[CVE-2026-89561][MODERATE REGULAR] ipv6: rpl: fix NULL dereference of idev in ipv6_rpl_srh_rcv()

[CVE-2026-89561][MODERATE REGULAR] ipv6: rpl: fix NULL dereference of idev in ipv6_rpl_srh_rcv()

[AL-KERNEL]

CVE: CVE-2026-89561
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: ipv6: rpl: fix NULL dereference of idev in ipv6_rpl_srh_rcv()
Commit: 63f50e9f90d0287ad66a0955ebfc2d3a9c044a1c
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=63f50e9f90d0287ad66a0955ebfc2d3a9c044a1c
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89561
Analysis date: Fri, 11 Sep 2026 22:43:03 -0400
ActionableScore: 3
ActionableScore lower bound: 1
Actionable bucket: Borderline / manual review recommended
Manual review required: YES

Summary:
No one-sentence report phrase was found.

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

The original Linux kernel CVE announcement for CVE-2026-89561 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89561

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: MODERATE REGULAR
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-2026-89561	MODERATE	https://www.kernelcve.org/list/?q=CVE-2026-89561 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:N/I:N/A:H';**CWE-476;CWE-362;CWE-703;Other CVSS 'AV:L/AC:H/PR:H/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '5.3';DESCR 'An IPv6 RPL source routing header packet can reach ipv6_rpl_srh_rcv after the inet6 device pointer was cleared during an interface state change, causing a NULL pointer dereference when rpl_seg_enabled is read. The issue is a race between packet receive processing and IPv6 device teardown caused by an MTU drop below IPV6_MIN_MTU or similar state transitions. For the CVSS the PR:N is used for the paranoid network assisted scenario because the packet sender only needs reachability to the affected IPv6 interface, while reliable triggering still depends on a high complexity race with local MTU or IPv6 device state changes. The practical base scenario is more restricted because changing MTU normally requires administrative privileges on the victim. Impact is denial of service via kernel crash only, with no supported confidentiality or integrity impact.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline / manual review recommended (with actual score 2)	YES	REMOTE READ KASAN NULLPTR NETWORK SKB HARDWARE LINUS IPV6 PACKET  DECREASED_TO_MODERATE_REGULAR_FROM_MODERATE7_BASED_ON_GUESSCVSS_AND_CIANNH	YES	NO	checked

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

ActionableScore=3
ActionableScoreLower=1
PrimitiveClass=CLASS_C
RealUAF=NO
HostAdminRequired=UNKNOWN
NamespaceScopedPrivilege=UNKNOWN
ImportantCandidate=NO
ManualReview=RECOMMENDED
AutoDowngradeAllowed=YES

1. ActionableScore

* Raw conservative score: 1
* Conservative score: 1
* Raw paranoid score: 3
* Paranoid score: 3
* Final recommended bucket: **Borderline / manual review recommended**::

No floor or cap was applied.

2. Signal breakdown

Conservative signals:

* +1 Reliable kernel crash / strong DoS: the supplied KASAN trace shows a NULL pointer dereference in `ipv6_rpl_srh_rcv()` from the IPv6 receive softirq path.
* +0 Real UAF: not awarded. The bug is a NULL dereference after `dev->ip6_ptr` becomes NULL, not a demonstrated stale freed object use.
* +0 Confidentiality / integrity: not awarded. The patch supports crash/DoS only and does not show disclosure, overwrite, controlled reclaim, or privilege escalation.
* +0 Remote reachable: not awarded conservatively because the demonstrated reliable reproducer also requires local interface MTU flapping or an equivalent IPv6 device state transition.
* +0 Hard race penalty: not subtracted conservatively because the race is reproducible by repeated flooding and repeated MTU transitions, rather than a proven one-shot uncontrollable timing window.
* +0 Host-admin penalty: not subtracted because host-admin-equivalent privilege is not positively established for every plausible manifestation. The reliable local reproducer likely needs CAP_NET_ADMIN for MTU changes, but remote packet participation is also material.

Paranoid additional signals:

* +1 Network-assisted packet-processing exposure: attacker-controlled IPv6 traffic with routing-header processing can be the packet reaching the vulnerable path while the interface state changes.
* +1 Networking receive path relevance: the fault occurs in IPv6 extension/routing-header receive processing, not in a purely local ioctl or admin-only cleanup path.
* +0 Direct remote +2: not awarded. The complete trigger is not shown to be remote-only because the NULL `idev` state depends on concurrent local device or MTU state changes.
* +0 Strong LPE / memory corruption: not awarded. This is a NULL dereference, not UAF, OOB write, arbitrary write, refcount corruption, or controlled object replacement.
* +0 Extra availability: not awarded because the same kernel crash is already counted once.

Repeated-release/refcount checks:

* First release can end lifetime: no such release is shown.
* Exact same reference survives: not applicable.
* Second lifetime/refcount mutation: not present.
* Concrete repeated release/refcount +1: not awarded.
* Paranoid repeated-refcount leverage +1: not awarded.
* FLOOR 5: not applied.

3. Reachability analysis

The vulnerable path is reached by IPv6 packet receive processing for routing headers, specifically the RPL source routing header path. The supplied reproducer uses packet flooding while the receiving interface MTU is repeatedly changed between valid and below-`IPV6_MIN_MTU` values. That MTU transition can cause `addrconf_ifdown()` to clear `dev->ip6_ptr`, creating a window where a packet that passed an earlier `idev` check reaches `ipv6_rpl_srh_rcv()` with `idev == NULL`.

