[CVE-2026-89564][IMPORTANT] ip: orphan prefetched skbs before multicast forwarding

[CVE-2026-89564][IMPORTANT] ip: orphan prefetched skbs before multicast forwarding

[AL-KERNEL]

CVE: CVE-2026-89564
Priority: IMPORTANT
AL-KERNEL base severity: IMPORTANT
KPANIC flag: YES
Patch: ip: orphan prefetched skbs before multicast forwarding
Commit: a8af6fbac895f057c4b8ff8a2e3fb4c5827fe4ce
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=a8af6fbac895f057c4b8ff8a2e3fb4c5827fe4ce
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89564
Analysis date: Sat, 12 Sep 2026 06:57:46 -0400
ActionableScore: 6
ActionableScore lower bound: 5
Actionable bucket: Actionable Moderate / Strong Important candidate requiring manual review
Manual review required: YES

Summary:
No one-sentence report phrase was found.

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

The original Linux kernel CVE announcement for CVE-2026-89564 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89564

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-2026-89564	IMPORTANT	https://www.kernelcve.org/list/?q=CVE-2026-89564 CHECK WITH IMPACT FROM ORIG NN LOW	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-416;CWE-362;*CWE-672;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 use after free can occur in the IP multicast forwarding path because an skb may retain a prefetched skb sk association from bpf_sk_assign() or UDP early demux without holding a stable socket reference. If the multicast packet is not locally deliverable, IPv4 or IPv6 can forward or queue the original skb after the receive-side RCU section has ended. When the prefetched socket is later destroyed, freeing the skb can call sock_pfree and dereference the stale socket pointer. For the CVSS the PR:N is used because an attacker does not need credentials on the victim when the affected multicast forwarding path is reachable, although reliable triggering depends on multicast routing and a matching local BPF or UDP early demux state. The issue is not expected to be Internet wide in most deployments and is more likely reachable from an adjacent or internal multicast enabled network. Impact is at least denial of service via kernel crash and in the paranoid case may include confidentiality and integrity impact because the stale socket pointer is a UAF candidate rather than a pure NULL dereference.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate / Strong Important candidate requiring manual review (with actual score 6)	YES	REMOTE OOB BPF SIMPLEFIX NETWORK SKB IPV6 PACKET  KPANIC INCREASED_FROM_LOW_BASED_ON_REQUIREMANUALCHECK KPANIC INCREASED_TO_HIGH_BASED_ON_ACTIONABLESCOREHIGHERTHAN5	NO	NO	checked

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

ActionableScore=6
ActionableScoreLower=5
PrimitiveClass=CLASS_B
RealUAF=YES
HostAdminRequired=NO
NamespaceScopedPrivilege=NO
ImportantCandidate=YES
ManualReview=REQUIRED
AutoDowngradeAllowed=NO

1. ActionableScore

* Raw conservative score: 5
* Conservative score: 5. No floor or cap changed the score.
* Raw paranoid score: 6
* Paranoid score: 6. No floor or cap changed the score.
* Final recommended bucket: **Actionable Moderate / Strong Important candidate requiring manual review**::

2. Signal breakdown

Conservative signals:

* +2 Real UAF / active lifetime violation. The skb keeps skb->sk without a guaranteed socket reference, then the receive-side RCU section ends and the socket may be destroyed before later skb free.
* +1 Reliable kernel crash / strong DoS. The later skb free invokes sock_pfree() and dereferences stale skb->sk, which is a realistic kernel fault path.
* +2 Network reachable packet-processing path. Attacker-controlled multicast traffic can reach the vulnerable IPv4 or IPv6 multicast forwarding path on affected systems.

Paranoid additional signal:

* +1 Weak structured stale-object corruption concern. sock_pfree() does not merely test a stale pointer. It consumes a stale struct sock pointer and may mutate socket accounting state through it, leaving unresolved post-lifetime corruption leverage even though controlled reclaim is not demonstrated.

Non-awarded signals:

* +2 Strong exploitation leverage not awarded. Controlled reclaim, controlled replacement, arbitrary read/write, callback control, or demonstrated LPE are not established.
* Concrete repeated release/refcount signal not awarded. The patch establishes stale skb->sk dereference/use, but not a first put followed by a second put/release on the exact same dangling socket reference.
* Extra availability point not awarded. The crash/DoS is already counted once.
* Confidentiality and integrity direct-impact bonuses not awarded conservatively. The patch does not establish disclosure, arbitrary write, credential corruption, or a demonstrated privilege-escalation chain.
* Race penalty not applied. The bug involves RCU/lifetime timing, but the evidence does not prove a narrow one-shot race that should reduce the score.
* Rare-subsystem penalty not applied. Multicast forwarding requires deployment-specific configuration, but the vulnerable path is still an IP packet-processing path once enabled.

