[CVE-2026-89563][IMPORTANT] ip6_tunnel: use skb_cow_head() in ip6_tnl_xmit()

[CVE-2026-89563][IMPORTANT] ip6_tunnel: use skb_cow_head() in ip6_tnl_xmit()

[AL-KERNEL]

CVE: CVE-2026-89563
Priority: IMPORTANT
AL-KERNEL base severity: IMPORTANT
KPANIC flag: YES
Patch: ip6_tunnel: use skb_cow_head() in ip6_tnl_xmit()
Commit: 37df5bc6b5ccad88c2faa0f5466f150178882b74
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=37df5bc6b5ccad88c2faa0f5466f150178882b74
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89563
Analysis date: Sat, 12 Sep 2026 06:53:34 -0400
ActionableScore: 7
ActionableScore lower bound: 6
Actionable bucket: Strong Important candidate
Manual review required: YES

Summary:
A double-free or use-after-free in the IPv6 tunnel transmit path can occur when `ip6_tnl_xmit()` consumes a caller-owned skb after reallocating headroom and then later returns an error, allowing caller error paths to free the stale original skb again.

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

The original Linux kernel CVE announcement for CVE-2026-89563 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89563

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.

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

CVE-2026-89563	IMPORTANT	https://www.kernelcve.org/list/?q=CVE-2026-89563 CHECK WITH IMPACT FROM ORIG NN IMPORTANT	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:A/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:H';CWE-415;*CWE-416;*CWE-664;Other CVSS 'AV:A/AC:H/PR:N/UI:N/S:U/C:L/I:L/A:H';BEST CVSS score: '7.5';DESCR 'A double free or use after free can occur in the IPv6 tunnel transmit path because ip6_tnl_xmit() could replace the caller owned skb with skb_realloc_headroom() and consume the original skb before all later error paths had completed. If a later collect_md or encapsulation error occurs, the helper returns an error while the caller still holds the original stale skb pointer and may free it again. For the CVSS the PR:N is used because triggering may require only packet reachability to a host that already forwards or transmits traffic through the affected IPv6 tunnel configuration, not an account on the victim. The attack is not a generic public Internet vector in most deployments and usually requires access to an internal or adjacent network where IPv6 tunnels or GRE over IPv6 are configured. Impact is at least denial of service via kernel crash and in the paranoid case may include confidentiality or integrity impact because the bug class is memory corruption.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate (with actual score 7)	YES	REMOTE NETWORK SKB IPV6 KPANIC 	NO	NO	checked

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

ActionableScore=7
ActionableScoreLower=6
PrimitiveClass=CLASS_B
RealUAF=YES
HostAdminRequired=NO
NamespaceScopedPrivilege=NO
ImportantCandidate=YES
ManualReview=REQUIRED
AutoDowngradeAllowed=NO

## 1. ActionableScore

* Raw conservative score: 6
* Conservative score: 6. No floor or cap changed the score.
* Raw paranoid score: 7
* Paranoid score: 7. No floor or cap changed the score.
* Final recommended bucket: **Strong Important candidate**::

## 2. Signal breakdown

Conservative signals:

* +2 Real UAF / lifetime violation. `ip6_tnl_xmit()` can consume the original caller-owned skb via `consume_skb(skb)` after `skb_realloc_headroom()`, while callers still retain the original skb pointer.
* +1 Concrete repeated release / free. After `ip6_tnl_xmit()` returns an error on a later post-reallocation path, callers such as `ip6_tnl_start_xmit()` and IPv6 GRE error paths may free the same original skb again.
* +1 Reliable kernel crash / strong DoS. Double-free / UAF of an skb in the network transmit path is a realistic kernel crash condition.
* +2 Network-triggerable in configured deployments. An attacker may need only packet reachability to a system that already forwards or transmits traffic through the affected IPv6 tunnel path.

Paranoid additional signal:

* +1 Paranoid repeated lifetime mutation leverage. The second free is not merely a stale read. It is a concrete lifetime mutation through a reference whose original object may already have been consumed, leaving post-lifetime corruption leverage unresolved.

Non-awarded signals:

* Controlled reclaim is not demonstrated.
* Arbitrary kernel read/write is not demonstrated.
* Callback or function-pointer control is not demonstrated.
* Confidentiality and integrity are not independently awarded beyond the lifetime-corruption primitive.
* Extra Availability is not awarded because the crash/DoS is already counted once.
* Structured stale-object leverage is not separately awarded because the same fact is already counted as concrete repeated release/free.
* Weak LPE is not separately awarded for the same repeated-release fact.
* No race penalty is applied because this is an ordered error-path ownership bug, not a narrow timing race.

Repeated-release checks:

* First release can end lifetime: yes, `consume_skb()` consumes the original skb.
* Exact same reference survives: yes, callers still hold the original skb pointer.
* Second operation mutates lifetime/refcount: yes, caller error paths free the skb again.
* Concrete repeated release/refcount +1 awarded: yes.
* Paranoid repeated-refcount leverage +1 awarded: yes, because the second operation itself performs lifetime mutation through potentially freed storage.
* FLOOR 5 applied: not needed, raw conservative score is already above 5.

