[CVE-2026-63917][IMPORTANT] ip6: vti: Use ip6_tnl.net in vti6_changelink().

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

CVE: CVE-2026-63917
Priority: IMPORTANT
AL-KERNEL base severity: IMPORTANT
KPANIC flag: YES
Patch: ip6: vti: Use ip6_tnl.net in vti6_changelink().
Commit: 0cdce7618464f7fb06f461e8f4ad575cb1d570f4
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=0cdce7618464f7fb06f461e8f4ad575cb1d570f4
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-63917
Analysis date: Sun, 19 Jul 2026 12:53:46 -0400
ActionableScore: 7
ActionableScore lower bound: 6
Actionable bucket: Strong Important candidate / Actionable Moderate at minimum
Manual review required: YES

Summary:
A vti6 namespace-move bug can leave a stale tunnel entry in the creation netns, allowing a local unprivileged namespace user to trigger cleanup_net walking freed memory and causing at least a kernel crash, with UAF-based privilege escalation concerns requiring manual review.

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

The original Linux kernel CVE announcement for CVE-2026-63917 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-63917

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-2026-63917	IMPORTANT	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-672;*CWE-664;Other CVSS 'AV:L/AC:L/PR:L/UI:N/S:U/C:L/I:L/A:H';BEST CVSS score: '7';DESCR 'A stale per netns vti6 tunnel entry can remain after a vti6 device is moved between network namespaces because update and changelink paths used the current device namespace instead of the tunnel creation namespace. When the original namespace is later destroyed, cleanup_net may walk freed memory through that stale entry, creating a use after free condition. For the CVSS the PR:L is used because an unprivileged local user can reach the path through user and network namespaces when unprivileged namespaces are enabled. The issue is not directly network reachable and is triggered through local netlink namespace operations rather than packet traffic. Impact is at least local denial of service via kernel crash and in worst case may allow confidentiality or integrity impact due to kernel use after free. Container hosts have higher practical risk because the bug can cross tenant boundaries when unprivileged namespaces are available.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate / Actionable Moderate at minimum (with actual score 7)	YES	REMOTE DANGER UAF NETWORK HARDWARE IPV6 KPANIC  KPANIC INCREASED_TO_HIGH_BASED_ON_ACTIONABLESCOREHIGHEREQTHAN7	NO	NO	guess

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

ActionableScore=7
ActionableScoreLower=6

## 1. ActionableScore

* Conservative score: **6**
* Paranoid score: **7**
* Final recommended bucket: **Strong Important candidate / Actionable Moderate at minimum**::

## 2. Signal breakdown

Triggered signals:

* **Local unprivileged trigger: +1**
  The commit explicitly states reachability from an unprivileged user namespace using user and network namespaces.

* **Memory corruption, strong corruption primitive: +2**
  The stale per-netns vti6 hash entry can cause `cleanup_net()` to walk freed memory. This is a real UAF-class condition, not just a warning or validation failure.

* **Real lifetime corruption: +1**
  The bug is caused by inconsistent object lifetime and namespace ownership: `dev_net(dev)` changes after namespace move, while `t->net` remains the creation namespace.

* **Reliable kernel crash / strong DoS: +1**
  The provided reproducer reaches `kernel BUG at net/core/dev.c`, so availability impact is concrete.

* **Cross-boundary isolation/policy bypass: +1**
  Container hosts are explicitly relevant because unprivileged namespaces may allow a tenant to trigger a host-kernel UAF from a reduced-privilege context.

Paranoid-only signal:

* **Weak LPE concern: +1**
  The bug is a kernel UAF reachable by a local unprivileged namespace user. No controlled reclaim or write primitive is shown, so this is not strong LPE plausibility, but it is enough for manual-review escalation.

Not applied:

* **Remote reachable / network-triggerable: +0**
  This is a local netlink / namespace control-plane issue, not packet-triggered.

* **Requires admin/root/CAP_NET_ADMIN: +0 / not subtracted**
  Although creating and changing network devices normally requires `CAP_NET_ADMIN`, the commit states the path is reachable from an unprivileged user namespace.

