[CVE-2026-74489][MODERATE 7.0] wifi: mac80211: fix tid_tx use-after-free on BA session stop

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

CVE: CVE-2026-74489
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: wifi: mac80211: fix tid_tx use-after-free on BA session stop
Commit: 09fc36eec7841cdf732f4db39855e0c73bd075cc
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=09fc36eec7841cdf732f4db39855e0c73bd075cc
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74489
Analysis date: Sat, 15 Aug 2026 09:33:42 -0400
ActionableScore: 5
ActionableScore lower bound: 4
Actionable bucket: Actionable Moderate at minimum
Manual review required: YES

Summary:
A race in mac80211 BA session teardown can read `tid_tx->ndp` after `tid_tx` has been freed through `kfree_rcu()`, causing a confirmed UAF that is most likely a DoS but should receive manual review due to WiFi-adjacent exposure and kernel lifetime corruption.

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

The original Linux kernel CVE announcement for CVE-2026-74489 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74489

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-2026-74489	MODERATE	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:L/I:L/A:H';*CWE-416;CWE-362;*CWE-672;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '6.4';DESCR 'A use after free in mac80211 BA session teardown can occur because ieee80211_stop_tx_ba_cb frees tid_tx through ieee80211_remove_tid_tx and later reads tid_tx ndp after dropping sta lock. The read is outside an RCU read side critical section and can race with the normal RCU softirq free path in preemptible process context. The base impact is a local denial of service through a kernel crash, with high attack complexity because a narrow timing window must be hit. For the CVSS the PR:N is used in the paranoid score because an adjacent WiFi peer or AP may be able to influence BA session lifetime without local code execution on the victim. The issue is not a generic Internet reachable bug, but practical exposure may exist on systems using WiFi aggregation where an attacker is in radio range or controls the connected AP. Impact is at least denial of service. In worst case it may allow limited confidentiality or integrity impact due to the UAF class and should be reviewed manually.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate at minimum (with actual score 5)  SKIP CVE-2026-74489 UNKNOWN SKIP The Fixes patch not applied yet, so unlikely that actual: 98acd4c1d9f7dc9c426e840c16e81b57315ff84b YES  NO NO unknown 	MAYBE	REMOTE READ DANGER UAF TEST  INCREASED_TO_MODERATE7_FROM_MODERATE_BASED_ON_GUESSCVSS DECREASED_TO_MODERATEREG_BASED_ON_FALSEPOSCHECKOFKP	-	-	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=5
ActionableScoreLower=4

## 1. ActionableScore

* Conservative score: 4
* Paranoid score: 5
* Final recommended bucket: **Actionable Moderate at minimum**::

## 2. Signal breakdown

Conservative signals:

* Local unprivileged trigger: +1. A local unprivileged user may be able to influence BA session creation and teardown indirectly by generating WiFi traffic, though reliable triggering is not straightforward.
* Memory corruption, weak/indirect corruption candidate: +1. This is a confirmed UAF, but the observed stale access is a read of one byte from `tid_tx->ndp`, not a write, callback dispatch, type confusion, or controlled reclaim.
* Real lifetime corruption: +1. `tid_tx` is passed to `kfree_rcu()` via `ieee80211_remove_tid_tx()` and then dereferenced after the lifetime has ended.
* Reliable kernel crash / strong DoS concern: +1. KASAN confirms a slab-use-after-free. In production, impact is most defensibly DoS or unstable kernel behavior rather than proven LPE.
* Broad/default/common subsystem exposure: +1. `mac80211` is a common WiFi subsystem on client systems.
* Hard or unreliable race / special timing required: -1. The bug requires the RCU grace period/free path to land in a narrow window after `ieee80211_remove_tid_tx()` and before the `ndp` read.

Paranoid additional interpretation:

* Adjacent WiFi influence / practical exposure concern: +1. The commit says peer DELBA does not directly trigger this path, but BA session lifetime can be affected by realistic WiFi conditions such as idle timeout, PTK rekey, suspend, or hardware reconfiguration. This supports manual-review sensitivity but not full Internet remote reachability.

Not awarded:

* Strong LPE plausibility: +0. No attacker-controlled reclaim, object replacement, write-after-free, callback control, type confusion, or refcount takeover is shown.
* Confidentiality impact: +0 conservative. The stale read is limited to `ndp`, a boolean-like field, with no demonstrated disclosure path.
* Integrity impact: +0 conservative. No stale write or cross-object modification primitive is shown.
* Remote reachable / network-triggerable: +0 conservative. The commit explicitly states this is local teardown and not reached from a peer DELBA.

## 3. Reachability analysis

The most realistic trigger is local or system-driven BA session teardown in a WiFi client using `mac80211`. Conditions include session idle timeout, PTK rekey, suspend, hardware reconfiguration, or station teardown. A local unprivileged user may influence traffic patterns and aggregation state, but cannot necessarily force the exact teardown and RCU timing reliably.

A nearby WiFi peer or AP may have some indirect influence over session lifetime, especially in managed WiFi environments, but the patch text explicitly excludes direct triggering via a peer DELBA. Therefore this should be treated as adjacent or environment-mediated exposure, not generic remote network reachability.

