[CVE-2026-46099][IMPORTANT] net: ipv6: fix NOREF dst use in seg6 and rpl lwtunnels

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

CVE: CVE-2026-46099
Priority: IMPORTANT
AL-KERNEL base severity: IMPORTANT
KPANIC flag: YES
Patch: net: ipv6: fix NOREF dst use in seg6 and rpl lwtunnels
Commit: 6bd17925bd6866027a6555db17905b9fc073d38d
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=6bd17925bd6866027a6555db17905b9fc073d38d
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-46099
Analysis date: Wed, 27 May 2026 10:29:40 -0400
ActionableScore: 8
ActionableScore lower bound: 5
Actionable bucket: Strong Important candidate / Actionable Moderate at minimum
Manual review required: YES

Summary:
A network-influenced UAF can occur in IPv6 seg6/rpl lwtunnel input on PREEMPT_RT when a NOREF dst from `ip6_route_input()` is cached after concurrent `pcpu_rt` release, causing kernel crash and requiring review for possible broader memory-corruption impact.

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ABOUT THIS REPORT
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The original Linux kernel CVE announcement for CVE-2026-46099 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-46099

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
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CVE-2026-46099	IMPORTANT	CHECK	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:H';*CWE-416;CWE-362;*CWE-664;Other CVSS 'AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '8.1';DESCR 'A use-after-free can occur in the IPv6 seg6 and rpl lwtunnel input paths because ip6_route_input() installs a NOREF dst on the skb and the code later caches it with dst_cache_set_ip6(), which calls dst_hold(). On PREEMPT_RT, ksoftirqd can be preempted after the NOREF route lookup while another task releases the same pcpu route through a shared nexthop and FIB generation change, leaving the skb pointing to a dead dst. For the CVSS the PR:N is used because once the affected seg6 or rpl route is configured, packet input can be influenced remotely without local credentials. AC:H is used because the race requires PREEMPT_RT, shared nexthop state, a stale pcpu route release, and precise timing. Impact is at least remote or adjacent network denial of service via kernel crash. Because this is a real UAF in an IPv6 packet processing path, worst-case confidentiality and integrity impact should be reviewed manually even though reliable exploitation is not demonstrated.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate / Actionable Moderate at minimum (with actual score 7)	YES	REMOTE RACE UAF NETWORK SKB HARDWARE IPV6 KPANIC 	NO	NO	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=8
ActionableScoreLower=5

## 1. ActionableScore

* Conservative score: 5
* Paranoid score: 8
* Final recommended bucket: **Strong Important candidate / Actionable Moderate at minimum**::

## 2. Signal breakdown

Conservative signals:

* Remote reachable / network-triggerable: +2
  Once affected seg6 or rpl lwtunnel routing is configured, packet input can reach `seg6_input_core()` or `rpl_input()` without local credentials.

* Generic memory corruption, strong corruption primitive: +2
  The commit explicitly describes a use-after-free on a dead `dst` / `pcpu_rt`.

* Real lifetime corruption: +1
  The bug is a refcount/lifetime mismatch involving a NOREF dst later used after concurrent release.

* Reliable kernel crash / strong DoS: +1
  The described result is WARN / use-after-free and likely kernel crash.

* Availability impact realistic: +1
  A successful trigger can affect kernel networking and system availability.

Subtracting signals:

* Hard race / special timing required: -1
  Requires a PREEMPT_RT scheduling window between route lookup and dst caching.

* Configuration-dependent path: -1
  Requires PREEMPT_RT conditions, seg6 or rpl lwtunnel use, shared nexthop state, and concurrent FIB activity.

Paranoid additions:

* Privilege escalation plausible: +2
  This is a real UAF in a packet-processing path. No reliable exploit primitive is demonstrated, but UAF reclaim or object lifetime manipulation is sufficiently plausible for manual-review sensitivity.

* Confidentiality impact plausible: +1
  UAF may expose stale object state or misinterpret freed memory in worst-case analysis.

* Integrity impact plausible: +1
  UAF may allow corruption or misuse of reclaimed kernel objects in worst-case analysis.

## 3. Reachability analysis

A remote or adjacent network attacker can plausibly influence packet input into the affected seg6 or rpl lwtunnel paths if the target has such routes configured. The attacker does not need local credentials for packet delivery, but the vulnerable configuration itself is not universal.

The race requires PREEMPT_RT behavior where `ksoftirqd` is preemptible, a shared nexthop allowing another task to release the same `pcpu_rt`, and a timing window around `ip6_route_input()` and `dst_cache_set_ip6()`. Containers and namespaces mainly matter if they can configure routing or lwtunnel state, but packet triggering itself can be external once the route exists.

