[CVE-2026-89700][MODERATE REGULAR] nfsd: validate sockaddr length per family in listener_set

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

CVE: CVE-2026-89700
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: nfsd: validate sockaddr length per family in listener_set
Commit: 2aca70c18c5f5d06004122861fb151d5fcc57faf
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=2aca70c18c5f5d06004122861fb151d5fcc57faf
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89700
Analysis date: Fri, 11 Sep 2026 20:12:51 -0400
ActionableScore: 4
ActionableScore lower bound: 3
Actionable bucket: Borderline / manual review recommended
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-2026-89700 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89700

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 REGULAR
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
======================================================================

CVE-2026-89700	MODERATE	https://www.kernelcve.org/list/?q=CVE-2026-89700 CHECK WITH IMPACT FROM ORIG NN IMPORTANT	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:L/A:H';CWE-125;*CWE-20;CWE-754;Other CVSS 'AV:L/AC:L/PR:L/UI:N/S:U/C:L/I:N/A:L';BEST CVSS score: '6.6';DESCR 'nfsd listener_set accepted a short NFSD_A_SOCK_ADDR netlink attribute and only checked it against the generic sockaddr size. A CAP_NET_ADMIN caller could send a 16 byte address with sa_family set to AF_INET6, while later consumers treated it as sockaddr_in6 and performed a 12 byte OOB read in paths such as rpc_cmp_addr_port, svc_find_listener, and kernel_bind. For the CVSS the PR:L is used because triggering requires a local caller with CAP_NET_ADMIN or an equivalent delegated network administration capability, not full host administrative control. The issue is not directly network reachable from remote NFS clients because it is triggered through the local nfsd generic netlink control plane. Impact is at least a local denial of service or limited confidentiality exposure from an out of bounds kernel read, and the paranoid score also allows limited integrity impact and high availability impact because malformed listener configuration may be processed with side effects.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline / manual review recommended (with actual score 4)	YES	OOB DISK NETWORK LINUS  DECREASED_TO_MODERATE70_BASED_ON_ACTIONABLESCORELOWERTHAN6 DECREASED_TO_MODERATEREG_BASED_ON_FALSEPOSCHECKOFKP	YES	NO	checked

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

ActionableScore=4
ActionableScoreLower=3
PrimitiveClass=CLASS_C
RealUAF=NO
HostAdminRequired=UNKNOWN
NamespaceScopedPrivilege=UNKNOWN
ImportantCandidate=NO
ManualReview=RECOMMENDED
AutoDowngradeAllowed=YES

1. ActionableScore

* Raw conservative score: 3
* Conservative score: 3. No floor or cap changed the score.
* Raw paranoid score: 4
* Paranoid score: 4. No floor or cap changed the score.
* Final recommended bucket: **Borderline / manual review recommended**::

2. Signal breakdown

Conservative positive signals:

* +1. Weak / indirect corruption candidate. The patch establishes a concrete 12 byte OOB read when a short NFSD_A_SOCK_ADDR with sa_family AF_INET6 is later consumed as sockaddr_in6.
* +1. Invalid active control-plane state. The malformed listener entry could be processed by multiple consumers and the old code interleaved validation with listener configuration mutation.
* +1. Limited DoS concern. A kernel OOB read in this path may produce a KASAN report or fault depending on object layout and hardening, although a reliable production panic is not directly shown.

Conservative negative signals:

* 0. No host-admin-equivalent penalty was applied because the input establishes CAP_NET_ADMIN but does not fully resolve whether this is strictly host CAP_NET_ADMIN over host-owned nfsd state or a delegated namespace-scoped capability.
* 0. No race penalty. This is an input validation and control-plane ordering bug, not a timing race.

Paranoid additional signal:

* +1. The OOB bytes are consumed by active listener-management paths such as rpc_cmp_addr_port, svc_find_listener, and kernel_bind, so manual analysis is warranted to confirm whether the read can influence observable listener state or only cause rejection or crash.

Important non-awarded signals:

* Controlled reclaim not demonstrated.
* Arbitrary write not demonstrated.
* Real UAF not demonstrated.
* Concrete repeated release/refcount mutation not present.
* Remote reachability not established because the trigger is local nfsd generic netlink control plane, not remote NFS packet traffic.
* Confidentiality was not awarded because no user-visible disclosure path for the 12 OOB bytes is shown.
* Integrity was not awarded because the caller is already performing listener configuration and no unauthorized protected-state modification is established.
* Extra Availability was not awarded because no distinct persistent DoS mechanism beyond possible OOB-read fault or malformed state handling is established.
* Strong LPE or Class A was not awarded because there is no controlled corruption, authorization bypass, protected-resource access, or demonstrated privilege escalation.

Repeated release/refcount checklist:

* First release can end lifetime: no evidence.
* Exact same reference survives: not applicable.
* Second operation is a lifetime/refcount mutation: no.
* Concrete repeated release/refcount +1 awarded: no.
* Paranoid repeated-refcount leverage +1 awarded: no.
* FLOOR 5 applied: no.

