[CVE-2026-63829][IMPORTANT] net: ip_gre: require CAP_NET_ADMIN in the device netns for changelink

[CVE-2026-63829][IMPORTANT] net: ip_gre: require CAP_NET_ADMIN in the device netns for changelink

[AL-KERNEL]

CVE: CVE-2026-63829
Priority: IMPORTANT
AL-KERNEL base severity: IMPORTANT
KPANIC flag: YES
Patch: net: ip_gre: require CAP_NET_ADMIN in the device netns for changelink
Commit: 9831bc9ecb402957810c2045c663fbfe9b09e296
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=9831bc9ecb402957810c2045c663fbfe9b09e296
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-63829
Analysis date: Sun, 19 Jul 2026 10:13:53 -0400
ActionableScore: 7
ActionableScore lower bound: 6
Actionable bucket: Strong Important candidate / Actionable Moderate at minimum
Manual review required: YES

Summary:
GRE and ERSPAN changelink authorization checked CAP_NET_ADMIN only in dev_net(dev), allowing a caller with delegated network admin rights in one namespace to rewrite tunnel parameters owned by another namespace.

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

The original Linux kernel CVE announcement for CVE-2026-63829 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-63829

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-2026-63829	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:C/C:H/I:H/A:H';CWE-862;CWE-863;CWE-269;CWE-284;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:C/C:L/I:H/A:L';BEST CVSS score: '7.8';DESCR 'GRE and ERSPAN tunnel changelink authorization was incomplete because the rtnetlink path checked CAP_NET_ADMIN only in dev_net(dev), while the tunnel link namespace t net could be different. A caller with network administration rights in the device namespace but not in the tunnel link namespace could still rewrite tunnel parameters that belong to the other namespace. For the CVSS the PR:L is used because the attacker needs local code execution with limited CAP_NET_ADMIN style control in one network namespace, not full host administrative privileges. The issue is not directly network reachable and is triggered through local rtnetlink control plane operations rather than packet traffic. Impact is a cross namespace authorization bypass that can affect tunnel routing and encapsulation policy, with plausible confidentiality impact from traffic redirection, high integrity impact from unauthorized configuration changes, and availability impact from breaking tunnel connectivity.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate / Actionable Moderate at minimum (with actual score 7)	YES	REMOTE HARDWARE KPANIC  KPANIC INCREASED_TO_HIGH_BASED_ON_GUESSCVSS	NO	NO	checked

======================================================================
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

* Security-boundary bypass with practical sensitive-object access: +2
  The bug bypasses the intended CAP_NET_ADMIN check in the tunnel link netns and allows a caller privileged only in dev_net(dev) to modify a GRE or ERSPAN tunnel owned by another netns.

* Cross-boundary isolation/policy bypass: +1
  The affected condition is explicitly cross-netns: dev_net(dev) and t->net can differ after device creation or movement.

* Confidentiality impact plausible: +1
  Unauthorized tunnel reconfiguration can plausibly redirect, mirror, or alter encapsulated traffic paths, especially for ERSPAN/GRE deployments.

* Integrity impact plausible: +1
  The attacker can rewrite live tunnel parameters without authorization in the tunnel namespace.

* Availability impact realistic: +1
  Bad tunnel parameters can break tunnel connectivity or disrupt encapsulation policy.

* Practical exploit chain strongly implied: +1 for paranoid score only
  The commit directly describes the missing check and notes that parsers update live tunnel fields before the normal changelink path completes.

* Requires CAP_NET_ADMIN in one namespace: 0 penalty
  No full -2 penalty is applied because the issue bypasses a meaningful namespace/capability boundary. The attacker does not need full host root, only CAP_NET_ADMIN style control in the wrong namespace.

* Remote reachable: +0
  This is rtnetlink control-plane access, not packet-triggered remote reachability.

* Memory corruption / LPE primitive: +0
  No UAF, OOB write, type confusion, or arbitrary write is indicated.

