[CVE-2026-80842][IMPORTANT] net: bridge: mcast: fix use-after-free of a master VLAN's multicast context

[CVE-2026-80842][IMPORTANT] net: bridge: mcast: fix use-after-free of a master VLAN's multicast context

[AL-KERNEL]

CVE: CVE-2026-80842
Priority: IMPORTANT
AL-KERNEL base severity: IMPORTANT
KPANIC flag: YES
Patch: net: bridge: mcast: fix use-after-free of a master VLAN's multicast context
Commit: 3afaaee2f972aec9059110953adb62fa3cf5c4bd
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=3afaaee2f972aec9059110953adb62fa3cf5c4bd
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-80842
Analysis date: Fri, 04 Sep 2026 15:22:04 -0400
ActionableScore: 8
ActionableScore lower bound: 5
Actionable bucket: Strong Important candidate / Actionable Moderate at minimum
Manual review required: YES

Summary:
A race in Linux bridge multicast master VLAN teardown can leave BR_VLFLAG_MCAST_ENABLED set while the multicast context is deinitialized and freed, allowing the bridge transmit path to re arm timers and trigger a slab use after free write that can crash the kernel and may warrant privilege escalation review.

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

The original Linux kernel CVE announcement for CVE-2026-80842 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-80842

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.

======================================================================
AL-KERNEL CLASSIFICATION RESULT
======================================================================

CVE-2026-80842	IMPORTANT	https://www.kernelcve.org/list/?q=CVE-2026-80842 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-667;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 can occur in the Linux bridge multicast VLAN teardown path because a master VLAN may keep BR_VLFLAG_MCAST_ENABLED set while its multicast context is being deinitialized and later freed through RCU. A reader in the bridge transmit path can still treat the context as enabled and re arm multicast timers after br_multicast_ctx_deinit has stopped them, which leads to a slab use after free write in timer handling. For the CVSS the PR:L value is used for the paranoid score because reliable triggering usually needs local control over bridge VLAN or multicast configuration, but this control may be available to a less than fully privileged actor through CAP_NET_ADMIN in a network namespace or a delegated network management service. The issue is not directly network reachable as a pure remote packet processing bug because the vulnerable window depends on local control plane VLAN teardown, although IGMP traffic through the bridge device is part of the trigger path. Impact is at least local denial of service via kernel crash and in worst case may allow confidentiality and integrity impact due to kernel use after free memory corruption.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate / Actionable Moderate at minimum (with actual score 7)	YES	DANGER UAF HARDWARE LINUS TEST  KPANIC INCREASED_TO_HIGH_BASED_ON_GUESSCVSS	NO	NO	checked

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

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:

* Generic memory corruption, strong corruption primitive: +2. The patch and KASAN trace show a real slab-use-after-free with a write of size 8 from timer rearm code.
* Real lifetime corruption: +1. The bug is caused by an RCU lifetime teardown mismatch where a multicast context can be re-armed after deinit and later free.
* Reliable kernel crash / strong DoS: +1. The reported path reaches KASAN slab-use-after-free and can crash the kernel.
* Weak LPE concern: +1. The primitive is a UAF write in timer handling, but no controlled reclaim or stable overwrite target is demonstrated.
* Hard or unreliable race / special timing required: -1. Triggering depends on a race between master VLAN teardown and bridge transmit multicast processing.
* Requires CAP_NET_ADMIN in typical deployments: -1. Bridge VLAN control normally needs network administration privileges, but user namespaces or delegated network management may reduce practical privilege requirements.
* Broad networking subsystem exposure: +1. Linux bridge and VLAN multicast are common on virtualization, container, and network hosts.

Paranoid additional interpretation:

* Local trigger through delegated CAP_NET_ADMIN or user namespace setup: +1. In some deployments, an untrusted local user may obtain CAP_NET_ADMIN inside a network namespace and manipulate bridge/VLAN state.
* Privilege escalation plausible: +2. The UAF involves timer rearming after context teardown, which is a higher-risk lifetime pattern involving active kernel objects, even though a practical LPE primitive is not demonstrated.
* Confidentiality impact plausible: +1. Kernel UAF memory corruption may theoretically expose or influence kernel memory state.
* Integrity impact plausible: +1. The write-after-free may theoretically corrupt reclaimed kernel objects.

## 3. Reachability analysis

The bug is locally reachable through bridge VLAN multicast configuration and traffic paths. Reliable triggering appears to require control over bridge or VLAN teardown, normally requiring CAP_NET_ADMIN. However, this may be reachable by less-than-host-root actors in environments with unprivileged user namespaces, privileged containers, container root with CAP_NET_ADMIN, or delegated network management services.

