[CVE-2026-93212][LOW] nfsd: guard nfsd_serv deref in nfsd_file_net_dispose

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

CVE: CVE-2026-93212
Priority: LOW
AL-KERNEL base severity: LOW
KPANIC flag: NO
Patch: nfsd: guard nfsd_serv deref in nfsd_file_net_dispose
Commit: f4776c1c4b38fbc459420321c8ece87d0f7f95fb
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=f4776c1c4b38fbc459420321c8ece87d0f7f95fb
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-93212
Analysis date: Thu, 24 Sep 2026 11:56:28 -0400
ActionableScore: 1
ActionableScore lower bound: unknown
Actionable bucket: Ordinary Moderate
Manual review required: NO

Summary:
No one-sentence report phrase was found.

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

The original Linux kernel CVE announcement for CVE-2026-93212 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-93212

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: LOW
Manual review required: NO

A detailed explanation of the methodology and priority rules is included
at the end of this message.

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

CVE-2026-93212	LOW	https://www.kernelcve.org/list/?q=CVE-2026-93212 CHECK WITH IMPACT FROM ORIG NN LOW	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H';**CWE-476;CWE-362;CWE-703;Other CVSS 'AV:L/AC:H/PR:H/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '4.7';DESCR 'A race during nfsd per net teardown can cause nfsd_file_net_dispose to call svc_wake_up through a NULL nfsd_serv pointer while filecache disposal work is still running. This can trigger a kernel NULL pointer dereference and result in a local denial of service through an Oops or kernel panic. Remote NFS requests may contribute filecache activity but do not by themselves perform the required network namespace teardown, so the issue is not directly network reachable. For the CVSS the PR:L value is used for the paranoid score because delegated network administration or a reduced capability service or container role may be able to control a suitable namespace. Impact is limited to availability because the patch context shows a guarded NULL dereference and does not indicate use after free memory corruption information disclosure or privilege escalation. NO MANUAL CHECK REQUIRED COULD BE AUTOCLOSED.';NO MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate (with actual score 1)	YES	DISK NETWORK  DECREASED_TO_MODERATE_REGULAR_FROM_MODERATE7_BASED_ON_GUESSCVSS_AND_CIANNH DECREASED_TO_LOW_FROM_MODERATE_BASED_ON_ACTIONABLESCORELESSTHAN3	YES	NO	checked

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

ActionableScore=1
PrimitiveClass=CLASS_C
RealUAF=NO
HostAdminRequired=UNKNOWN
NamespaceScopedPrivilege=UNKNOWN
ImportantCandidate=NO
ManualReview=NO
AutoDowngradeAllowed=YES

1. ActionableScore

* Raw conservative score: 1
* Conservative score: 1
* Raw paranoid score: 1
* Paranoid score: 1
* Final recommended bucket: **Ordinary Moderate**::

No floor or cap was applied.

2. Signal breakdown

* +1 Reliable kernel crash / strong DoS. The patch and commit explicitly establish that `nn->nfsd_serv` can be NULL while `nfsd_file_net_dispose()` calls `svc_wake_up()`. Dereferencing a NULL `svc_serv` can produce a kernel Oops or panic.
* +0 Remote reachability. Remote NFS traffic can create filecache activity, but it does not itself initiate the required per network namespace teardown or clear `nn->nfsd_serv`.
* +0 Local unprivileged trigger. The supplied patch does not establish that an ordinary unprivileged local user can create the complete condition involving an active nfsd service and its per network teardown.
* +0 Host-admin penalty. Host-admin-equivalent privilege is plausible for nfsd setup and teardown, but is not positively established by the supplied call path. Therefore no unsupported negative penalty is applied.
* +0 Hard race penalty. This is an ordering and concurrency issue during teardown, but the supplied evidence does not establish an exceptionally narrow or externally uncontrollable timing window.
* +0 Real UAF. Not awarded. The comment explicitly states that the filecache laundrette is shut down after `nn->nfsd_serv` is cleared but before the `svc_serv` object is freed. The demonstrated defect is a NULL pointer dereference, not a dereference after the `svc_serv` lifetime ends.
* +0 Controlled reclaim, arbitrary write, structured stale-object leverage, confidentiality, integrity, and privilege escalation. None is established.
* +0 Extra Availability impact. Not awarded because the kernel crash or Oops is already represented by the single DoS point.

3. Reachability analysis

The immediate trigger requires a running nfsd service, pending filecache disposal entries, and teardown ordering in which `nn->nfsd_serv` has been cleared while the laundrette or service disposal path still runs. The vulnerable path is local control-plane and teardown behavior, not normal NFS packet processing.

