[CVE-2026-64119][MODERATE 7.0] l2tp: use list_del_rcu in l2tp_session_unhash

[CVE-2026-64119][MODERATE 7.0] l2tp: use list_del_rcu in l2tp_session_unhash

[AL-KERNEL]

CVE: CVE-2026-64119
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: l2tp: use list_del_rcu in l2tp_session_unhash
Commit: 5e40919a40cb3e590ed45c2a54a4a2518aa88a99
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=5e40919a40cb3e590ed45c2a54a4a2518aa88a99
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64119
Analysis date: Sun, 19 Jul 2026 15:13:01 -0400
ActionableScore: 5
ActionableScore lower bound: unknown
Actionable bucket: Actionable Moderate at minimum
Manual review required: YES

Summary:
An RCU list handling bug in l2tp_session_unhash() lets a local unprivileged user with namespace CAP_NET_ADMIN repeatedly race L2TP session get/create/delete operations and pin a host CPU indefinitely, stalling RCU grace periods and causing a host-level denial of service.

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

The original Linux kernel CVE announcement for CVE-2026-64119 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64119

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-2026-64119	MODERATE	CHECK WITH IMPACT FROM ORIG NN LOW	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H';CWE-362;CWE-667;*CWE-835;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '5.5';DESCR 'An RCU list handling bug in l2tp_session_unhash can make l2tp_session_get_by_ifname loop forever when L2TP session get create and delete operations race on the same tunnel. The deleted entry can become self linked because list_del_init is used on a list walked with list_for_each_entry_rcu, so an in flight reader may keep resolving the same session and never reach the list head. This can pin a host CPU with BH and preemption disabled and can stall RCU grace periods, causing a local denial of service. For the CVSS the PR:L is used because an unprivileged local user may obtain CAP_NET_ADMIN inside a new user and network namespace with unshare -Urn when l2tp_core is loaded. The issue is not directly network reachable and does not show a concrete memory corruption primitive, so confidentiality and integrity remain N while availability is H.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate at minimum (with actual score 5)  SKIP CVE-2026-64119 UNKNOWN SKIP The Fixes patch not applied yet, so unlikely that actual: 89b768ec2dfefaeba5212de14fc71368e12d06ba YES  NO NO unknown 	MAYBE	INIT HARDWARE LINUS  KPANIC INCREASED_FROM_LOW_BASED_ON_REQUIREMANUALCHECK	-	-	checked

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

ActionableScore=5

## 1. ActionableScore

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

## 2. Signal breakdown

Triggered signals:

* Local unprivileged trigger: +1
  The commit explicitly states that CAP_NET_ADMIN in a user/network namespace is enough and that a standard unprivileged namespace sandbox can reach the trigger when l2tp_core is loaded.

* Real lifetime / RCU list handling issue: +1
  The bug is caused by using list_del_init() on a list walked with list_for_each_entry_rcu(). This is an RCU consistency/lifetime semantics violation, even though the described failure mode is not a freed-object UAF.

* Reliable kernel DoS / strong DoS: +1
  The reader can loop forever in l2tp_session_get_by_ifname(), pinning a CPU with BH and preemption disabled.

* Availability impact realistic and host-wide: +1
  The impact is not just a transient local hang. The stuck RCU read-side section can stall RCU grace periods on the host and the wedged thread cannot be killed until syscall return.

* Cross-boundary isolation/resource impact: +1
  A process in a reduced namespace context can affect host CPU and RCU progress, impacting workloads outside the namespace.

Not counted:

* Remote reachable: +0
  This is a local generic-netlink control-plane trigger, not a packet-processing remote path.

* Privilege escalation plausible: +0
  No reclaim primitive, write-after-free, type confusion, callback control, refcount takeover, or arbitrary write is shown.

* Generic strong memory corruption: +0
  The patch and description support an RCU list iteration infinite loop, not a concrete UAF exploitation primitive.

* Hard race penalty: 0
  Although concurrency is required, the description implies the race is practically reachable using repeated GET/CREATE/DELETE operations, so I would not subtract for unreliable timing.

* Requires admin/CAP_NET_ADMIN penalty: 0
  In the relevant practical model, CAP_NET_ADMIN is obtainable in a user/network namespace by an unprivileged local user, so treating this as PR:H would understate risk.

Call-site confidence: high. The commit explains both the writer path l2tp_session_unhash() and the RCU reader path l2tp_session_get_by_ifname().