This is not a pure remote network-triggerable bug. IGMP traffic through the bridge device participates in the trigger path, but the vulnerable window depends on local control-plane VLAN removal or multicast state teardown.

The affected subsystem is not always configured on ordinary desktops, but Linux bridge with VLANs is common on virtualization hosts, container platforms, and network appliances.

## 4. Severity interpretation

This should not be treated as an ordinary Moderate or auto-closed DoS-only issue. The realistic demonstrated impact is local kernel crash via slab-use-after-free, but the primitive is stronger than a NULL dereference because it includes a write-after-free in timer handling after RCU teardown.

Theoretical privilege escalation is plausible but not proven. There is no demonstrated attacker-controlled reclaim, arbitrary write, or public exploit primitive in the supplied material. Conservative handling is Actionable Moderate. Paranoid handling is Strong Important candidate because the bug combines race, RCU lifetime corruption, timer rearm, and a UAF write.

## 5. One-sentence report phrase

A race in Linux bridge multicast master VLAN teardown can leave BR_VLFLAG_MCAST_ENABLED set while the multicast context is deinitialized and freed, allowing the bridge transmit path to re arm timers and trigger a slab use after free write that can crash the kernel and may warrant privilege escalation review.

## 6. Manual review recommendation

YES REQUIRES MANUAL CHECK
MANUAL CHECK REQUIRED

Reason: explicit slab-use-after-free write, RCU lifetime race, and timer rearm after teardown make this more than a simple crash-only bug, even though practical LPE is not demonstrated.

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

Patch: net: bridge: mcast: fix use-after-free of a master VLAN's multicast context
Commit: 3afaaee2f972aec9059110953adb62fa3cf5c4bd
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=3afaaee2f972aec9059110953adb62fa3cf5c4bd

Commit description:

br_multicast_toggle_one_vlan() clears BR_VLFLAG_MCAST_ENABLED under
br->multicast_lock before stopping a VLAN's multicast context.  That is
the teardown handshake: lockless readers gate on the flag through
br_multicast_ctx_should_use() -> br_multicast_ctx_vlan_disabled(), so
once it is cleared under the lock no reader can arm the context again.

For a master VLAN the handshake never runs.  __vlan_del() clears
BRIDGE_VLAN_INFO_BRENTRY before calling br_vlan_put_master(), so
br_multicast_toggle_one_vlan(masterv, false) returns early on
!br_vlan_is_brentry(vlan): the flag stays set and br->multicast_lock is
never taken.  br_vlan_put_master() then drains the context in
br_multicast_ctx_deinit() and frees the VLAN through call_rcu(), while a
reader still inside rcu_read_lock() sees the context as enabled and
re-arms it.  The port and port-VLAN branch of the function has no
br_vlan_is_brentry() test and flips the flag under br->multicast_lock,
so it is not affected.

The reader is the bridge transmit path.  For a master VLAN
br_multicast_rcv() selects brmctx = &vlan->br_mcast_ctx with
pmctx = NULL, so IGMP sent to the bridge device re-arms the context's
timers after br_multicast_ctx_deinit() has already stopped them.

  BUG: KASAN: slab-use-after-free in detach_if_pending+0x412/0x4a0
  Write of size 8 at addr ffff88810ac39918 by task brmc/601
   __mod_timer+0x51a/0xc50
   br_multicast_host_join+0x25b/0x390
   __br_multicast_add_group+0x468/0x530
   br_ip4_multicast_add_group+0x1a0/0x260
   br_multicast_rcv+0x2cda/0x61e0
   br_dev_xmit+0x6c4/0x1540
  Allocated by task 610:
   br_vlan_add+0x111/0xb40
   br_vlan_info+0x370/0x3e0
  Freed by task 0:
   kfree+0x1a7/0x4f0
   rcu_core+0x7dc/0x10a0

Only test br_vlan_is_brentry() when enabling, like the
br_multicast_ctx_vlan_global_disabled() test next to it.  Disabling then
always clears BR_VLFLAG_MCAST_ENABLED under br->multicast_lock before
br_multicast_ctx_deinit() drains the context.

Fixes: 7b54aaa ("net: bridge: multicast: add vlan state initialization and control")
Cc: stable@vger.kernel.org
Signed-off-by: Norbert Szetei <norbert@doyensec.com>
Acked-by: Nikolay Aleksandrov <razor@blackwall.org>
Link: https://patch.msgid.link/D400F6C7-543A-4B79-9E5B-D1D8974DE5C9@doyensec.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=3afaaee2f972aec9059110953adb62fa3cf5c4bd

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

The original Linux kernel CVE announcement for CVE-2026-80842 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-80842

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: 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).