3. Reachability analysis

The evaluated attacker is a network sender able to reach an IPv4 or IPv6 multicast forwarding path on the target. The strongest realistic exposure is adjacent or internal network reachability rather than Internet-wide reachability, because multicast forwarding is normally used in controlled routing domains.

Host administrator privileges are not required by the packet sender. However, affected exposure depends on prior system state: multicast routing/forwarding must be active, and the skb must have a prefetched socket association from bpf_sk_assign() or UDP early demux. The BPF path may depend on privileged local setup, but the UDP early demux path is a normal networking optimization and does not by itself imply attacker host privileges.

Namespace-scoped privilege is not the relevant model for this patch. No evidence is supplied that the complete trigger depends on attacker-created namespaces or namespace-local capabilities.

The trigger depends on socket lifetime and RCU timing, but the skb can be queued or forwarded after the receive-side RCU section ends. This makes the lifetime mismatch concrete rather than a purely speculative race.

4. Primitive and severity interpretation

The exact primitive is a stale post-lifetime use of skb->sk. The stale object is an active structured object, struct sock, not fixed stale storage. The stale use occurs when skb cleanup invokes sock_pfree() after the prefetched socket may already have been destroyed.

This is Class B: a real kernel lifetime/UAF primitive requiring exploitation-leverage review. It is stronger than a pure NULL dereference because the stale pointer refers to a previously valid socket object and sock_pfree() may access or mutate fields in that object. It is not Class A because the evidence does not establish controlled reclaim, directed corruption, arbitrary kernel read/write, or a demonstrated privilege-escalation chain.

The demonstrated consequence is kernel crash / denial of service. The speculative consequence is post-free corruption through stale socket accounting or related sock_pfree() behavior if the freed socket storage is reused in an attacker-useful way. That stronger path is unresolved, not proven.

ImportantCandidate=YES because this is a network-reachable active-object UAF candidate in a core packet-processing path with unresolved post-lifetime corruption implications. AutoDowngradeAllowed=NO because automatically closing it as ordinary DoS would discard a concrete kernel lifetime primitive with meaningful unresolved exploitation leverage.

Likely operational severity is Actionable Moderate at minimum, with Strong Important review justified for exposed multicast routing deployments.

5. One-sentence report phrase

A network reachable UAF can occur when multicast forwarding keeps an skb with a stale prefetched socket association past the receive-side RCU lifetime, leading to sock_pfree() dereferencing the destroyed socket and causing at least kernel DoS with unresolved memory corruption risk.

6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: real network-reachable active-object UAF involving stale struct sock use, with demonstrated crash and unresolved post-lifetime corruption leverage.

7. Call-site confidence

Call-site confidence: high

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

Patch: ip: orphan prefetched skbs before multicast forwarding
Commit: a8af6fbac895f057c4b8ff8a2e3fb4c5827fe4ce
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=a8af6fbac895f057c4b8ff8a2e3fb4c5827fe4ce

Commit description:

IPv4 and IPv6 input preserve an skb->sk association installed by
bpf_sk_assign() so that local delivery can use the selected socket under
RCU. Both address families can also prefetch a socket in UDP early demux.
In both paths (BPF and UDP early demux) a reference is not guaranteed to
be held on the socket.

When a multicast packet is not locally deliverable, IPv6 hands the
original skb to ip6_mr_input(). IPv4's ip_mr_input() similarly keeps the
original skb when local delivery is not needed. Either path can put the
skb on an unresolved multicast route queue or forward it after the
receive-side RCU section ends.

After the prefetched socket is destroyed, a later skb free invokes
sock_pfree() and dereferences the stale skb->sk. Orphan the skb before
each non-local multicast forwarding path. Local delivery retains the
original skb; the existing skb_clone() calls provide multicast forwarding
with a socket-free clone.

Fixes: cf7fbe6 ("bpf: Add socket assign support")
Fixes: 08842c4 ("udp: no longer touch sk->sk_refcnt in early demux")
Cc: stable@vger.kernel.org
Reported-by: Vega <vega@nebusec.ai>
Signed-off-by: Zhiling Zou <zhilinz@nebusec.ai>
Reported-by: Vega <vega@nebusec.ai>
Signed-off-by: Zhiling Zou <zhilinz@nebusec.ai>
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Link: https://patch.msgid.link/0c52eb3d7532aaf8bccf37e0f7c922143c639735.1786552223.git.zhilinz@nebusec.ai
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  net/ipv4/ipmr.c
  net/ipv6/ip6_input.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=a8af6fbac895f057c4b8ff8a2e3fb4c5827fe4ce

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

The original Linux kernel CVE announcement for CVE-2026-89564 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89564

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: 5
  Paranoid ActionableScore: 6

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).