[CVE-2026-89702][MODERATE 7.0] nfsd: size fh_verify server sockaddr slot by xpt_locallen

[CVE-2026-89702][MODERATE 7.0] nfsd: size fh_verify server sockaddr slot by xpt_locallen

[AL-KERNEL]

CVE: CVE-2026-89702
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: nfsd: size fh_verify server sockaddr slot by xpt_locallen
Commit: 95d064f9828a20ccca5ae90a17be3e2076f25272
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=95d064f9828a20ccca5ae90a17be3e2076f25272
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89702
Analysis date: Fri, 11 Sep 2026 23:51:15 -0400
ActionableScore: 6
ActionableScore lower bound: 5
Actionable bucket: Actionable Moderate / Strong Important candidate requiring manual review
Manual review required: YES

Summary:
No one-sentence report phrase was found.

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

The original Linux kernel CVE announcement for CVE-2026-89702 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89702

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.

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

CVE-2026-89702	MODERATE	https://www.kernelcve.org/list/?q=CVE-2026-89702 CHECK WITH IMPACT FROM ORIG NN LOW	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:H';*CWE-787;CWE-131;*CWE-200;Other CVSS 'AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:L/A:H';BEST CVSS score: '8.1';DESCR 'A server sockaddr slot size mismatch in the NFSD fh_verify tracepoints can make __assign_sockaddr copy xpt_local using xpt_locallen into space reserved with xpt_remotelen. On NFS/UDP, xpt_remotelen can remain 0 while xpt_locallen is 16 or 28 bytes, so each affected fh_verify trace event can write past the reserved ring-buffer slot. The reverse size mismatch can also expose stale ring-buffer bytes to trace consumers. For the CVSS the PR:N is used in the paranoid vector because a remote NFS client can trigger fh_verify over NFS/UDP once the service and tracepoint are reachable, although enabling the tracepoint itself is an administrative configuration. The issue is network reachable only on systems running NFSD over UDP with the affected tracepoints enabled, and NFS services are usually deployed on trusted internal networks rather than the public Internet. Impact is at least denial of service from kernel trace buffer corruption and in worst case may allow confidentiality or integrity impact due to a concrete OOB write primitive.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate / Strong Important candidate requiring manual review (with actual score 6)  SKIP CVE-2026-89702 UNKNOWN SKIP No affected files built, so skip this CVE NO  - - unknown 	MAYBE	OOB DISK TRACE LEAK NETWORK KPANIC  KPANIC INCREASED_TO_HIGH_BASED_ON_GUESSCVSS DECREASED_TO_MODERATE7_BASED_ON_FALSEPOSCHECKOFDH	-	-	checked

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

ActionableScore=6
ActionableScoreLower=5
PrimitiveClass=CLASS_B
RealUAF=NO
HostAdminRequired=UNKNOWN
NamespaceScopedPrivilege=NO
ImportantCandidate=YES
ManualReview=REQUIRED
AutoDowngradeAllowed=NO

1. ActionableScore

* Raw conservative score: 5
* Conservative score: 5
* Raw paranoid score: 6
* Paranoid score: 6
* Final recommended bucket: **Actionable Moderate / Strong Important candidate requiring manual review**::

No floor or cap changed the score.

2. Signal breakdown

Conservative positive signals:

* +1 Constrained kernel buffer overwrite. The tracepoint reserves the server sockaddr field using xpt_remotelen but copies xpt_local using xpt_locallen, so memcpy can write past the reserved trace ring-buffer slot.
* +1 Memory layout invariant restoration. The patch directly restores consistency between declared dynamic trace field size and copied sockaddr length.
* +2 Remote reachable in the affected configuration. A remote NFSv2/v3 over UDP request can reach fh_verify and trigger the affected trace event when NFSD over UDP is running and the tracepoint is enabled.
* +1 Reliable kernel corruption / DoS concern. The write-past-end affects kernel trace ring-buffer storage and can plausibly corrupt tracing memory, supporting at least availability impact.