3. Reachability analysis

The trigger is a local generic netlink operation for nfsd listener configuration. The supplied description states that a CAP_NET_ADMIN caller can send a short NFSD_A_SOCK_ADDR with AF_INET6 and cause the OOB read. Ordinary unprivileged reachability is not established.

The exact capability namespace for the complete nfsd listener_set operation is not fully resolved from the supplied patch text. Therefore HostAdminRequired is UNKNOWN and NamespaceScopedPrivilege is UNKNOWN. CAP_NET_ADMIN may be delegated in some environments, but the prompt requires positive evidence before treating the complete trigger as namespace-scoped.

No direct network trigger from remote NFS clients is established. Prior nfsd availability or configuration may be relevant operationally, but the malformed request itself is a local control-plane request. No special hardware or race timing is required.

4. Primitive and severity interpretation

The exact primitive is a bounded 12 byte kernel OOB read caused by validating NFSD_A_SOCK_ADDR only against generic sockaddr size and then consuming it as sockaddr_in6 based on sa_family. This is not a UAF and not a write primitive.

PrimitiveClass is CLASS_C because the established behavior is primarily invalid read and invalid control-plane state handling without an independently useful exploitation primitive. The demonstrated consequence is OOB read and possible malformed listener side effects. A crash is plausible under debugging or unlucky layout, but reliable host-wide DoS is not fully demonstrated.

The practical severity is borderline Moderate. It should not be treated as an Important candidate on the current evidence because there is no disclosure path, no arbitrary or directed write, no authorization bypass, and no privilege escalation chain. Manual review is still recommended because the OOB read is concrete and reaches active listener-management consumers.

ImportantCandidate is NO because the primitive is limited and no meaningful exploitation leverage beyond invalid read / possible DoS is established. AutoDowngradeAllowed is YES because this is not Class A, not UAF, not repeated lifetime mutation, not shared-page ownership violation, and not a demonstrated security-boundary bypass.

5. One-sentence report phrase

A CAP_NET_ADMIN caller can provide a short AF_INET6 nfsd listener sockaddr through generic netlink, causing a 12 byte kernel OOB read in listener handling paths and possible local DoS or malformed listener state.

6. Manual review recommendation

MANUAL CHECK RECOMMENDED

A concrete OOB read is established in an nfsd control-plane path, but current evidence does not show disclosure, write corruption, UAF, LPE, or direct network reachability.

7. Call-site confidence

Call-site confidence: high

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

Patch: nfsd: validate sockaddr length per family in listener_set
Commit: 2aca70c18c5f5d06004122861fb151d5fcc57faf
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=2aca70c18c5f5d06004122861fb151d5fcc57faf

Commit description:

nfsd_sock_nl_policy declares NFSD_A_SOCK_ADDR as a bare NLA_BINARY
attribute with no minimum length. A CAP_NET_ADMIN caller can send a
16-byte NFSD_A_SOCK_ADDR with sa_family=AF_INET6, causing a 12-byte
OOB read across three consumers (rpc_cmp_addr_port, svc_find_listener,
kernel_bind).

nfsd_nl_listener_set_doit() also parsed and validated each listener
entry inline in two separate loops, interleaved with mutating the
running listener configuration. The validation was duplicated, used an
open-coded "nla_len < sizeof(struct sockaddr)" check that was too short
for AF_INET6, and handled a malformed entry inconsistently depending on
which loop noticed it.

Add an nfsd_nl_validate_listeners() helper that walks the entire list
once and confirms each entry parses, carries both an address and a
transport name, and is long enough for its address family
(sizeof(struct sockaddr_in) for AF_INET, sizeof(struct sockaddr_in6)
for AF_INET6, -EAFNOSUPPORT otherwise). Call it before taking
nfsd_mutex or creating the serv, so a malformed request fails cleanly
with no side effects.

Since every entry is known valid by the time the two existing loops
run, drop the redundant presence and per-family length checks from
both, leaving only the nla_parse_nested() call needed to extract the
data.

Fixes: 16a4711 ("NFSD: add listener-{set,get} netlink command")
Cc: stable@vger.kernel.org
Assisted-by: Claude:claude-opus-4-8
Signed-off-by: Jeff Layton <jlayton@kernel.org>
Link: https://patch.msgid.link/20260615-nfsd-testing-v5-1-188d75aedda0@kernel.org
Signed-off-by: Chuck Lever <cel@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  fs/nfsd/nfsctl.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=2aca70c18c5f5d06004122861fb151d5fcc57faf

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

The original Linux kernel CVE announcement for CVE-2026-89700 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89700

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:L/I:N/A:L
  The Best / paranoid CVSS vector: AV:L/AC:L/PR:L/UI:N/S:U/C:L/I:L/A:H
  The Best / paranoid CVSS score: 6.6

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: 4

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 REGULAR

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).