A purely remote attacker is not proven to control the complete trigger sequence, because changing MTU or inducing the relevant device state transition normally requires local network administration privilege. A remote or adjacent attacker may still provide the packet stream that reaches the vulnerable code if the local state transition occurs, so the practical exposure is network-assisted rather than clearly direct remote-only.

Ordinary unprivileged local reachability is not established. Namespace-scoped capability reachability is also not established because the input does not prove that the complete trigger, including interface state changes and RPL packet delivery, can be performed inside attacker-created namespaces. No special hardware is required. The race is repeatable under flood plus MTU-flap conditions, but it still depends on a concurrent state transition.

4. Primitive and severity interpretation

The exact primitive is a NULL pointer dereference of `idev` when reading `idev->cnf.rpl_seg_enabled` in `ipv6_rpl_srh_rcv()`. This is Class C crash/DoS behavior. It is not an active structured UAF and not fixed stale storage. The patch moves the `idev` check into `ipv6_rthdr_rcv()` before dispatching to `ipv6_srh_rcv()` or `ipv6_rpl_srh_rcv()`, so both paths drop the packet when IPv6 is disabled or the inet6 device pointer is unavailable.

Demonstrated consequence is kernel crash or Oops via NULL dereference. Speculative consequences beyond DoS are not supported by the patch. There is no evidence of controlled reclaim, attacker-controlled post-free contents, arbitrary read/write, authorization bypass, or privilege escalation.

ImportantCandidate=NO because the established primitive is crash-only NULL dereference without unresolved memory-corruption leverage. ManualReview=RECOMMENDED because this is network-assisted IPv6 packet handling and a race with device state, so exposure assumptions should be verified for the target environment. AutoDowngradeAllowed=YES because no Class A or meaningful Class B primitive remains unresolved.

Likely operational severity is ordinary Moderate to borderline Moderate, depending on whether untrusted network traffic can reach an affected IPv6 interface during local MTU or device state transitions.

5. One-sentence report phrase

An IPv6 RPL routing header packet can hit a race where the inet6 device pointer is cleared before `ipv6_rpl_srh_rcv()` reads `rpl_seg_enabled`, causing a NULL pointer dereference and kernel DoS, but exploitation beyond crash is not supported by the patch.

6. Manual review recommendation

MANUAL CHECK RECOMMENDED

Reason: network-assisted packet input is involved, but the complete reliable trigger appears to require concurrent local interface state manipulation, and the demonstrated primitive is NULL dereference DoS only.

7. Call-site confidence

Call-site confidence: high

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

Patch: ipv6: rpl: fix NULL dereference of idev in ipv6_rpl_srh_rcv()
Commit: 63f50e9f90d0287ad66a0955ebfc2d3a9c044a1c
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=63f50e9f90d0287ad66a0955ebfc2d3a9c044a1c

Commit description:

ipv6_rpl_srh_rcv() dereferences idev from __in6_dev_get() without a NULL
check when reading idev->cnf.rpl_seg_enabled.

When the device's MTU drops below IPV6_MIN_MTU, addrconf_ifdown() clears
dev->ip6_ptr through RCU_INIT_POINTER(). A packet that passed the idev
check in ip6_rcv_core() can then reach ipv6_rpl_srh_rcv() with
dev->ip6_ptr already NULL.

Reproduced by flooding the receiving interface with ping6 traffic while
flapping its MTU between 1500 and 1200:

 BUG: KASAN: null-ptr-deref in ipv6_rpl_srh_rcv+0xb3/0x1070
 Read of size 4 at addr 00000000000006b4 by task ping6/394

 CPU: 2 UID: 0 PID: 394 Comm: ping6 Not tainted 7.2.0-rc7-micro-vm-dev-00095-g24ef02f934ee #240 PREEMPT(full)
 Call Trace:
  <IRQ>
  kasan_report+0xc6/0x100
  ipv6_rpl_srh_rcv+0xb3/0x1070
  ip6_protocol_deliver_rcu+0x759/0x9a0
  ip6_input_finish+0xa8/0x1b0
  ip6_input+0xe1/0x490
  ipv6_rcv+0x33d/0x460
  __netif_receive_skb_one_core+0xd6/0x130
  process_backlog+0x2cc/0xa00
  __napi_poll.constprop.0+0x56/0x270
  net_rx_action+0x327/0x730
  handle_softirqs+0x11e/0x630
  do_softirq+0xb3/0xf0
  </IRQ>

Both ipv6_rpl_srh_rcv() and ipv6_srh_rcv() are called only from
ipv6_rthdr_rcv(), which already has an idev lookup.

Fix the NULL dereference on the RPL path by checking idev in
ipv6_rthdr_rcv(), before it calls either function. The callees take idev as
an argument and no longer call __in6_dev_get(), so the packet is now
dropped in one place, with SKB_DROP_REASON_IPV6DISABLED on both paths.

Fixes: 8610c7c ("net: ipv6: add support for rpl sr exthdr")
Cc: stable@vger.kernel.org
Signed-off-by: Andrea Mayer <andrea.mayer@uniroma2.it>
Tested-by: Xiang Mei <xmei5@asu.edu>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://patch.msgid.link/20260817132644.2223-1-andrea.mayer@uniroma2.it
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  net/ipv6/exthdrs.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=63f50e9f90d0287ad66a0955ebfc2d3a9c044a1c

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

The original Linux kernel CVE announcement for CVE-2026-89561 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89561

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:H/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS vector: AV:A/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS score: 5.3

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: 1
  Paranoid ActionableScore: 3

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: MODERATE
KPANIC detected for this report: NO
Published priority for this report (same as in Subject): MODERATE REGULAR

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).