## 3. Reachability analysis

The evaluated attacker is a network or network-adjacent packet sender against a host that already has an affected IPv6 tunnel or IPv6 GRE transmit path configured. The attacker does not need host administrator privileges in that deployment model, but the vulnerable tunnel configuration is normally created by a prior privileged administrator or network management system.

This is not a generic public Internet vector for all Linux hosts. Practical exposure depends on whether packets from the attacker can be routed, bridged, encapsulated, or otherwise transmitted through the affected IPv6 tunnel path. The required conditions include a need for extra skb headroom or writability and a later error after the old skb has been consumed, such as a collect metadata tunnel rejecting an unsupported encapsulation state or `ip6_tnl_encap()` failing.

No namespace-scoped privilege is required for the packet-sender model. No special hardware is required. No hard race is established.

## 4. Primitive and severity interpretation

The exact primitive is an skb ownership/lifetime bug that can become a double-free or use-after-free. The stale object is an active kernel networking object, but the scored primitive is specifically the repeated lifetime mutation on the same skb reference, not generic complexity of `struct sk_buff`.

This is stronger than an ordinary stale cleanup dereference:

A. Ordinary stale cleanup dereference would be a stale pointer used only for a read or immediate fault.

B. Here, the original skb can be consumed inside `ip6_tnl_xmit()`, the caller still retains that exact stale pointer, and a later caller-side error path can free it again. That is a second lifetime mutation through a potentially dangling object.

PrimitiveClass is CLASS_B because a real kernel lifetime corruption primitive exists, but controlled reclaim, arbitrary write, or demonstrated privilege escalation is not established. Demonstrated consequence is kernel crash / DoS. Speculative consequence is broader memory corruption impact if heap reuse or refcount/lifetime corruption becomes exploitable, but that is not proven by the supplied patch.

ImportantCandidate is YES because network-reachable skb double-free/UAF with concrete repeated lifetime mutation should not be treated as ordinary crash-only cleanup without review. AutoDowngradeAllowed is NO because automatic downgrade would discard an unresolved concrete kernel lifetime-corruption primitive.

Likely operational severity is high-end Actionable Moderate to Strong Important candidate, depending on real deployment exposure of IPv6 tunnel/GRE paths.

## 5. One-sentence report phrase

A double-free or use-after-free in the IPv6 tunnel transmit path can occur when `ip6_tnl_xmit()` consumes a caller-owned skb after reallocating headroom and then later returns an error, allowing caller error paths to free the stale original skb again.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: concrete skb double-free / UAF with network-reachable deployment scenarios and unresolved post-lifetime corruption leverage.

## 7. Call-site confidence

Call-site confidence: high

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

Patch: ip6_tunnel: use skb_cow_head() in ip6_tnl_xmit()
Commit: 37df5bc6b5ccad88c2faa0f5466f150178882b74
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=37df5bc6b5ccad88c2faa0f5466f150178882b74

Commit description:

ip6_tnl_xmit() may need to expand headroom before it can push the
outer IPv6 and optional encap headers. It currently does that with
skb_realloc_headroom(), copies skb->sk ownership, consumes the original
skb, and then continues processing with the replacement skb kept only in
its local variable.

That is safe only if the helper cannot fail afterwards. But this helper
still has post-reallocation error exits. collect_md tunnels reject
non-NONE encap after the replacement, and ip6_tnl_encap() can also fail
later. In those cases the helper returns an error to its callers while
the caller still only has the original skb pointer.

Both ip6_tnl_start_xmit() and the IPv6 GRE paths free the caller skb on
error, so they can end up freeing an skb that ip6_tnl_xmit() already
consumed.

Use skb_cow_head() instead. It provides the required headroom and
writability without privately replacing the caller-owned skb, so later
error returns cannot leave callers with a stale pointer.

The Ethernet users, ip6gretap and ip6erspan, clear IFF_TX_SKB_SHARING
and already call skb_cow_head() before entering ip6_tnl_xmit(). They do
not rely on the removed skb_shared() reallocation. This also makes the
IPv6 tunnel path consistent with ip_tunnel_xmit().

Fixes: 058214a ("ip6_tun: Add infrastructure for doing encapsulation")
Cc: stable@vger.kernel.org
Reported-by: Vega <vega@nebusec.ai>
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: Zhiling Zou <zhilinz@nebusec.ai>
Link: https://patch.msgid.link/30807a062ccc5c9c8a5ec2c5eb805ef279c50bdd.1786452593.git.zhilinz@nebusec.ai
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  net/ipv6/ip6_tunnel.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=37df5bc6b5ccad88c2faa0f5466f150178882b74

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

The original Linux kernel CVE announcement for CVE-2026-89563 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89563

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:A/AC:H/PR:N/UI:N/S:U/C:L/I:L/A:H
  The Best / paranoid CVSS vector: AV:A/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:H
  The Best / paranoid CVSS score: 7.5

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: 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).