[CVE-2026-80838][MODERATE 7.0] vxlan: keep the last remote linked during FDB flush

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

CVE: CVE-2026-80838
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: vxlan: keep the last remote linked during FDB flush
Commit: 2a7c2f00843225d5f037676bca649321f3d024c7
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=2a7c2f00843225d5f037676bca649321f3d024c7
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-80838
Analysis date: Fri, 04 Sep 2026 14:53:59 -0400
ActionableScore: 5
ActionableScore lower bound: 4
Actionable bucket: Actionable Moderate / Strong Important candidate requiring manual review
Manual review required: YES

Summary:
No one-sentence report phrase was found.

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

The original Linux kernel CVE announcement for CVE-2026-80838 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-80838

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-2026-80838	MODERATE	https://www.kernelcve.org/list/?q=CVE-2026-80838 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-362;*CWE-416;*CWE-787;Other CVSS 'AV:L/AC:H/PR:H/UI:N/S:U/C:L/I:L/A:H';BEST CVSS score: '7';DESCR 'A race in the VXLAN FDB flush path can temporarily leave a reachable non-nexthop FDB entry with an empty remotes list. An RCU reader can still find the parent FDB entry during this window and first_remote_rcu can derive an invalid vxlan_rdst pointer from the empty list head. The receive learning path may then read from and write through that invalid remote pointer which makes this a memory corruption candidate rather than a simple resource leak. For the CVSS the PR:L is used for the paranoid case because delegated CAP_NET_ADMIN or container-root network administration can be enough to perform FDB flush operations in some deployments. The issue is not directly network-triggerable by packets alone but VXLAN receive processing can participate in the race once the local control plane operation is triggered. Impact is at least denial of service and in the worst defensible case may include confidentiality and integrity impact due to invalid pointer read/write.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate / Strong Important candidate requiring manual review (with actual score 5)  SKIP CVE-2026-80838 UNKNOWN SKIP The Fixes patch not applied yet, so unlikely that actual: url YES  NO NO unknown 	MAYBE	READ INIT SIMPLEFIX NETWORK  KPANIC INCREASED_TO_MODERATE7_BASED_ON_GUESSCVSS	-	-	checked

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

ActionableScore=5
ActionableScoreLower=4
PrimitiveClass=CLASS_B
RealUAF=NO
HostAdminRequired=UNKNOWN
NamespaceScopedPrivilege=UNKNOWN
ImportantCandidate=YES
ManualReview=REQUIRED
AutoDowngradeAllowed=NO

1. ActionableScore

* Raw conservative score: 4
* Conservative score: 4. No floor or cap applied.
* Raw paranoid score: 5
* Paranoid score: 5. No floor or cap applied.
* Final recommended bucket: **Actionable Moderate / Strong Important candidate requiring manual review**::

2. Signal breakdown

Conservative signals:

* +2. Concrete Class B kernel corruption candidate. The patch establishes that an RCU reader can find a still reachable VXLAN FDB entry whose remotes list has become empty, and first_remote_rcu() can derive an invalid vxlan_rdst pointer.
* +1. Reliable kernel crash or strong DoS is plausible. The invalid remote pointer can be read from and written to in the receive learning path, which can realistically fault or corrupt kernel state.
* +1. Packet-processing path participation. The dangerous stale or invalid state is consumed from the VXLAN receive learning path, even though packet traffic alone is not shown to trigger the full bug.

Paranoid additional signal:

* +1. Weak exploitation leverage concern. The receive learning path may write through a bogus structured vxlan_rdst pointer, leaving unresolved memory-corruption leverage beyond an ordinary NULL dereference or passive invalid read.

Non-awarded or withheld signals:

* +2 direct remote reachability not awarded. A local control-plane FDB flush appears necessary.
* Local unprivileged trigger not awarded. Below-host-admin reachability is not positively established.
* Host-admin penalty not applied. Host-admin-equivalent privilege is not positively established from the supplied evidence.
* NamespaceScopedPrivilege=YES not awarded. The complete required FDB flush and VXLAN setup chain is not proven namespace-scoped.
* RealUAF=YES not awarded. The supplied text establishes an invalid remote pointer after the last remote is unlinked while the parent remains reachable, but it does not directly prove dereference of the freed vxlan_rdst object itself.
* Controlled reclaim not demonstrated.
* Arbitrary or directed kernel write not demonstrated.
* Strong LPE leverage not awarded.
* Confidentiality and integrity consequences are not scored as established. They remain unresolved paranoid concerns because the patch-supported primitive is invalid structured pointer read/write, not a demonstrated disclosure or controlled overwrite.
* Extra Availability not awarded because the same crash or corruption symptom is already counted.
* Concrete repeated release/refcount signal not awarded. There is no demonstrated first put/free followed by a second lifetime-mutating operation on the exact same dangling reference.
* Race penalty not applied. Although the bug is an RCU race window, the evidence does not prove a one-shot or practically unretryable race, and the control-plane operation plus receive activity may be repeatable.

