[CVE-2026-89487][IMPORTANT] openvswitch: only skb_tx_error() a packet we are about to drop

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

CVE: CVE-2026-89487
Priority: IMPORTANT
AL-KERNEL base severity: IMPORTANT
KPANIC flag: YES
Patch: openvswitch: only skb_tx_error() a packet we are about to drop
Commit: 6767d70cf46f65807a6a4c4406a518e6c12e36ae
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=6767d70cf46f65807a6a4c4406a518e6c12e36ae
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89487
Analysis date: Sat, 12 Sep 2026 04:16:08 -0400
ActionableScore: 7
ActionableScore lower bound: 6
Actionable bucket: Strong Important candidate
Manual review required: YES

Summary:
No one-sentence report phrase was found.

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

The original Linux kernel CVE announcement for CVE-2026-89487 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89487

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-2026-89487	IMPORTANT	https://www.kernelcve.org/list/?q=CVE-2026-89487 CHECK WITH IMPACT FROM ORIG NN LOW	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-664;CWE-703;CWE-284;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:H/A:H';BEST CVSS score: '7';DESCR 'Open vSwitch could call skb_tx_error() on an skb that was not going to be dropped after a failed userspace upcall. This clears zerocopy metadata including SKBFL_SHARED_FRAG while the skb may still continue through later flow actions. For MSG_ZEROCOPY traffic carrying page-cache backed fragments, later local ESP in UDP processing can then decrypt in place instead of forcing copy on write, creating an unauthorized page-cache write primitive. For the CVSS the PR:L is used because a local user or local process is normally required to create the relevant zerocopy skb and drive it through an existing OVS and XFRM configuration. The issue is not a simple remote packet only trigger, but it can affect systems where untrusted local workloads can send traffic through Open vSwitch. Impact is at least local denial of service and high integrity impact, with possible privilege escalation risk from the page-cache write primitive. YES REQUIRES MANUAL CHECK.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate (with actual score 7)	YES	ERRORPATH UAF SKB PACKET  KPANIC INCREASED_TO_MODERATE7_FROM_LOW_BASED_ON_GUESSCVSS_AND_ACTIONABLESCOREHIGHER4 KPANIC INCREASED_TO_HIGH_BASED_ON_ACTIONABLESCOREHIGHERTHAN5	NO	NO	checked

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

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

1. ActionableScore

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

2. Signal breakdown

Conservative positive signals:

* +2: Protected page-cache or shared-page ownership corruption primitive. The supplied context explicitly says that stripping SKBFL_SHARED_FRAG from a live MSG_ZEROCOPY skb can make later ESP in UDP decrypt in place over page-cache fragments that the sender does not own.
* +1: Local unprivileged trigger. The supplied context explicitly describes an unprivileged page-cache write primitive.
* +1: Integrity impact. The primitive is an unauthorized write to page-cache backed data owned outside the sender authority.
* +1: Practical exploit chain strongly implied. The chain is failed OVS USERSPACE upcall, skb_tx_error on live skb, SKBFL_SHARED_FRAG removal, later OVS actions, ESP in UDP local delivery, in-place AEAD over non-owned page-cache pages.
* +1: Important filesystem/storage memory ownership path. Page-cache backed fragments and COW avoidance are security-relevant storage ownership semantics.

Paranoid additional signal:

* +1: Local unprivileged high-control data-plane API. The attacker-influenced traffic path involves MSG_ZEROCOPY skb fragments and page-cache backed payload state, which materially affects the ownership/COW failure. This is counted only in the paranoid score because complete deployment reachability through OVS and XFRM depends on configuration.

Non-awarded signals:

* RealUAF not awarded. This is an skb ownership and COW metadata corruption issue, not a post-free stale object use.
* Controlled reclaim not demonstrated.
* Arbitrary kernel read/write not demonstrated.
* Remote reachability not established. The trigger is not a simple network packet only attack.
* Extra availability not awarded. The key consequence is unauthorized page-cache write, not merely crash.
* Separate COW +2 and page-cache corruption +2 are not stacked because they describe the same ownership violation.
* Privilege penalty not applied because supplied evidence explicitly states an unprivileged page-cache write primitive.
* Prior privileged setup penalty not applied as a score reduction. OVS and XFRM configuration are material deployment prerequisites, but the supplied evidence does not prove that the attacker must be host admin to perform the complete trigger once the vulnerable path is present.

3. Reachability analysis

The evaluated attacker is a local unprivileged process or workload that can create the relevant MSG_ZEROCOPY traffic and send it through an existing Open vSwitch and XFRM ESP in UDP path. The supplied context directly states an unprivileged page-cache write primitive, so HostAdminRequired is NO for the analyzed exploitation model.

This is not established as a direct remote attack. Packet processing is involved, but the primitive requires a local skb ownership scenario, failed USERSPACE upcall behavior, and later local ESP in UDP processing. A purely remote network sender is not shown to be able to satisfy the page-cache backed zerocopy and OVS action sequence.

Namespace scoped privilege is not the basis of the score. No complete namespace-only capability chain is needed or established from the supplied data, so NamespaceScopedPrivilege is NO.

