[CVE-2026-99117][MODERATE 7.0] net: bridge: mcast: don't truncate the port group walk on teardown

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

CVE: CVE-2026-99117
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: net: bridge: mcast: don't truncate the port group walk on teardown
Commit: 8243d587a081e790ad96e80d48eef44d9f7ef005
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=8243d587a081e790ad96e80d48eef44d9f7ef005
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-99117
Analysis date: Wed, 28 Oct 2026 23:32:20 -0400
ActionableScore: 5
ActionableScore lower bound: 4
Actionable bucket: Actionable Moderate / Strong Important candidate requiring manual review
Manual review required: YES

Summary:
Namespaced bridge multicast teardown can leave an active MDB group pointing to a freed port, which a later MDB netlink dump dereferences and can crash the host kernel.

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ABOUT THIS REPORT
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The original Linux kernel CVE announcement for CVE-2026-99117 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-99117

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: 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-2026-99117	MODERATE	https://www.kernelcve.org/list/?q=CVE-2026-99117 CHECK WITH IMPACT FROM ORIG NN MODERATE | AEGIS IMPACT MODERATE_KPANIC AEGIS_AS=4 LEGACY=MODERATE_KPANIC RESULT=SAME (final reconciliation agrees; DECISION: both MODERATE_KPANIC)	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:L/PR:L/UI:N/S:U/C:L/I:N/A:H';*CWE-416;*CWE-664;*CWE-672;Other CVSS 'AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '6.1';DESCR 'A bridge multicast teardown walk can terminate early when deleting one port group also removes the node saved as the next iterator entry. Remaining groups can retain a dangling key.port pointer after the bridge port is freed, and a later MDB netlink dump reaches a slab use after free in __mdb_fill_info. The demonstrated primitive is a UAF dereference with a reliable kernel crash. Limited disclosure from reused stale object contents is plausible but not established, and there is no supported arbitrary write or privilege escalation primitive. For the CVSS the PR:L value reflects the need for local bridge and multicast configuration control such as CAP_NET_ADMIN in a network namespace. Incoming multicast traffic alone does not establish remote reachability because reliable exploitation also requires the affected port teardown sequence. Automated impact decision path. The original SimpleNet classifier selected MODERATE. The primary LLM estimated paranoid CVSS 6.1 and the production ActionableScore returned 5 with recommendation Actionable Moderate / Strong Important candidate requiring manual review. The production rules then evaluated CVSS thresholds and ActionableScore thresholds. Recorded rule transitions were CVSS and initial triage stage MODERATE to IMPORTANT because the CVSS score, vector or bug class threshold rule was applied. KP and FD specialist checks stage IMPORTANT to MODERATE_KPANIC because KP specialist did not establish a qualifying panic. The legacy KP specialist produced a review artifact and its decision was processed by the applicable KP rules. The legacy FD or DH downgrade specialist has no review artifact for this run. Exact triggered rule markers were INCREASED_TO_HIGH_BASED_ON_GUESSCVSS and DECREASED_TO_MODERATE7_BASED_ON_FALSEPOSCHECKOFKP. The independent weaker Aegis shadow evaluated CVSS 3.3 and ActionableScore 4 with primitive class CLASS_B and manual review REQUIRED. Its independently reconciled impact was MODERATE_KPANIC. The shadow agreed with the final production impact and aggregation did not change that impact. The final reported impact is MODERATE_KPANIC.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate / Strong Important candidate requiring manual review (with actual score 5)	MAYBE	INIT LEAK UAF LINUS  KPANIC INCREASED_TO_HIGH_BASED_ON_GUESSCVSS DECREASED_TO_MODERATE7_BASED_ON_FALSEPOSCHECKOFKP	-	-	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=5
ActionableScoreLower=4
PrimitiveClass=CLASS_B
RealUAF=YES
HostAdminRequired=NO
NamespaceScopedPrivilege=YES
ImportantCandidate=YES
ManualReview=REQUIRED
AutoDowngradeAllowed=NO
RequiresHostUID0=NO
AuthorizationDecisionBypass=NO
ProtectedResourceImpact=NONE
B4PendingReview=NO
B4PendingConfirmed=NO

1. ActionableScore

* Raw base score: 3.
* Conservative score: 4. Applied Real-UAF floor 4: this is active structured bridge state, not fixed stale storage.
* Raw paranoid score: 4.
* Paranoid score: 5. Applied floor 5 because the below-host-admin path reaches a later structured consumer that reads the dangling port through an MDB dump; non-crash leverage remains unresolved.
* Final recommended bucket: **Actionable Moderate / Strong Important candidate requiring manual review**::

2. Signal breakdown

* +2 Real UAF: truncated teardown leaves `pg->key.port` dangling after `del_nbp()` frees the port.
* +1 Reliable kernel crash: KASAN demonstrates the stale dereference in `__mdb_fill_info()`.
* +1 Attacker-influenced lifetime sequence: an actor controlling bridge multicast configuration can create affected groups, tear down the port, and request the later MDB dump.
* -1 Reduced/namespaced privilege: supplied evidence grounds the sequence in network-namespace `CAP_NET_ADMIN`, below host-administrator-equivalent power.
* Paranoid +1 structured stale-object leverage: `__mdb_fill_info()` consumes fields through the freed port while constructing netlink dump output, rather than merely detecting stale state.
* No arbitrary write, controlled reclaim, callback control, LPE, or confirmed disclosure is shown. The crash receives no second availability point.

