[CVE-2026-74609][MODERATE 7.0] tipc: read le->link under the node lock in tipc_node_link_down()

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

CVE: CVE-2026-74609
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: tipc: read le->link under the node lock in tipc_node_link_down()
Commit: 2be741ad565c610871a6a95062c12c39da7168fd
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=2be741ad565c610871a6a95062c12c39da7168fd
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74609
Analysis date: Sat, 22 Aug 2026 14:45:14 -0400
ActionableScore: 6
ActionableScore lower bound: 4
Actionable bucket: Strong Important candidate / Actionable Moderate at minimum
Manual review required: YES

Summary:
A race in TIPC `tipc_node_link_down()` can leave a cached `tipc_link` pointer valid past bearer teardown, causing a use-after-free read and possible write-after-free during link reset. For the CVSS the PR:L value is defensible in delegated CAP_NET_ADMIN or container-root deployments, while typical host deployments require CAP_NET_ADMIN. Impact is at least kernel crash DoS and may warrant manual review for privilege-escalation potential.

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

The original Linux kernel CVE announcement for CVE-2026-74609 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74609

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.

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

CVE-2026-74609	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:L/I:L/A:H';BEST CVSS score: '7';DESCR 'TIPC has a race in tipc_node_link_down() because le link was read before taking the node lock while another path can disable a bearer and free the same tipc_link under that lock. An in flight receive path or supervision timer can then dereference freed memory in tipc_link_is_establishing() and may write to the freed object through tipc_link_reset(). For the CVSS the PR:L value is used in the paranoid score because delegated CAP_NET_ADMIN, container root, or a network management service account may be enough to disable a TIPC bearer in some deployments. The issue is not directly reachable by packets alone, but remote TIPC traffic over UDP or another bearer can help create the racing link activity when TIPC is enabled. Impact is at least denial of service via kernel crash and in worst case may allow 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 5)	YES	READ DANGER TIPC UAF HARDWARE KPANIC 	NO	NO	checked

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

ActionableScore=6
ActionableScoreLower=4

## 1. ActionableScore

* Conservative score: **4**
* Paranoid score: **6**
* Final recommended bucket: **Strong Important candidate / Actionable Moderate at minimum**::

## 2. Signal breakdown

Conservative signals:

* **Memory corruption, strong corruption primitive: +2**. The patch and KASAN trace show a real slab use-after-free in `tipc_link_is_establishing()`, and the commit also identifies a possible UAF write through `tipc_link_reset()`.
* **Real lifetime corruption: +1**. `le->link` is read before locking, while another path can free the same `tipc_link` with plain `kfree()`.
* **Reliable kernel crash / strong DoS: +1**. KASAN captured a slab-use-after-free in the timer path, and a non-KASAN system can plausibly crash.
* **Weak LPE concern: +1**. The UAF includes a potential write-after-free path, but there is no demonstrated attacker-controlled reclaim, object replacement, callback control, or arbitrary write.
* **Hard or unreliable race / special timing required: -1**. Triggering requires a race between link-down processing or timer activity and bearer teardown.
* **Requires CAP_NET_ADMIN in typical deployments: -2**. The freeing path is via `TIPC_NL_BEARER_DISABLE`, normally requiring network administration privileges.
* **Rare AND difficult-to-reach subsystem/configuration: -1**. TIPC must be enabled and configured, and many general-purpose systems do not use it by default.

Paranoid additions or refinements:

* **Local delegated privilege concern: +1 effective uplift**. In containerized or delegated-network-admin environments, CAP_NET_ADMIN may be available to a non-host-root actor, and a kernel crash or UAF affects the host kernel.
* **Cross-boundary isolation concern: +1**. If reachable from a network namespace or container with delegated network administration, the bug may cross the container or namespace security boundary by affecting the host kernel.

## 3. Reachability analysis

The direct freeing path requires disabling a TIPC bearer through netlink, which is normally a local privileged control-plane operation requiring CAP_NET_ADMIN. The racing use path can occur from `tipc_rcv()` on `TIPC_LINK_DOWN_EVT` or from the link supervision timer, so remote TIPC traffic can help create link activity but does not by itself provide the full trigger without the local teardown action. TIPC is not a universal default-exposed protocol, and practical exploitation requires TIPC to be enabled and a bearer to exist. Namespace and container setups matter: if a container root or service account has delegated CAP_NET_ADMIN and can manage TIPC bearers, the practical privilege requirement is lower than full host root.

Call-site confidence: **high**, because the commit message gives both racing call paths, the freeing path, and the KASAN trace.

