[CVE-2026-64423][MODERATE 7.0] ipv4: igmp: remove multicast group from hash table on device destruction

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

CVE: CVE-2026-64423
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: ipv4: igmp: remove multicast group from hash table on device destruction
Commit: 412ba7def06ffe974ba9a1d862b022362c54ffa5
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=412ba7def06ffe974ba9a1d862b022362c54ffa5
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64423
Analysis date: Sat, 25 Jul 2026 06:23:32 -0400
ActionableScore: 5
ActionableScore lower bound: 3
Actionable bucket: Actionable Moderate at minimum
Manual review required: NO

Summary:
An RCU race in IPv4 IGMP device teardown can leave a multicast membership reachable from `mc_hash` after it is scheduled for freeing, allowing concurrent packet receive processing to dereference freed memory and crash the kernel.

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

The original Linux kernel CVE announcement for CVE-2026-64423 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64423

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 7.0
Manual review required: NO

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-2026-64423	MODERATE	CHECK WITH IMPACT FROM ORIG NN MODERATE	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:H';*CWE-416;CWE-362;*CWE-672;Other CVSS 'AV:L/AC:H/PR:H/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '5.8';DESCR 'An RCU race in IPv4 IGMP device teardown can leave a multicast membership object reachable from mc_hash after its reference count is dropped and it is scheduled for RCU freeing. Concurrent packet receive processing can still find that object through ip_check_mc_rcu and dereference freed memory, causing a kernel use after free. For the CVSS the PR:L is used in the paranoid score because some deployments allow an untrusted local user to obtain delegated CAP_NET_ADMIN in a network namespace and create or destroy local network devices. The issue is not directly Internet reachable by packet traffic alone because reliable triggering requires local control of device teardown, although incoming or locally generated packets can exercise the reader side of the race. Impact is at least local denial of service via kernel crash and in worst case may allow limited confidentiality or integrity impact due to UAF read of reused memory, so manual review is recommended.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate at minimum (with actual score 4)	YES	REMOTE READ DANGER UAF NETWORK HARDWARE  DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN5	NO	NO	checked

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

ActionableScore=5
ActionableScoreLower=3

## 1. ActionableScore

* Conservative score: 3
* Paranoid score: 5
* Final recommended bucket: **Actionable Moderate at minimum**

## 2. Signal breakdown

Conservative signals:

* Memory corruption, weak primitive: +1. The patch fixes a KASAN confirmed RCU use after free read in `ip_check_mc_rcu()`, but the shown primitive is stale read/deref, not write-after-free or controlled reuse.
* Real lifetime corruption: +1. The object remains reachable from `mc_hash` after refcount drop and RCU reclamation scheduling.
* Reliable kernel crash / strong DoS: +1. KASAN shows slab-use-after-free in the IPv4 receive path.
* Common networking core or packet-processing path: +1. The reader side is reached through IPv4 RX processing, `udp_v4_early_demux()` and `ip_check_mc_rcu()`.
* Hard or unreliable race / special timing required: -1. Reliable triggering requires overlap between device teardown and RCU packet receive traversal.
* Requires admin/root/CAP_NET_ADMIN in typical deployments: -2. Destroying network devices normally requires administrative network privileges.

Paranoid additional interpretation:

* Local unprivileged trigger: +1. In deployments with unprivileged user namespaces or delegated network namespaces, a local user may obtain effective `CAP_NET_ADMIN` in a namespace and exercise device creation/destruction paths.
* Broad/default/common subsystem: +1. IPv4 multicast and IGMP support are common kernel networking components, even though the teardown trigger is privileged or namespace-dependent.

No LPE bonus is awarded. The patch does not show attacker-controlled reclaim, stale object replacement, write-after-free, callback control, type confusion, or refcount takeover.

## 3. Reachability analysis

The bug is not directly Internet reachable by packets alone. Packet traffic exercises the reader side of the race, but reliable triggering also requires local control over network device destruction while multicast membership objects exist.

In typical host deployments, the trigger requires root or `CAP_NET_ADMIN`, so conservative reachability is privileged local. In containerized or namespace-enabled environments, the effective privilege requirement may be lower if untrusted users can create network namespaces and destroy devices inside them. That makes PR:L plausible for paranoid triage, but not enough to treat it as remotely exploitable.

The affected code is in a common IPv4 networking path, but the vulnerable window is a teardown race around `ip_mc_destroy_dev()` and RCU readers of `mc_hash`.