Conservative total: 5

Paranoid positive signals:

* +2 Concrete OOB write with unresolved exploitation leverage. The paranoid interpretation treats the write past the reserved trace ring-buffer slot as an active kernel memory corruption primitive, although payload and target control are constrained.
* +1 Memory layout invariant restoration. Allocation size and copy size are mismatched in an active trace event record.
* +2 Remote reachable in the affected configuration. Remote NFS/UDP traffic can trigger the vulnerable fh_verify tracepoint path once the vulnerable tracing configuration is active.
* +1 Reliable kernel corruption / DoS concern. Kernel trace ring-buffer corruption can lead to denial of service or unstable kernel behavior.

Paranoid total: 6

Important non-awarded signals:

* RealUAF not awarded. No stale post-lifetime object use is established.
* Concrete repeated release/refcount not awarded. No first release, surviving exact reference, and second lifetime mutation are present.
* Strong CLASS_A not awarded. The overwrite is concrete, but the copied payload is the server local sockaddr and target placement is constrained to trace event storage.
* Controlled reclaim not demonstrated.
* Arbitrary write not demonstrated.
* Privilege escalation not demonstrated.
* Confidentiality point not awarded as an independent score. The reverse mismatch can leak stale ring-buffer contents to trace consumers, but the supplied evidence does not establish that an unprivileged attacker can read trace output.
* Extra availability not awarded because the same trace-buffer corruption / crash concern is already counted once.
* Race penalty not applied. This is a deterministic size mismatch, not a timing race.
* Host-admin penalty not applied. Host-admin-equivalent triggering by the attacker is not positively established, but tracepoint enablement is a privileged prior configuration.

3. Reachability analysis

The vulnerable operation is in NFSD tracepoints nfsd_fh_verify and nfsd_fh_verify_err. The supplied context states that the write-past-end case is reachable on NFS/UDP because UDP setup calls svc_xprt_set_local but does not call svc_xprt_set_remote, leaving xpt_remotelen at 0 while xpt_locallen is normally 16 or 28 bytes.

The packet-level trigger can be remote: an NFSv2/v3 over UDP client can cause fh_verify activity. However, the tracepoint assignment path is relevant only when the affected tracepoints are enabled. Enabling kernel tracepoints is normally an administrative action, so the complete deployment prerequisite is privileged configuration, while the per-request trigger can be remote and unauthenticated from the NFS client side.

HostAdminRequired is UNKNOWN rather than YES because the attacker does not necessarily need host-admin privilege if the service and tracepoint are already enabled by monitoring or debugging policy. It is also not NO because the supplied evidence does not prove that an ordinary attacker can enable the tracepoint or control the full configuration.

NamespaceScopedPrivilege is NO because no namespace-scoped capability path is relevant to the supplied trigger. No special hardware is required. No race or narrow timing condition is required.

4. Primitive and severity interpretation

The exact primitive is a trace event dynamic field size mismatch that causes __assign_sockaddr to copy xpt_locallen bytes into a slot reserved using xpt_remotelen. On NFS/UDP this can be a 16 or 28 byte copy into a zero-byte server sockaddr slot.

This is not a UAF and not a repeated-release/refcount lifetime bug. It is a concrete OOB write into kernel trace ring-buffer event storage, with a reverse-direction stale ring-buffer disclosure condition to trace consumers.

The primitive is CLASS_B because real kernel memory corruption is established, but the exploitation leverage is constrained and unresolved. The copied bytes are not an arbitrary attacker-controlled payload, and the supplied evidence does not show controlled target selection, controlled reclaim, arbitrary read/write, callback overwrite, or demonstrated privilege escalation.

Demonstrated consequence is kernel trace-buffer corruption and possible trace data leak to trace consumers. Practical operational severity is at least Actionable Moderate because this is a concrete memory layout violation reachable from an NFS/UDP request in affected tracing configurations. It is an ImportantCandidate because a network-triggerable OOB write in kernel tracing memory should not be auto-downgraded without manual review.

