[CVE-2026-68409][IMPORTANT] wifi: mac80211: defer link RX stats percpu free to RCU [ Upstream

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

CVE: CVE-2026-68409
Priority: IMPORTANT
AL-KERNEL base severity: IMPORTANT
KPANIC flag: YES
Patch: wifi: mac80211: defer link RX stats percpu free to RCU [ Upstream
Commit: 2aa1789880fa5e41049b0f6a74a4fc2fa1997610
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=2aa1789880fa5e41049b0f6a74a4fc2fa1997610
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-68409
Analysis date: Mon, 10 Aug 2026 09:57:22 -0400
ActionableScore: 7
ActionableScore lower bound: 5
Actionable bucket: Strong Important candidate / Actionable Moderate at minimum
Manual review required: YES

Summary:
A mac80211 MLO link removal race can free `pcpu_rx_stats` before all RCU RX readers have drained, allowing adjacent WiFi traffic to hit a write-after-free in the RX fast path and potentially cause kernel memory corruption or a crash.

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

The original Linux kernel CVE announcement for CVE-2026-68409 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-68409

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: IMPORTANT
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-2026-68409	IMPORTANT	CHECK WITH IMPACT FROM ORIG NN MODERATE	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:A/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:H';*CWE-416;CWE-362;*CWE-664;Other CVSS 'AV:A/AC:H/PR:N/UI:N/S:U/C:L/I:L/A:H';BEST CVSS score: '7.5';DESCR 'mac80211 MLO link removal can race with the RX fast path because sta_remove_link freed pcpu_rx_stats immediately while the link_sta container was deferred with RCU. An RX reader that already resolved link_sta under rcu_read_lock can still write to the freed percpu stats buffer, creating a use after free on kernel memory. For the CVSS the PR:N is used for the paranoid score because an adjacent WiFi peer can provide the RX traffic side of the race without a local user account, although reliable triggering depends on MLO link removal, uses_rss, and a narrow timing window. The issue is not Internet remote but can be adjacent network reachable over WiFi. Impact is at least denial of service via kernel crash and in the worst case may allow confidentiality or integrity impact due to memory corruption, so it should be reviewed manually.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate / Actionable Moderate at minimum (with actual score 7)	YES	REMOTE INIT SIMPLEFIX RACE KPANIC  KPANIC INCREASED_TO_HIGH_BASED_ON_GUESSCVSS	NO	NO	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=7
ActionableScoreLower=5

## 1. ActionableScore

* Conservative score: 5
* Paranoid score: 7
* Final recommended bucket: **Strong Important candidate / Actionable Moderate at minimum**::

## 2. Signal breakdown

Conservative signals:

* Remote or adjacent network-triggerable: +2
  The RX fast path is driven by WiFi traffic. This is not Internet-remote, but an adjacent WiFi peer can plausibly provide the packet traffic side of the race.

* Memory corruption, strong corruption primitive: +2
  The bug is a real UAF: `pcpu_rx_stats` is freed while an RCU reader may still hold `link_sta` and write RX statistics through the stale pointer.

* Real lifetime corruption: +1
  This is an RCU lifetime mismatch between the `link_sta` container and its embedded/percpu data lifetime.

* Availability impact realistic: +1
  A write to freed percpu kernel memory can plausibly crash the kernel.

Negative signals:

* Hard or unreliable race / special timing required: -1
  The commit explicitly says the race is hard to win in practice.

* Configuration/runtime dependency: -1
  The issue requires MLO link removal and `uses_rss`, because `pcpu_rx_stats` must exist.

Conservative total: 5

Paranoid additional signals:

* Weak LPE concern: +1
  The stale access is a write-after-free into kernel percpu memory. There is no demonstrated reclaim or controlled overwrite, but this is more than a pure stale read or NULL dereference.

* Common packet RX path relevance: +1
  The affected access is in the mac80211 RX fast path, which increases triage sensitivity even though the vulnerable scenario is MLO-specific.

Paranoid total: 7

## 3. Reachability analysis

An adjacent WiFi peer may influence the RX traffic side of the race, but reliable triggering also depends on concurrent MLO link removal and the `uses_rss` configuration that allocates `pcpu_rx_stats`. No local account on the victim is required for the packet-input side, so the paranoid interpretation treats this as adjacent-network reachable rather than local-only. Namespaces and containers are not the primary privilege boundary here. The vulnerable subsystem is common on WiFi-capable systems, but the exact vulnerable path is limited by MLO and RSS-related runtime conditions.

Call-site confidence: high. The commit message and patch directly show the lifetime mismatch: `sta_info_free_link()` frees `pcpu_rx_stats` immediately, while `alloc` remains RCU-deferred, and the RX path can still use the stale `link_sta`.

## 4. Severity interpretation

This is stronger than an ordinary Moderate because it is a real RCU UAF with writable stale access in a packet receive path. Realistic exploitation is constrained by timing and MLO-specific conditions, so the conservative score stays in Actionable Moderate range. The paranoid score reaches Strong Important candidate because adjacent-network influence plus write-after-free into freed kernel percpu memory is a class that should not be auto-closed without manual review. There is no evidence of a reliable LPE primitive, controlled reclaim, callback hijack, or arbitrary write, so this should not be treated as Critical.

## 5. One-sentence report phrase

A mac80211 MLO link removal race can free `pcpu_rx_stats` before all RCU RX readers have drained, allowing adjacent WiFi traffic to hit a write-after-free in the RX fast path and potentially cause kernel memory corruption or a crash.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

This is a real UAF race with stale writable access in a network receive path. Even though exploitation is timing-dependent and configuration-specific, the memory lifetime primitive is strong enough to require manual review rather than automatic closure.

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

Patch: wifi: mac80211: defer link RX stats percpu free to RCU [ Upstream
Commit: 2aa1789880fa5e41049b0f6a74a4fc2fa1997610
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=2aa1789880fa5e41049b0f6a74a4fc2fa1997610

Changed files:
  net/mac80211/sta_info.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=2aa1789880fa5e41049b0f6a74a4fc2fa1997610

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

The original Linux kernel CVE announcement for CVE-2026-68409 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-68409

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:A/AC:H/PR:N/UI:N/S:U/C:L/I:L/A:H
  The Best / paranoid CVSS vector: AV:A/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:H
  The Best / paranoid CVSS score: 7.5

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: 5
  Paranoid ActionableScore: 7

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: IMPORTANT
KPANIC detected for this report: YES
Published priority for this report (same as in Subject): IMPORTANT

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).