[CVE-2026-72255][IMPORTANT] netfilter: nf_queue: pin bridge device while NFQUEUE holds fake dst

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

CVE: CVE-2026-72255
Priority: IMPORTANT
AL-KERNEL base severity: IMPORTANT
KPANIC flag: YES
Patch: netfilter: nf_queue: pin bridge device while NFQUEUE holds fake dst
Commit: 430521af7fe8a9c08f5a2554224a35f11f51d99e
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=430521af7fe8a9c08f5a2554224a35f11f51d99e
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-72255
Analysis date: Sat, 15 Aug 2026 08:04:25 -0400
ActionableScore: 7
ActionableScore lower bound: 5
Actionable bucket: Strong Important candidate / Actionable Moderate at minimum
Manual review required: YES

Summary:
NFQUEUE can retain a bridge fake dst whose backing storage is embedded in struct net_bridge after bridge teardown starts, creating a UAF-style lifetime bug that can lead to kernel crash and may have higher impact under worst-case object reuse conditions.

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ABOUT THIS REPORT
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The original Linux kernel CVE announcement for CVE-2026-72255 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-72255

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-2026-72255	IMPORTANT	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:H/I:H/A:H';*CWE-416;CWE-362;*CWE-664;*CWE-672;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:H';BEST CVSS score: '7';DESCR 'A use after free candidate exists in the netfilter NFQUEUE bridge path because a bridged skb can carry a fake dst backed by storage embedded in struct net_bridge. When such a packet is queued to NFQUEUE, skb_dst_force can turn the fake dst into a real referenced dst while the queued packet remains alive beyond bridge teardown. Later dst release or queued packet cleanup may touch bridge private fake rtable or metrics storage after the bridge device has started unregistering. For the CVSS the PR:L is used for the paranoid score because a local user with delegated network administration in a namespace or a reduced capability service may be able to create the bridge NFQUEUE setup and control queue lifetime. The issue is not a simple remote packet only trigger, but bridged network traffic can participate once NFQUEUE and br_netfilter are configured. Impact is at least local denial of service via kernel crash and in worst case may allow confidentiality or integrity impact due to UAF style memory corruption. YES REQUIRES MANUAL CHECK.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate / Actionable Moderate at minimum (with actual score 7)	YES	REMOTE NETFILTER UAF SKB HARDWARE TEST IPV6 PACKET  KPANIC INCREASED_TO_MODERATE7_BASED_ON_GUESSCVSS KPANIC INCREASED_TO_HIGH_BASED_ON_ACTIONABLESCOREHIGHEREQTHAN6	NO	NO	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=7
ActionableScoreLower=5

## 1. ActionableScore

* Conservative score: 5
* Paranoid score: 7
* Final recommended bucket: **Strong Important candidate / Actionable Moderate at minimum**::

## 2. Signal breakdown

Conservative signals:

* Local unprivileged or reduced privilege trigger: +1. In normal host setup bridge and NFQUEUE configuration need CAP_NET_ADMIN, but user namespaces or delegated network administration may make this reachable below full host root.
* Memory corruption, weak or indirect corruption candidate: +1. The patch describes a stale lifetime relation around fake dst storage embedded in struct net_bridge.
* Real lifetime corruption: +1. NFQUEUE can keep a referenced fake dst alive after bridge teardown begins.
* Reliable kernel crash / strong DoS: +1. The commit explicitly says fake rtable and metrics storage can be freed under dst_release().
* Common networking / packet-processing path: +1. This is netfilter NFQUEUE plus br_netfilter packet handling.
* Capability requirement penalty: not fully applied. CAP_NET_ADMIN is often required, but delegated namespaces reduce practical privilege assumptions.

Paranoid additional interpretation:

* Generic memory corruption / UAF candidate: +2 instead of +1. The fake rtable backing storage can be freed while a queued skb still holds a dst reference, making this more than bookkeeping.
* Confidentiality impact plausible: +1. Possible only under a worst-case UAF reuse interpretation.
* Integrity impact plausible: +1. Possible only under a worst-case UAF reuse interpretation.
* Coordination / lifetime timing constraint: -1. Triggering requires NFQUEUE retention plus bridge teardown timing and is not a simple remote packet-only trigger.