## 4. Severity interpretation

This is stronger than an ordinary Moderate cleanup because it is a confirmed RCU lifetime bug with KASAN use-after-free in a packet receive path. Realistic impact is mainly local denial of service through kernel crash or instability.

Theoretical memory-corruption concern exists because stale RCU readers can dereference freed slab memory, but the provided patch and trace show a read-side UAF only. There is no demonstrated reclaim control, write primitive, object replacement, function pointer dispatch, or privilege escalation path. Therefore it fits **Actionable Moderate**, not a strong Important candidate.

## 5. One-sentence report phrase

An RCU race in IPv4 IGMP device teardown can leave a multicast membership reachable from `mc_hash` after it is scheduled for freeing, allowing concurrent packet receive processing to dereference freed memory and crash the kernel.

## 6. Manual review recommendation

MANUAL CHECK RECOMMENDED

Manual review is recommended because this is a confirmed networking RCU use-after-free in an RX path, but the current evidence supports DoS and weak memory-corruption concern rather than a demonstrated privilege-escalation primitive.

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

Patch: ipv4: igmp: remove multicast group from hash table on device destruction
Commit: 412ba7def06ffe974ba9a1d862b022362c54ffa5
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=412ba7def06ffe974ba9a1d862b022362c54ffa5

Commit description:

When a device is destroyed under RTNL, ip_mc_destroy_dev() iterates through
the multicast list and calls ip_ma_put() on each membership, scheduling
them for RCU reclamation. However, they are not unlinked from the device's
multicast hash table (mc_hash).

Since the device remains published in dev->ip_ptr until after
ip_mc_destroy_dev() completes, concurrent RCU readers traversing mc_hash
can still locate and access the multicast group after its refcount is
decremented. If the RCU callback runs and frees the group while a reader is
accessing it, a use-after-free occurs.

Fix this by unlinking the multicast group from mc_hash using
ip_mc_hash_remove() before scheduling it for reclamation.

BUG: KASAN: slab-use-after-free in ip_check_mc_rcu+0x149/0x3f0
Read of size 4 at addr ffff888009bf1408 by task mausezahn/2276

Call Trace:
 <IRQ>
 dump_stack_lvl+0x67/0x90
 print_report+0x175/0x7c0
 kasan_report+0x147/0x180
 ip_check_mc_rcu+0x149/0x3f0
 udp_v4_early_demux+0x36d/0x12d0
 ip_rcv_finish_core+0xb8b/0x1390
 ip_rcv_finish+0x54/0x120
 NF_HOOK+0x213/0x2b0
 __netif_receive_skb+0x126/0x340
 process_backlog+0x4f2/0xf00
 __napi_poll+0x92/0x2c0
 net_rx_action+0x583/0xc60
 handle_softirqs+0x236/0x7f0
 do_softirq+0x57/0x80
 </IRQ>

Allocated by task 2239:
 kasan_save_track+0x3e/0x80
 __kasan_kmalloc+0x72/0x90
 ____ip_mc_inc_group+0x31a/0xa40
 __ip_mc_join_group+0x334/0x3f0
 do_ip_setsockopt+0x16fa/0x2010
 ip_setsockopt+0x3f/0x90
 do_sock_setsockopt+0x1ad/0x300

Freed by task 0:
 kasan_save_track+0x3e/0x80
 kasan_save_free_info+0x40/0x50
 __kasan_slab_free+0x3a/0x60
 __rcu_free_sheaf_prepare+0xd4/0x220
 rcu_free_sheaf+0x36/0x190
 rcu_core+0x8d9/0x12f0
 handle_softirqs+0x236/0x7f0

Fixes: e989707 ("igmp: hash a hash table to speedup ip_check_mc_rcu()")
Cc: stable@vger.kernel.org
Signed-off-by: Yuyang Huang <yuyanghuang@google.com>
Reviewed-by: Kuniyuki Iwashima <kuniyu@google.com>
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Link: https://patch.msgid.link/20260701235014.73505-1-yuyanghuang@google.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  net/ipv4/igmp.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=412ba7def06ffe974ba9a1d862b022362c54ffa5

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

The original Linux kernel CVE announcement for CVE-2026-64423 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64423

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:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:H
  The Best / paranoid CVSS score: 5.8

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: 3
  Paranoid ActionableScore: 5

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 7.0

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).