless readers may still hold references.

* Reliable kernel crash / strong DoS: +1
  A stale or freed `net_shaper` object can be reached by lookup or dump paths, plausibly causing kernel crash.

* Privileged kernel/device-management lifetime path: +1
  The affected code manages network shaper objects in a trusted kernel networking control-plane path.

* Availability impact realistic: +1 in paranoid scoring only
  If triggered successfully, the impact is host-wide kernel memory safety failure rather than a contained userspace error.

Negative / limiting signals:

* Hard or unreliable race: -1
  The bug requires a timing window between reader interruption, writer replacement, and rollback/free.

* Requires CAP_NET_ADMIN in typical deployments: conservative penalty applied as -1 rather than -2
  Net shaper control is normally privileged, but CAP_NET_ADMIN may be delegated to containers or network namespaces, so this is not equivalent to full host root in all deployments.

Caps / constraints:

* Race-UAF downgrade rule applies.
  The patch shows UAF/lifetime corruption, but does not show attacker-controlled reclaim, callback control, write-after-free, refcount takeover, or arbitrary write. Therefore conservative scoring is capped at 4 and paranoid scoring at 5.

## 3. Reachability analysis

The bug is triggered through net shaper control-plane operations, likely via netlink configuration paths, not by ordinary packet traffic. In typical host deployments this requires CAP_NET_ADMIN or an equivalent network administration capability. In containerized or namespace-enabled environments, delegated CAP_NET_ADMIN may lower the practical privilege barrier, especially if untrusted workloads can manage network objects. The path is not remotely network-triggerable by packets alone. Realistic exploitation requires racing lookup or dump readers against configuration rollback or replacement.

Call-site confidence: high.
The provided patch includes lookup, commit, rollback, and dump paths, and the lifetime consequence is directly visible through the `kfree()` to `kfree_rcu()` change.

## 4. Severity interpretation

This is stronger than an ordinary Moderate because the patch fixes an explicit UAF-class lifetime race in kernel networking object management. However, it is not a confirmed Important-class LPE because there is no demonstrated attacker-controlled reclaim, object replacement, writable stale access, callback dispatch, or arbitrary write primitive. Realistic impact is at least local kernel crash / DoS. Paranoid handling should keep it as Actionable Moderate because UAF in an RCU-visible networking object should not be auto-closed without manual review.

## 5. One-sentence report phrase

A race in net shaper VALID state handling can expose lockless readers to a stale or freed `net_shaper` object after XArray entry replacement, creating a UAF risk with at least local kernel DoS impact and requiring manual review for possible escalation.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: the commit explicitly states possible UAF, the fix changes object lifetime handling to `kfree_rcu()`, and the affected code has lockless reader paths. No confirmed LPE primitive is shown, but this is a memory lifetime bug in kernel networking control-plane state and should not be auto-closed.

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

Patch: net: shaper: rework the VALID marking (again) [ Upstream
Commit: 2417df5e7bb4184b9d3a2988036bf2c46e594545
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=2417df5e7bb4184b9d3a2988036bf2c46e594545

Changed files:
  include/net/net_shaper.h
  net/shaper/shaper.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=2417df5e7bb4184b9d3a2988036bf2c46e594545

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

The original Linux kernel CVE announcement for CVE-2026-64027 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64027

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:L/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: 4
  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: YES
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).