## 3. Reachability analysis

The issue is not directly remotely exploitable by arbitrary packets alone. A practical trigger needs a bridge, br_netfilter, an NFQUEUE rule, queued bridged packets, and bridge device teardown while the queue entry still exists. In typical deployments this setup requires CAP_NET_ADMIN, but containers, user namespaces, delegated network management services, or reduced-capability root contexts can make PR:L more realistic than full PR:H. The affected path is in netfilter and bridge packet handling, so it deserves scrutiny even though the exact runtime configuration is not default everywhere.

Call-site confidence: high, because the patch shows nf_queue_entry lifetime handling, dev_hold/dev_put placement, and nfnetlink_queue device reaping behavior.

## 4. Severity interpretation

This is not an ordinary Moderate resource leak or validation-only fix. It is a network stack lifetime bug where a queued skb can retain a dst reference whose backing bridge-private storage may be freed during netdevice teardown. Realistic impact is likely kernel crash or host DoS. Theoretical memory corruption impact is plausible because the stale object is tied to freed bridge-private storage, but there is no demonstrated attacker-controlled reclaim, object replacement, callback control, or arbitrary write primitive in the patch. Therefore the conservative result is Actionable Moderate, while the paranoid result is a Strong Important candidate requiring manual review.

## 5. One-sentence report phrase

NFQUEUE can retain a bridge fake dst whose backing storage is embedded in struct net_bridge after bridge teardown starts, creating a UAF-style lifetime bug that can lead to kernel crash and may have higher impact under worst-case object reuse conditions.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

This should not be auto-closed because the patch fixes a real lifetime issue involving dst references, netdevice teardown, and bridge-private storage in a common packet-processing subsystem.

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

Patch: netfilter: nf_queue: pin bridge device while NFQUEUE holds fake dst
Commit: 430521af7fe8a9c08f5a2554224a35f11f51d99e
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=430521af7fe8a9c08f5a2554224a35f11f51d99e

Commit description:

The br_netfilter fake rtable is embedded in struct net_bridge and is
attached to bridged packets with skb_dst_set_noref(). If such a packet is
queued to NFQUEUE, __nf_queue() upgrades that fake dst with
skb_dst_force().

At that point the queued skb can hold a real dst reference after bridge
teardown has started. The problem is not that every bridged packet needs
its own dst reference. The problem is that NFQUEUE can keep the bridge
private fake dst alive after unregister begins.

Fix this by keeping the bridge fake dst model unchanged and pinning the
bridge master device only while the packet sits in NFQUEUE. Record the
bridge device in nf_queue_entry when the queued skb carries a bridge fake
dst, take a device reference for the queue lifetime, and drop it when the
queue entry is freed.

Also make sure queued entries are reaped when that bridge device goes
down, and drop the redundant nf_bridge_info_exists() test from the fake
dst detection.

This keeps netdev_priv(br->dev) alive until verdict completion, so the
embedded fake rtable and its metrics backing storage cannot be freed out
from under dst_release(). It also avoids the constant refcount bump and
avoids using ipv4-specific dst helpers for IPv6 bridge traffic.

Fixes: 34666d4 ("netfilter: bridge: move br_netfilter out of the core")
Cc: stable@kernel.org
Reported-by: Yuan Tan <yuantan098@gmail.com>
Reported-by: Yifan Wu <yifanwucs@gmail.com>
Reported-by: Juefei Pu <tomapufckgml@gmail.com>
Reported-by: Xin Liu <bird@lzu.edu.cn>
Signed-off-by: Haoze Xie <royenheart@gmail.com>
Signed-off-by: Ren Wei <n05ec@lzu.edu.cn>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  include/net/netfilter/nf_queue.h
  net/netfilter/nf_queue.c
  net/netfilter/nfnetlink_queue.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=430521af7fe8a9c08f5a2554224a35f11f51d99e

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

The original Linux kernel CVE announcement for CVE-2026-72255 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-72255

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:L/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: 5
  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).