## 3. Reachability analysis

The bug is locally reachable through L2TP generic-netlink operations. The important detail is that these commands require GENL_UNS_ADMIN_PERM, which allows CAP_NET_ADMIN in the network namespace rather than full host root. On systems where unprivileged user namespaces are enabled and l2tp_core is loaded, a local unprivileged user can plausibly create a namespace context and trigger the affected path.

The issue is not directly network-triggerable by L2TP packets alone. It requires local control-plane operations to create, delete, and query sessions concurrently. Exposure is conditional on l2tp_core being available or loaded, but this is still an ordinary local kernel interface rather than a rare hardware-only path.

## 4. Severity interpretation

This behaves stronger than an ordinary Moderate DoS because the trigger can come from a reduced-privilege local namespace context and can pin a host CPU indefinitely while stalling RCU grace periods. It does not currently justify Important-class memory-corruption treatment because the patch does not show attacker-controlled reclaim, stale object reuse after free, type confusion, arbitrary write, or callback control.

Realistic impact is local host denial of service. Theoretical memory-corruption or LPE impact is not supported by the patch context. The right practical triage category is Actionable Moderate at minimum, with manual review due to local unprivileged namespace reachability and RCU-wide availability effects.

## 5. One-sentence report phrase

An RCU list handling bug in l2tp_session_unhash() lets a local unprivileged user with namespace CAP_NET_ADMIN repeatedly race L2TP session get/create/delete operations and pin a host CPU indefinitely, stalling RCU grace periods and causing a host-level denial of service.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: this is a local unprivileged namespace-reachable RCU bug with persistent host-level availability impact. It is probably DoS-only, but it should not be auto-closed because the trigger crosses a namespace privilege boundary and can affect the host outside the attacker namespace.

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

Patch: l2tp: use list_del_rcu in l2tp_session_unhash
Commit: 5e40919a40cb3e590ed45c2a54a4a2518aa88a99
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=5e40919a40cb3e590ed45c2a54a4a2518aa88a99

Commit description:

An unprivileged local user can pin a host CPU indefinitely in
l2tp_session_get_by_ifname() by issuing L2TP_CMD_SESSION_GET on
L2TP_ATTR_IFNAME concurrently with L2TP_CMD_SESSION_CREATE and
L2TP_CMD_SESSION_DELETE on the same tunnel. All three commands take
GENL_UNS_ADMIN_PERM, so CAP_NET_ADMIN in the netns user namespace
suffices; on any host that has l2tp_core loaded the trigger is
reachable from a standard `unshare -Urn` sandbox.

l2tp_session_unhash() removes a session from tunnel->session_list
with list_del_init(), but that list is walked by
l2tp_session_get_by_ifname() with list_for_each_entry_rcu() under
rcu_read_lock_bh(). list_del_init() leaves the deleted entry's
next/prev self-pointing; a reader that has loaded the entry and
then advances pos->list.next reads &session->list, container_of()s
back to the same session, and list_for_each_entry_rcu() never
reaches the list head. The CPU stays in strcmp() inside the
walker, with BH and preemption disabled, so RCU grace periods on
the host stall behind it and the wedged thread cannot be killed
(SIGKILL is delivered on syscall return).

Use list_del_rcu() to match the existing list_add_rcu() in
l2tp_session_register(); the deleted session remains visible to
in-flight walkers with consistent next/prev pointers until
kfree_rcu() in l2tp_session_free() releases it. tunnel->session_list
has exactly one list_del_init() call site; the list_del_init
(&session->clist) at l2tp_core.c:533 operates on the per-collision
list, which is not walked under RCU. list_empty(&session->list) is
not used anywhere in net/l2tp/ after the unhash point, so dropping
the post-delete self-init is safe; the fix has no userspace-visible
behavior change.

Fixes: 89b768e ("l2tp: use rcu list add/del when updating lists")
Cc: stable@vger.kernel.org # 6.11+
Signed-off-by: Michael Bommarito <michael.bommarito@gmail.com>
Link: https://patch.msgid.link/20260518183447.64078-1-michael.bommarito@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  net/l2tp/l2tp_core.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=5e40919a40cb3e590ed45c2a54a4a2518aa88a99

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

The original Linux kernel CVE announcement for CVE-2026-64119 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64119

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:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS score: 5.5

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: unknown
  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).