## 4. Severity interpretation

This is not an ordinary Moderate resource leak or validation-only bug. It is a race-dependent lifetime bug with a confirmed UAF read and a plausible UAF write path. Realistic exploitation evidence supports at least kernel crash DoS, while privilege escalation remains theoretical because there is no demonstrated reclaim, type confusion, callback hijack, or controlled overwrite primitive. Conservative handling is borderline manual review, but the paranoid score makes it an actionable Moderate and possible Important candidate due to UAF plus write-after-free potential in networking code.

## 5. One-sentence report phrase

A race in TIPC `tipc_node_link_down()` can leave a cached `tipc_link` pointer valid past bearer teardown, causing a use-after-free read and possible write-after-free during link reset. For the CVSS the PR:L value is defensible in delegated CAP_NET_ADMIN or container-root deployments, while typical host deployments require CAP_NET_ADMIN. Impact is at least kernel crash DoS and may warrant manual review for privilege-escalation potential.

## 6. Manual review recommendation

**MANUAL CHECK REQUIRED**. YES REQUIRES MANUAL CHECK because this is a confirmed race-based UAF with both read and possible write access to freed memory, and namespace or delegated CAP_NET_ADMIN deployments may reduce the practical privilege barrier.

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

Patch: tipc: read le->link under the node lock in tipc_node_link_down()
Commit: 2be741ad565c610871a6a95062c12c39da7168fd
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=2be741ad565c610871a6a95062c12c39da7168fd

Commit description:

tipc_node_link_down() caches the link pointer before taking n->lock:

	struct tipc_link *l = le->link;		/* unlocked */

	if (!l)
		return;
	tipc_node_write_lock(n);
	if (!tipc_link_is_establishing(l)) {	/* deref l */
	...
		tipc_link_reset(l);		/* write into l */
	if (delete) {
		kfree(l);
		le->link = NULL;

The delete=true caller frees that very object under n->lock, so the lock
does not protect the cached pointer against it:

 - CPU A, delete=false: tipc_rcv() on TIPC_LINK_DOWN_EVT, or the link
   supervision timer via tipc_node_timeout(), reads l unlocked and then
   dereferences it under n->lock;
 - CPU B, delete=true: netlink TIPC_NL_BEARER_DISABLE -> bearer_disable()
   -> tipc_node_delete_links() -> tipc_node_link_down(n, bearer_id, true)
   -> kfree(l).

The link is freed with plain kfree(), not kfree_rcu(), and for UDP bearers
disable_media() only schedules the asynchronous cleanup_bearer() work, so
its synchronize_net() runs after the links are already gone.  An in-flight
CPU A that has read l therefore dereferences freed memory once B frees it:
a use-after-free read in tipc_link_is_establishing(), and a use-after-free
write via tipc_link_reset() on the establishing branch.

The following trace was captured on 7.2.0-rc5-00284-gaf39eb111ce6:

  BUG: KASAN: slab-use-after-free in tipc_link_is_establishing (net/tipc/link.c:285)
  Read of size 4 at addr ffff88802e2aa068 by task swapper/2/0
   tipc_link_is_establishing (net/tipc/link.c:285)
   tipc_node_link_down (net/tipc/node.c:1076)
   tipc_node_timeout (net/tipc/node.c:843)
  Allocated by task 9549:
   tipc_link_create (net/tipc/link.c:490)
   tipc_node_check_dest (net/tipc/node.c:1279)
   tipc_disc_rcv (net/tipc/discover.c:252)
   tipc_udp_recv (net/tipc/udp_media.c:389)
  Freed by task 9549:
   tipc_node_link_down (net/tipc/node.c:1084)
   tipc_node_delete_links (net/tipc/node.c:1320)
   bearer_disable (net/tipc/bearer.c:414)
   __tipc_nl_bearer_disable (net/tipc/bearer.c:992)

Move the le->link read inside tipc_node_write_lock(), so it is serialised
against the kfree() in the delete path.  A racing teardown now either has
not run yet, and we see a valid link, or has already run, and we see NULL.

Fixes: 73f646c ("tipc: delay ESTABLISH state event when link is established")
Cc: stable@kernel.org
Reported-by: TencentOS Corvus AI <corvus@tencent.com>
Assisted-by: tencentos-corvus-ai:kimi-k3
Signed-off-by: Jun Yang <junvyyang@tencent.com>
Reviewed-by: Tung Nguyen <tung.quang.nguyen@est.tech>
Link: https://patch.msgid.link/20260810102147.48191-1-juny24602@gmail.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  net/tipc/link.c
  net/tipc/node.c
  net/tipc/discover.c
  net/tipc/udp_media.c
  net/tipc/bearer.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=2be741ad565c610871a6a95062c12c39da7168fd

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

The original Linux kernel CVE announcement for CVE-2026-74609 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74609

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: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: 6

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).