3. Reachability analysis

The likely trigger requires a filtered VXLAN FDB bulk flush that matches all remotes of a non-nexthop FDB entry, plus an RCU-side receive learning path that observes the parent FDB entry before it is removed from lookup structures. This is not directly reachable by network packets alone, because a local or delegated control-plane operation appears necessary to perform the flush.

The relevant operation likely involves network administration rights such as CAP_NET_ADMIN, but the supplied evidence does not establish whether this must be host or init namespace CAP_NET_ADMIN, or whether an attacker-controlled network namespace can perform the complete sequence. Therefore HostAdminRequired is UNKNOWN and NamespaceScopedPrivilege is UNKNOWN.

Prior VXLAN configuration is likely required, but the supplied patch does not prove whether the attacker can instantiate the VXLAN device, populate the FDB entry, and perform the filtered flush below host-admin privilege. No special hardware is required. The race is real, but not scored as a hard timing penalty because the operation appears retryable in principle.

4. Primitive and severity interpretation

The exact primitive is an RCU-visible invariant violation in VXLAN FDB handling. A non-nexthop FDB entry remains reachable through the FDB hash table while its remotes list can temporarily become empty. first_remote_rcu() then applies list_entry_rcu() to the empty list head and constructs an invalid vxlan_rdst pointer.

This is an active structured-object corruption candidate, not fixed stale storage. The invalid pointer is consumed by the receive learning path, and the commit explicitly states that this path can read from and write to it. That makes the issue stronger than a simple crash-only invalid dereference, but weaker than a demonstrated controlled UAF reclaim, arbitrary write, or proven privilege escalation.

No concrete repeated release/refcount operation exists. No first lifetime operation followed by a second lifetime mutation on the same dangling reference is established.

PrimitiveClass is CLASS_B because there is a concrete invalid structured pointer with read/write behavior in kernel networking code, but no strong Class A primitive such as controlled reclaim, directed write, callback control, or demonstrated LPE. ImportantCandidate is YES because unresolved write-through-invalid-pointer behavior in a packet-processing path should not be automatically downgraded. AutoDowngradeAllowed is NO because the established primitive is a meaningful kernel memory-corruption candidate requiring manual review.

Likely operational severity is Actionable Moderate, with Strong Important candidacy depending on whether further review confirms attacker-controlled corruption, namespace-scoped reachability, or useful post-corruption leverage.

5. One-sentence report phrase

A VXLAN FDB flush race can leave a reachable FDB entry with an empty remotes list, allowing the receive learning path to derive and read or write through an invalid vxlan_rdst pointer, making this an actionable kernel memory corruption candidate requiring manual review.

6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: the patch establishes invalid structured pointer read/write in a network receive path, with unresolved exploitation leverage and unresolved namespace or delegated CAP_NET_ADMIN reachability.

7. Call-site confidence

Call-site confidence: medium

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

Patch: vxlan: keep the last remote linked during FDB flush
Commit: 2a7c2f00843225d5f037676bca649321f3d024c7
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=2a7c2f00843225d5f037676bca649321f3d024c7

Commit description:

A non-nexthop FDB entry is expected to have at least one remote while it
remains reachable through the FDB hash table. A filtered bulk flush
violates this invariant when every remote matches: It unlinks the last
remote in vxlan_fdb_dst_destroy() and only afterwards tells vxlan_flush()
to destroy the parent FDB entry.

An RCU reader can find the parent during this interval.
first_remote_rcu() then applies list_entry_rcu() to the empty list head,
producing an invalid remote pointer that the receive learning path can
read from and write to.

When a matching remote is the sole remaining remote, leave it linked and
ask the caller to destroy the entire FDB entry. vxlan_fdb_destroy() keeps
the remote attached while sending the deletion notification and removing
the parent from the lookup structures.

Fixes: c499fcc ("vxlan: vxlan_core: Support FDB flushing by destination VNI")
Cc: stable@vger.kernel.org
Signed-off-by: Kyle Zeng <kylebot@openai.com>
Co-developed-by: David Lee <david.lee@trailofbits.com>
Signed-off-by: David Lee <david.lee@trailofbits.com>
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Link: https://patch.msgid.link/20260810144115.821654-1-david.lee@trailofbits.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  drivers/net/vxlan/vxlan_core.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=2a7c2f00843225d5f037676bca649321f3d024c7

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

The original Linux kernel CVE announcement for CVE-2026-80838 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-80838

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: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: 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).