## 4. Severity interpretation

This is at least Actionable Moderate because it combines network influence with a real UAF in IPv6 packet processing. It is stronger than a simple warning, NULL dereference, or resource leak.

Realistic exploitation is constrained by PREEMPT_RT, seg6/rpl configuration, shared nexthop state, and race timing, so the conservative score stays below clear Critical territory. The paranoid interpretation is a Strong Important candidate because network-influenced UAF bugs in routing/lifetime code have historically been under-triaged if treated as DoS-only.

## 5. One-sentence report phrase

A network-influenced UAF can occur in IPv6 seg6/rpl lwtunnel input on PREEMPT_RT when a NOREF dst from `ip6_route_input()` is cached after concurrent `pcpu_rt` release, causing kernel crash and requiring review for possible broader memory-corruption impact.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: this is a real UAF in an IPv6 packet-processing path with possible remote packet influence, even though practical exploitation requires a narrow PREEMPT_RT and routing configuration race.

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

Patch: net: ipv6: fix NOREF dst use in seg6 and rpl lwtunnels
Commit: 6bd17925bd6866027a6555db17905b9fc073d38d
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=6bd17925bd6866027a6555db17905b9fc073d38d

Commit description:

seg6_input_core() and rpl_input() call ip6_route_input() which sets a
NOREF dst on the skb, then pass it to dst_cache_set_ip6() invoking
dst_hold() unconditionally.
On PREEMPT_RT, ksoftirqd is preemptible and a higher-priority task can
release the underlying pcpu_rt between the lookup and the caching
through a concurrent FIB lookup on a shared nexthop.
Simplified race sequence:

  ksoftirqd/X                       higher-prio task (same CPU X)
  -----------                       --------------------------------
  seg6_input_core(,skb)/rpl_input(skb)
    dst_cache_get()
      -> miss
    ip6_route_input(skb)
      -> ip6_pol_route(,skb,flags)
         [RT6_LOOKUP_F_DST_NOREF in flags]
        -> FIB lookup resolves fib6_nh
           [nhid=N route]
        -> rt6_make_pcpu_route()
           [creates pcpu_rt, refcount=1]
             pcpu_rt->sernum = fib6_sernum
             [fib6_sernum=W]
           -> cmpxchg(fib6_nh.rt6i_pcpu,
                      NULL, pcpu_rt)
              [slot was empty, store succeeds]
      -> skb_dst_set_noref(skb, dst)
         [dst is pcpu_rt, refcount still 1]

                                    rt_genid_bump_ipv6()
                                      -> bumps fib6_sernum
                                         [fib6_sernum from W to Z]
                                    ip6_route_output()
                                      -> ip6_pol_route()
                                        -> FIB lookup resolves fib6_nh
                                           [nhid=N]
                                        -> rt6_get_pcpu_route()
                                             pcpu_rt->sernum != fib6_sernum
                                             [W <> Z, stale]
                                          -> prev = xchg(rt6i_pcpu, NULL)
                                          -> dst_release(prev)
                                             [prev is pcpu_rt,
                                              refcount 1->0, dead]

    dst = skb_dst(skb)
    [dst is the dead pcpu_rt]
    dst_cache_set_ip6(dst)
      -> dst_hold() on dead dst
      -> WARN / use-after-free

For the race to occur, ksoftirqd must be preemptible (PREEMPT_RT without
PREEMPT_RT_NEEDS_BH_LOCK) and a concurrent task must be able to release
the pcpu_rt. Shared nexthop objects provide such a path, as two routes
pointing to the same nhid share the same fib6_nh and its rt6i_pcpu
entry.

Fix seg6_input_core() and rpl_input() by calling skb_dst_force() after
ip6_route_input() to force the NOREF dst into a refcounted one before
caching.
The output path is not affected as ip6_route_output() already returns a
refcounted dst.

Fixes: af4a220 ("ipv6: sr: use dst_cache in seg6_input")
Fixes: a7a29f9 ("net: ipv6: add rpl sr tunnel")
Cc: stable@vger.kernel.org
Signed-off-by: Andrea Mayer <andrea.mayer@uniroma2.it>
Reviewed-by: Simon Horman <horms@kernel.org>
Reviewed-by: Justin Iurman <justin.iurman@gmail.com>
Link: https://patch.msgid.link/20260421094735.20997-1-andrea.mayer@uniroma2.it
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  net/ipv6/rpl_iptunnel.c
  net/ipv6/seg6_iptunnel.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=6bd17925bd6866027a6555db17905b9fc073d38d

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

The original Linux kernel CVE announcement for CVE-2026-46099 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-46099

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:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS vector: AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:H
  The Best / paranoid CVSS score: 8.1

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: 8

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).