A remote NFS client could potentially contribute workload that leaves entries on `l->freeme`, but remote traffic alone does not clear `nn->nfsd_serv`, stop the service, or destroy the relevant network namespace. Therefore direct network reachability is not established.

The supplied evidence does not resolve whether the complete setup, nfsd operation, and teardown sequence can be performed by a below-host-admin container or delegated network-management actor. `HostAdminRequired=UNKNOWN` and `NamespaceScopedPrivilege=UNKNOWN` are therefore used. The presence of per-network nfsd state alone does not prove that all required nfsd lifecycle operations are namespace-scoped and attacker-reachable.

No special hardware, boot option, or sysctl prerequisite is identified. The bug is not a UAF and does not involve repeated release, refcount mutation, object replacement, or post-free object reuse.

4. Primitive and severity interpretation

The exact primitive is an invalid NULL pointer dereference in `svc_wake_up()` after `nn->nfsd_serv` is cleared during per-network teardown. The patched code caches the pointer and skips the wakeup when it is NULL.

This is not an active structured-object UAF. The `svc_serv` object is explicitly described as not yet freed when the pointer is cleared. There is no stale post-lifetime reference, no second `put` or release, no refcount mutation through freed storage, and no controlled replacement or reuse mechanism.

The demonstrated consequence is a local kernel Oops or panic and consequent denial of service. Speculative consequences such as information disclosure, integrity loss, arbitrary write, or privilege escalation are not supported by the patch context.

The vulnerability is `CLASS_C` because it is a crash-capable invalid-state dereference without an independently useful memory-corruption, lifetime, ownership, or authorization primitive. `ImportantCandidate=NO` because the patch establishes only a guarded NULL dereference in a teardown path. `AutoDowngradeAllowed=YES` because no unresolved meaningful exploitation leverage remains beyond the crash condition.

Likely operational severity is Low-like or ordinary Moderate at most, depending on whether the affected system exposes nfsd lifecycle control to less privileged actors.

5. One-sentence report phrase

A race during nfsd per network teardown can cause `nfsd_file_net_dispose()` to pass a NULL `nfsd_serv` pointer to `svc_wake_up()`, resulting in a local kernel crash or denial of service.

6. Manual review recommendation

NO MANUAL CHECK NEEDED

The patch establishes a NULL pointer dereference during teardown and does not show a UAF, repeated lifetime mutation, controlled corruption, remote trigger, security-boundary bypass, or privilege-escalation primitive.

7. Call-site confidence

Call-site confidence: medium

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

Patch: nfsd: guard nfsd_serv deref in nfsd_file_net_dispose
Commit: f4776c1c4b38fbc459420321c8ece87d0f7f95fb
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=f4776c1c4b38fbc459420321c8ece87d0f7f95fb

Commit description:

nfsd_file_net_dispose() is the consumer side of l->freeme: the nfsd
service thread loop calls it to drain entries that the filecache
garbage collector and shrinker append via
nfsd_file_dispose_list_delayed().  During per-net teardown,
nn->nfsd_serv is cleared before the filecache laundrette is shut
down, so the service thread can still run a dispose pass that finds
more than eight entries on l->freeme and dereferences a NULL
svc_serv:

    nfsd service thread loop
      nfsd_file_net_dispose(nn)
        if (!list_empty(&l->freeme)) {
            ...
            svc_wake_up(nn->nfsd_serv);   /* nn->nfsd_serv == NULL */
        }

The sibling helper nfsd_file_dispose_list_delayed() already documents
this ordering and caches nn->nfsd_serv into a local before testing it
for NULL.  nfsd_file_net_dispose() was introduced with the same raw
svc_wake_up(nn->nfsd_serv) call and never picked up the guard.

Fix by loading nn->nfsd_serv into a local svc_serv pointer and only
calling svc_wake_up() when it is non-NULL, matching the pattern in
nfsd_file_dispose_list_delayed().

Fixes: ffb4025 ("nfsd: Don't leave work of closing files to a work queue")
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/20260602-nfsd-testing-v2-4-e4ea62e3cd5c@kernel.org
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  fs/nfsd/filecache.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=f4776c1c4b38fbc459420321c8ece87d0f7f95fb

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

The original Linux kernel CVE announcement for CVE-2026-93212 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-93212

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:N/I:N/A:H
  The Best / paranoid CVSS vector: AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS score: 4.7

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: unknown
  Paranoid ActionableScore: 1

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: LOW
KPANIC detected for this report: NO
Published priority for this report (same as in Subject): LOW

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).