3. Reachability analysis

The grounded actor has bridge and multicast management rights in an attacker-controlled network namespace, including port teardown and MDB netlink dumping. This supports below-host-admin, namespace-scoped reachability, but not remote packet-only reachability. No special hardware, boot option, prior host provisioning, or hard race is shown; the defect follows deterministic iterator truncation during teardown.

4. Primitive and severity reading

This is an active structured-object UAF: a surviving MDB port group retains a pointer to a freed bridge port, and a later kernel dump operation dereferences it. It is broader than a cleanup-only stale fault because corrupted active MDB state escapes teardown and is subsequently consumed.

The proven result is a host-kernel UAF and crash. Disclosure through dump serialization or useful reclaimed-object reuse is plausible but unconfirmed, so this remains CLASS_B rather than CLASS_A. The concrete namespace-to-host-kernel active-state chain triggers V33 review preservation: ImportantCandidate is YES and automatic downgrade is unsafe despite no proven non-crash exploit.

There is no authorization decision bypass; B4-PENDING does not apply.

5. One-sentence report phrase

Namespaced bridge multicast teardown can leave an active MDB group pointing to a freed port, which a later MDB netlink dump dereferences and can crash the host kernel.

6. Manual review advice

MANUAL CHECK REQUIRED

Review whether freed `net_bridge_port` fields can be reclaimed or serialized before the demonstrated fault, and whether the complete namespace trigger is enabled in the target product.

7. Call-site confidence

Call-site confidence: high

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

Patch: net: bridge: mcast: don't truncate the port group walk on teardown
Commit: 8243d587a081e790ad96e80d48eef44d9f7ef005
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=8243d587a081e790ad96e80d48eef44d9f7ef005

Commit description:

__br_multicast_disable_port_ctx() and br_multicast_del_port() walk
port->mglist with hlist_for_each_entry_safe(). However,
br_multicast_find_del_pg() can also delete other entries from the same
list through br_multicast_fwd_src_remove() or __fwd_del_star_excl().

If such an entry is the iterator's saved next node, hlist_del_init()
clears its ->next and terminates the walk early. The reproducer triggers
this in both teardown walks, leaving port groups in the bridge mdb with
a dangling ->key.port after del_nbp() frees the port:

  BUG: KASAN: slab-use-after-free in __mdb_fill_info+0x1191/0x1320
   __mdb_fill_info+0x1191/0x1320
   br_mdb_dump+0x594/0xe40
   rtnl_mdb_dump+0x1cf/0x5d0

Use hlist_del_init_rcu() to unlink the group while preserving ->next.
br_multicast_del_pg() and the teardown walks run under
br->multicast_lock. The GC worker must acquire the same lock before
detaching the group for destruction, so the node remains alive while
the walk uses the preserved pointer.

Preserving ->next means a walk can now reach a group that an earlier
iteration already deleted as a side effect. That group is off mp->ports,
so br_multicast_find_del_pg() would fall through its port scan and hit
the trailing WARN_ON(1). Skip such groups at the top of that helper: a
port group is put on port->mglist when it is created and only unlinked
when it is deleted, so hlist_unhashed() identifies exactly this case.

Fixes: b081236 ("net: bridge: mcast: install S,G entries automatically based on reports")
Cc: stable@vger.kernel.org
Suggested-by: Nikolay Aleksandrov <razor@blackwall.org>
Reported-by: TencentOS Corvus AI <corvus@tencent.com>
Signed-off-by: Jun Yang <junvyyang@tencent.com>
Reviewed-by: Nikolay Aleksandrov <razor@blackwall.org>
Link: https://patch.msgid.link/20260831111330.199543-1-junvyyang@tencent.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  net/bridge/br_multicast.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=8243d587a081e790ad96e80d48eef44d9f7ef005

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

The original Linux kernel CVE announcement for CVE-2026-99117 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-99117

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:L/PR:L/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS vector: AV:L/AC:L/PR:L/UI:N/S:U/C:L/I:N/A:H
  The Best / paranoid CVSS score: 6.1

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: 4
  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: YES
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).