AutoDowngradeAllowed is NO because automatic closure would discard a concrete kernel OOB write primitive with unresolved exploitation leverage. The issue is not proven Strong Important, but it is also not safe to treat as ordinary crash-only behavior.

5. One-sentence report phrase

NFSD fh_verify tracepoints can reserve the server sockaddr trace field with xpt_remotelen but copy xpt_local using xpt_locallen, allowing remote NFS/UDP activity to corrupt trace ring-buffer storage when the affected tracepoints are enabled.

6. Manual review recommendation

MANUAL CHECK REQUIRED

A concrete OOB write into kernel trace ring-buffer storage is established, and the path can be triggered by remote NFS/UDP requests in affected tracing configurations, but the exact exploitation leverage beyond DoS requires manual assessment.

7. Call-site confidence

Call-site confidence: high

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

Patch: nfsd: size fh_verify server sockaddr slot by xpt_locallen
Commit: 95d064f9828a20ccca5ae90a17be3e2076f25272
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=95d064f9828a20ccca5ae90a17be3e2076f25272

Commit description:

The nfsd_fh_verify and nfsd_fh_verify_err tracepoints declare the
server sockaddr slot sized by xpt_remotelen but fill it from
xpt_local using xpt_locallen:

    TP_STRUCT__entry(
            ...
            __sockaddr(server, rqstp->rq_xprt->xpt_remotelen)
            ...
    )
    TP_fast_assign(
            ...
            __assign_sockaddr(server, &rqstp->rq_xprt->xpt_local,
                              rqstp->rq_xprt->xpt_locallen);
            ...
    )

When xpt_locallen exceeds xpt_remotelen, __assign_sockaddr's memcpy
writes past the reserved ring-buffer slot. In the reverse direction
(xpt_locallen < xpt_remotelen) the slot is oversized and the
unwritten tail leaks prior ring-buffer contents to trace consumers.

The write-past-end case is reachable on NFS/UDP. svc_xprt_set_remote()
is only called from svc_tcp_accept() (net/sunrpc/svcsock.c) and from
the RDMA connect path; svc_create_socket() for UDP calls only
svc_xprt_set_local(), so xpt_remotelen stays 0 for the xprt's
lifetime. Every fh_verify trace for an NFSv2/v3-over-UDP request
then copies 16 or 28 bytes from xpt_local into a zero-byte slot.

The other NFSD tracepoints that record the server address
(NFSD_TRACE_PROC_CALL_FIELDS, NFSD_TRACE_PROC_RES_FIELDS,
SVC_RQST_ENDPOINT_FIELDS) already size the server slot by
xpt_locallen; nfsd_fh_verify and nfsd_fh_verify_err were the only
exceptions.

Fix by sizing the server slot with xpt_locallen so the declared slot
matches the copy length. The client slot and its assignment already
agree on xpt_remotelen and are left untouched.

Fixes: 0513828 ("NFSD: Instrument fh_verify()")
Fixes: 948755e ("NFSD: Replace dprintk() call site in fh_verify()")
Cc: stable@vger.kernel.org
Assisted-by: kres:claude-opus-4-7
Signed-off-by: Chris Mason <clm@meta.com>
Link: https://patch.msgid.link/20260531-nfsd-testing-v1-1-7bfa481b0540@kernel.org
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  net/sunrpc/svcsock.c
  fs/nfsd/trace.h

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=95d064f9828a20ccca5ae90a17be3e2076f25272

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

The original Linux kernel CVE announcement for CVE-2026-89702 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89702

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:N/AC:H/PR:N/UI:N/S:U/C:L/I:L/A:H
  The Best / paranoid CVSS vector: AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:H
  The Best / paranoid CVSS score: 8.1

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: 5
  Paranoid ActionableScore: 6

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).