Namespaces and containers usually do not help much here unless the container can influence the host WiFi interface or generate traffic through it. The path is common on WiFi-enabled systems but irrelevant on systems without `mac80211` WiFi usage.

Call-site confidence: high. The commit describes both callers, the freeing path, the stale dereference, and the KASAN report.

## 4. Severity interpretation

This is stronger than an ordinary Moderate cleanup because it is a confirmed lifetime bug with KASAN-detected use-after-free in a common kernel WiFi subsystem. However, it does not currently look like an Important-class LPE candidate on the evidence provided, because the stale access is a read-only one-byte field access after RCU free and there is no demonstrated reclaim, overwrite, callback, or object confusion primitive.

Conservative handling is Borderline / manual review recommended with score 4. Paranoid handling reaches Actionable Moderate with score 5 because UAF bugs in packet-adjacent subsystems are often under-triaged and the WiFi adjacent exposure may be practically relevant on client systems.

## 5. One-sentence report phrase

A race in mac80211 BA session teardown can read `tid_tx->ndp` after `tid_tx` has been freed through `kfree_rcu()`, causing a confirmed UAF that is most likely a DoS but should receive manual review due to WiFi-adjacent exposure and kernel lifetime corruption.

## 6. Manual review recommendation

MANUAL CHECK RECOMMENDED

Reason: this is a confirmed UAF and RCU lifetime bug in `mac80211`, but the demonstrated primitive is limited to a stale one-byte read with race timing requirements and no proven LPE primitive.

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

Patch: wifi: mac80211: fix tid_tx use-after-free on BA session stop
Commit: 09fc36eec7841cdf732f4db39855e0c73bd075cc
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=09fc36eec7841cdf732f4db39855e0c73bd075cc

Commit description:

ieee80211_stop_tx_ba_cb() hands tid_tx to kfree_rcu() through
ieee80211_remove_tid_tx(), and then reads tid_tx->ndp after dropping
sta->lock:

	ieee80211_remove_tid_tx(sta, tid);	/* kfree_rcu(tid_tx, rcu_head) */
	...
	spin_unlock_bh(&sta->lock);

	if (start_txq)
		ieee80211_agg_start_txq(sta, tid, false);

	if (send_delba)
		ieee80211_send_delba(..., tid_tx->ndp);

That read is not covered by an RCU read-side critical section, and it runs
in preemptible process context: both callers hold the wiphy mutex, reaching
it either from the ieee80211_ba_session_work() wiphy work or from
ieee80211_sta_tear_down_BA_sessions() during station teardown.
Softirqs can run in that window too, both from the local_bh_enable() that
ends ieee80211_agg_start_txq() and from any interrupt exit, so the RCU
callback can free tid_tx before the read.

Driving the function from a test module with the grace period forced into
that window, KASAN reports the read, and the free arrives on the ordinary
RCU softirq path:

  BUG: KASAN: slab-use-after-free in ieee80211_stop_tx_ba_cb+0x3cd/0x400
  Read of size 1 at addr ffff888002b9f52e by task kworker/0:1/10
  [...]
  Freed by task 57:
   __kasan_slab_free+0x47/0x70
   __rcu_free_sheaf_prepare+0x70/0x250
   rcu_free_sheaf_nobarn+0x18/0x40
   rcu_core+0x426/0x1310
   handle_softirqs+0x144/0x590
   __irq_exit_rcu+0xea/0x150
   irq_exit_rcu+0x9/0x20
   sysvec_apic_timer_interrupt+0x6b/0x80
   asm_sysvec_apic_timer_interrupt+0x1a/0x20

send_delba is only set when tx_stop is set, which happens for
AGG_STOP_LOCAL_REQUEST alone, so this is reached on local teardown -
session idle timeout, PTK rekey, suspend, HW reconfig - and not from a
peer's DELBA.

Read ndp into a local before the session is freed, while sta->lock is still
held. tid_tx->ndp has a single writer, in
ieee80211_tx_ba_session_handle_start(), which cannot run concurrently here:
both paths are serialised by the wiphy mutex, and the session is already
marked HT_AGG_STATE_STOPPING at this point. tid_tx->ndp is also the only
tid_tx dereference left after ieee80211_remove_tid_tx() in this function.

Fixes: 98acd4c ("wifi: mac80211: add support for NDP ADDBA/DELBA for S1G")
Assisted-by: Codex:gpt-5.6-sol
Assisted-by: Kimi:K3
Cc: stable@vger.kernel.org
Signed-off-by: Zhao Li <enderaoelyther@gmail.com>
Link: https://patch.msgid.link/20260728112156.96822-1-enderaoelyther@gmail.com
[move/change the comment a bit to be more general not just on ndp,
 initialize ndp directly]
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  net/mac80211/agg-tx.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=09fc36eec7841cdf732f4db39855e0c73bd075cc

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

The original Linux kernel CVE announcement for CVE-2026-74489 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74489

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:A/AC:H/PR:N/UI:N/S:U/C:L/I:L/A:H
  The Best / paranoid CVSS score: 6.4

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: 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).