[CVE-2026-46120][IMPORTANT] ip6_gre: Use cached t->net in ip6erspan_changelink().

[CVE-2026-46120][IMPORTANT] ip6_gre: Use cached t->net in ip6erspan_changelink().

[AL-KERNEL]

CVE: CVE-2026-46120
Priority: IMPORTANT
AL-KERNEL base severity: IMPORTANT
KPANIC flag: YES
Patch: ip6_gre: Use cached t->net in ip6erspan_changelink().
Commit: eca62bb0569de4d43a4dac06a2092a9d4ca1d702
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=eca62bb0569de4d43a4dac06a2092a9d4ca1d702
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-46120
Analysis date: Thu, 28 May 2026 06:35:47 -0400
ActionableScore: 8
ActionableScore lower bound: 7
Actionable bucket: Strong Important candidate / Actionable Moderate at minimum
Manual review required: YES

Summary:
A local unprivileged user namespace can trigger stale ip6erspan tunnel state across network namespace migration, leading to a slab use-after-free during namespace teardown and a host kernel crash.

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

The original Linux kernel CVE announcement for CVE-2026-46120 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-46120

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.

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

CVE-2026-46120	IMPORTANT	CHECK	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H';*CWE-416;*CWE-664;*CWE-672;CWE-362;Other CVSS 'AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '7.8';DESCR 'ip6erspan_changelink used dev_net(dev) instead of the cached tunnel net namespace, so after IFLA_NET_NS_FD migration the tunnel could be reinserted into the wrong per netns ip6gre hash while a stale entry remained in the original namespace. Destroying that namespace later can walk the stale entry and trigger a slab use-after-free followed by a kernel BUG in unregister_netdevice_many_notify. For the CVSS the PR:L is used for the paranoid score because the commit states this is reachable from an unprivileged user namespace using unshare with user and net namespaces, not only from full host root. The issue is not directly network reachable by packet traffic and is triggered through local netlink namespace and tunnel lifecycle operations. Impact is at least local denial of service, and worst case may include confidentiality or integrity impact because this is a real kernel use-after-free in network namespace teardown.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate / Actionable Moderate at minimum (with actual score 8)	YES	REMOTE DANGER UAF NETWORK HARDWARE IPV6 KPANIC  KPANIC INCREASED_TO_HIGH_BASED_ON_GUESSCVSS	NO	NO	checked

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

ActionableScore=8
ActionableScoreLower=7

## 1. ActionableScore

* Conservative score: **7**
* Paranoid score: **8**
* Final recommended bucket: **Strong Important candidate / Actionable Moderate at minimum**::

## 2. Signal breakdown

* **Local unprivileged trigger: +1**
  The commit explicitly says the bug is reachable from an unprivileged user namespace using `unshare --user --map-root-user --net`.

* **Generic memory corruption, strong corruption primitive: +2**
  The stale tunnel entry is later walked during netns teardown and produces a slab use-after-free.

* **Real lifetime corruption: +1**
  This is a stale object lifetime bug across network namespace and tunnel hash ownership.

* **Reliable kernel crash / strong DoS: +1**
  The described result includes KASAN slab-use-after-free followed by kernel BUG with `LIST_POISON1` in `unregister_netdevice_many_notify()`.

* **Cross-boundary isolation / namespace impact: +1**
  A user namespace and network namespace workflow can affect the host kernel and crash it, so this crosses an intended containment boundary.

* **Local unprivileged high-control kernel control-plane API: +1**
  The trigger uses local netlink / rtnl tunnel lifecycle operations with user-controlled namespace migration and changelink state.

* **Confidentiality or integrity impact plausible, paranoid: +1**
  A real slab UAF in network namespace teardown can sometimes be more than DoS, although no controlled reclaim or write primitive is demonstrated in the patch.

No remote packet trigger is shown. No demonstrated arbitrary write or working LPE chain is shown.

## 3. Reachability analysis

A local unprivileged user can trigger the issue if unprivileged user namespaces are enabled and the required IPv6 GRE / ERSPAN functionality is available. The attacker creates user and network namespaces, manipulates an ip6erspan tunnel, migrates it with `IFLA_NET_NS_FD`, and then destroys the original namespace so that teardown walks the stale hash entry. This does not require full host root because CAP_NET_ADMIN is obtained inside the created namespace. The issue is not directly reachable through ordinary network packets.

## 4. Severity interpretation

This behaves like a strong Important candidate rather than ordinary Moderate. The realistic demonstrated impact is local host DoS from a namespace-reachable slab UAF. The theoretical higher impact is plausible because it is a real kernel UAF in network object lifetime handling, but the patch does not prove a reliable privilege escalation primitive. Manual review is required to decide whether the stale entry can be shaped into a stronger reclaim or object-confusion primitive.

## 5. One-sentence report phrase

A local unprivileged user namespace can trigger stale ip6erspan tunnel state across network namespace migration, leading to a slab use-after-free during namespace teardown and a host kernel crash.

## 6. Manual review recommendation

**MANUAL CHECK REQUIRED**

This is a real slab UAF reachable from an unprivileged user namespace, not a simple privileged DoS or cleanup bug. It should not be auto-closed without review.

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

Patch: ip6_gre: Use cached t->net in ip6erspan_changelink().
Commit: eca62bb0569de4d43a4dac06a2092a9d4ca1d702
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=eca62bb0569de4d43a4dac06a2092a9d4ca1d702

Commit description:

After commit 5e72ce3 ("net: ipv6: Use link netns in newlink() of
rtnl_link_ops"), ip6erspan_newlink() correctly resolves the per-netns
ip6gre hash via link_net. ip6erspan_changelink() was not converted in
that series and still uses dev_net(dev), which diverges from the
device's creation netns after IFLA_NET_NS_FD migration.

This re-inserts the tunnel into the wrong per-netns hash. The
original netns keeps a stale entry. When that netns is later
destroyed, ip6gre_exit_rtnl_net() walks the stale entry, producing a
slab-use-after-free reported by KASAN, followed by a kernel BUG at
net/core/dev.c (LIST_POISON1) in unregister_netdevice_many_notify().

Reachable from an unprivileged user namespace (unshare --user
--map-root-user --net).

ip6gre_changelink() earlier in the same file already uses the cached
t->net; only ip6erspan_changelink() has the wrong shape.

Fixes: 2d66503 ("net: ip6_gre: Fix ip6erspan hlen calculation")
Cc: stable@vger.kernel.org # v5.15+
Signed-off-by: Maoyi Xie <maoyi.xie@ntu.edu.sg>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Kuniyuki Iwashima <kuniyu@google.com>
Link: https://patch.msgid.link/20260430103318.3206018-1-maoyi.xie@ntu.edu.sg
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  net/core/dev.c
  net/ipv6/ip6_gre.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=eca62bb0569de4d43a4dac06a2092a9d4ca1d702

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

The original Linux kernel CVE announcement for CVE-2026-46120 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-46120

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:L/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:H/I:H/A:H
  The Best / paranoid CVSS score: 7.8

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: 7
  Paranoid ActionableScore: 8

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).