* **Availability impact realistic: +0 additional**
  DoS is already counted via reliable kernel crash, so it is not double-counted.

## 3. Reachability analysis

A local user can trigger the bug when unprivileged user namespaces and network namespaces are available. The path involves creating a vti6 tunnel, moving it between network namespaces, changing its parameters, and then tearing down namespaces so that cleanup walks a stale tunnel entry.

This is not remotely reachable through ordinary network packets. It is a local kernel control-plane issue through namespace and netlink operations.

On systems where unprivileged user namespaces are disabled, practical reachability is lower and may require `CAP_NET_ADMIN`. On container hosts or systems allowing unprivileged user namespaces, the practical risk is higher because a reduced-privilege tenant may be able to reach the vulnerable path.

Call-site confidence: **high**. The commit message directly explains the stale per-netns hash entry and the cleanup path walking freed memory.

## 4. Severity interpretation

This behaves more like an **Important-class candidate** than an ordinary Moderate because it combines local unprivileged reachability, namespace crossing, a real stale-pointer/UAF condition, and a demonstrated kernel BUG.

The realistic demonstrated impact is a local host-kernel crash. The theoretical higher impact is privilege escalation or cross-container impact due to UAF, but the patch does not show attacker-controlled reclaim, object replacement, callback control, type confusion, or arbitrary write. Therefore the conservative score stays at Actionable Moderate / Important boundary, while the paranoid score reaches Strong Important candidate.

## 5. One-sentence report phrase

A vti6 namespace-move bug can leave a stale tunnel entry in the creation netns, allowing a local unprivileged namespace user to trigger cleanup_net walking freed memory and causing at least a kernel crash, with UAF-based privilege escalation concerns requiring manual review.

## 6. Manual review recommendation

**MANUAL CHECK REQUIRED**

Reason: the bug is a real kernel UAF/lifetime issue, explicitly reachable from unprivileged user namespaces, and has cross-tenant relevance on container hosts. Even though only DoS is demonstrated, the UAF primitive should not be auto-closed without manual review.

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

Patch: ip6: vti: Use ip6_tnl.net in vti6_changelink().
Commit: 0cdce7618464f7fb06f461e8f4ad575cb1d570f4
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=0cdce7618464f7fb06f461e8f4ad575cb1d570f4

Commit description:

ip netns add ns1
ip netns add ns2
ip -n ns1 link add vti6_test type vti6 remote ::1 local ::2 key 7
ip -n ns1 link set vti6_test netns ns2
ip -n ns2 link set vti6_test type vti6 remote ::3 local ::4 key 9
ip netns del ns2
ip netns del ns1
[  132.495484] ------------[ cut here ]------------
[  132.497609] kernel BUG at net/core/dev.c:12376!

Commit 61220ab ("vti6: Enable namespace changing") dropped
NETIF_F_NETNS_LOCAL from vti6 devices. A vti6 tunnel can then
move through IFLA_NET_NS_FD. After the move dev_net(dev) points
at the new netns while t->net stays at the creation netns.

vti6_changelink() and vti6_update() still use dev_net(dev) and
dev_net(t->dev). They unlink from one per netns hash and relink
into another. The creation netns is left with a stale entry.
cleanup_net() of that netns later walks freed memory.

Reachable from an unprivileged user namespace (unshare --user
--map-root-user --net). Cross tenant scope on container hosts.

Fixes: 61220ab ("vti6: Enable namespace changing")
Reported-by: Maoyi Xie <maoyi.xie@ntu.edu.sg>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Cc: stable@vger.kernel.org # v5.15+
Signed-off-by: Kuniyuki Iwashima <kuniyu@google.com>
Link: https://patch.msgid.link/20260521130555.3421684-2-maoyixie.tju@gmail.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  net/core/dev.c
  net/ipv6/ip6_vti.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=0cdce7618464f7fb06f461e8f4ad575cb1d570f4

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

The original Linux kernel CVE announcement for CVE-2026-63917 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-63917

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: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: 6
  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).