There is likely prior deployment setup in practice: Open vSwitch must be in use, relevant flow actions must allow a failed USERSPACE upcall followed by continued forwarding, and XFRM ESP in UDP processing must be present. These are important exposure constraints, but they do not erase the established unprivileged page-cache write primitive.

No special rare hardware is required. No hard race is established. The issue is an ordered ownership/lifetime semantics bug in skb error handling, not a narrow timing race.

4. Primitive and severity interpretation

The exact primitive is unauthorized page-cache backed write caused by clearing zerocopy/shared-frag metadata on an skb that is still live and later processed. queue_userspace_packet() only borrows the skb, but its error path called skb_tx_error(skb). skb_tx_error() is only safe when the skb is about to be freed. Because the skb continues through later Open vSwitch actions, clearing SKBFL_SHARED_FRAG can suppress the copy-on-write protection that ESP input relies on before in-place decrypt.

This is active ownership and COW state corruption, not a UAF. It is not cleanup-only and not merely a crash-only stale pointer issue. It creates a Dirty-Pipe-like page-cache ownership violation where attacker-authorized traffic processing can modify storage-backed pages the attacker should not own.

PrimitiveClass is CLASS_A because attacker-directed page-cache or protected shared-page corruption is an Important-preserving primitive. The demonstrated consequence is high integrity impact. The speculative consequence is local privilege escalation or broader protected-file modification depending on what page-cache backed data can be targeted, but arbitrary kernel R/W and a complete LPE chain are not demonstrated in the supplied text.

ImportantCandidate is YES because the primitive is a concrete unprivileged page-cache write ownership violation. AutoDowngradeAllowed is NO because automatic Moderate closure would discard a meaningful established corruption primitive with strong security-boundary relevance.

Operationally this should be treated as at least Actionable Moderate and more likely a Strong Important candidate requiring manual review, especially for systems running Open vSwitch with untrusted local workloads and relevant XFRM/ESP traffic paths.

5. One-sentence report phrase

Open vSwitch could clear SKBFL_SHARED_FRAG on a live MSG_ZEROCOPY skb after a failed userspace upcall, allowing later ESP in UDP processing to write in place to page-cache backed fragments not owned by the sender and creating an unprivileged page-cache write primitive.

6. Manual review recommendation

MANUAL CHECK REQUIRED

A concrete unprivileged page-cache ownership violation is established, with high integrity impact and possible privilege-escalation relevance depending on targetability and deployment configuration.

7. Call-site confidence

Call-site confidence: high

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

Patch: openvswitch: only skb_tx_error() a packet we are about to drop
Commit: 6767d70cf46f65807a6a4c4406a518e6c12e36ae
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=6767d70cf46f65807a6a4c4406a518e6c12e36ae

Commit description:

queue_userspace_packet() borrows the packet skb -- it only copies it into
a private netlink message (user_skb) and does not own it; on return
do_execute_actions() keeps forwarding it through the flow's remaining
actions. Its error path nevertheless calls skb_tx_error(skb), which via
skb_zcopy_clear() does skb_shinfo(skb)->flags &= ~SKBFL_ALL_ZEROCOPY,
stripping SKBFL_SHARED_FRAG from that live skb (skb_tx_error()'s kerneldoc
says "skb must be freed afterwards").

For a MSG_ZEROCOPY skb carrying page-cache frags, SKBFL_SHARED_FRAG is
what makes esp_input() skb_cow_data() before in-place AEAD; once it is
stripped a later local ESP-in-UDP delivery decrypts in place over pages
the sender does not own -- an unprivileged page-cache write (the
"Fragnesia" primitive).
do_execute_actions() ignores output_userspace()'s return value, so any
action after a failed USERSPACE upcall inherits the stripped skb.

Move the skb_tx_error() to the flow-miss drop path - the "default"
branch of ovs_dp_process_packet()'s switch(error), before kfree_skb().

The call has been here since commit 36d5fe6 ("core, nfqueue,
openvswitch: Orphan frags in skb_zerocopy and handle errors") but was
harmless until esp_input() began relying on SKBFL_SHARED_FRAG to gate
in-place decrypt; only then did stripping it on a still-forwarded skb
become a page-cache write primitive.

Fixes: 36d5fe6 ("core, nfqueue, openvswitch: Orphan frags in skb_zerocopy and handle errors")
Fixes: f4c50a4 ("xfrm: esp: avoid in-place decrypt on shared skb frags")
Cc: stable@vger.kernel.org
Assisted-by: Claude:claude-opus-5
Signed-off-by: Norbert Szetei <norbert@doyensec.com>
Reviewed-by: Ilya Maximets <i.maximets@ovn.org>
Tested-by: Jongmin Jang <payload.jang@gmail.com>
Link: https://patch.msgid.link/55A52703-7548-4A55-A9CE-2A37145BDCAD@doyensec.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  net/openvswitch/datapath.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=6767d70cf46f65807a6a4c4406a518e6c12e36ae

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

The original Linux kernel CVE announcement for CVE-2026-89487 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89487

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:L/I:H/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).