Call-site confidence: high. The affected changelink entry points and the new authorization check are visible in the patch.

## 3. Reachability analysis

The bug is triggered locally through RTM_NEWLINK/changelink operations for ip_gre or erspan devices. The realistic attacker is a process with CAP_NET_ADMIN in dev_net(dev), for example a container or delegated network namespace administrator, but without CAP_NET_ADMIN in the tunnel link namespace t->net. Namespace delegation is central to the impact because the bug exists only when those namespaces differ. The path is not remotely reachable through packet traffic. Exploitation requires GRE or ERSPAN tunnel objects in a cross-netns configuration, so it is configuration-dependent but plausible in containerized or delegated networking environments.

## 4. Severity interpretation

This behaves like an Important-candidate access-control and namespace-boundary bypass, not like memory corruption. The realistic impact is unauthorized tunnel reconfiguration with possible traffic redirection, policy bypass, and tunnel disruption. There is no demonstrated kernel memory corruption or direct privilege escalation primitive, so it should not be treated as a memory-safety Important. However, because it breaks a capability boundary across network namespaces and can affect network traffic policy, it should not be auto-closed as an ordinary Moderate.

## 5. One-sentence report phrase

GRE and ERSPAN changelink authorization checked CAP_NET_ADMIN only in dev_net(dev), allowing a caller with delegated network admin rights in one namespace to rewrite tunnel parameters owned by another namespace.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

This needs manual review because it is a namespace capability-boundary bypass with practical network policy impact, and the real severity depends on whether the affected product allows delegated CAP_NET_ADMIN or container-controlled tunnel device movement.

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

Patch: net: ip_gre: require CAP_NET_ADMIN in the device netns for changelink
Commit: 9831bc9ecb402957810c2045c663fbfe9b09e296
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=9831bc9ecb402957810c2045c663fbfe9b09e296

Commit description:

A tunnel changelink() operates on at most two netns, dev_net(dev) and
the tunnel link netns t->net. They differ once the device is created in
or moved to a netns other than the one the request runs in. The rtnl
changelink path checks CAP_NET_ADMIN only against dev_net(dev), so a
caller privileged there but not in t->net can rewrite a tunnel that
lives in t->net.

Add rtnl_dev_link_net_capable() next to rtnl_get_net_ns_capable() in
net/core/rtnetlink.c. It requires CAP_NET_ADMIN in the link netns and is
skipped when the link netns is dev_net(dev), where the rtnl path already
checked it. The other patches in this series use the same helper.

Gate ipgre_changelink() and erspan_changelink() with it, at the top of
the op before any attribute is parsed, because the parsers update live
tunnel fields first. ipgre_netlink_parms() sets t->collect_md before
ip_tunnel_changelink() runs.

Commit 8b484ef ("ip6: vti: Use ip6_tnl.net in
vti6_siocdevprivate().") added the same check on the ioctl path. This
adds it on RTM_NEWLINK.

Reported-by: Xiao Liang <shaw.leon@gmail.com>
Closes: https://lore.kernel.org/netdev/CABAhCOSzP1vaThGV35_VnsRCb=87_CPjPVsTHbq905k8A+BuUg@mail.gmail.com/
Fixes: b57708a ("gre: add x-netns support")
Cc: stable@vger.kernel.org
Signed-off-by: Maoyi Xie <maoyixie.tju@gmail.com>
Reviewed-by: Kuniyuki Iwashima <kuniyu@google.com>
Link: https://patch.msgid.link/20260612085941.3158249-2-maoyixie.tju@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  net/core/rtnetlink.c
  include/net/rtnetlink.h
  net/ipv4/ip_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=9831bc9ecb402957810c2045c663fbfe9b09e296

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

The original Linux kernel CVE announcement for CVE-2026-63829 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-63829

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:C/C:L/I:H/A:L
  The Best / paranoid CVSS vector: AV:L/AC:H/PR:L/UI:N/S:C/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: 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).