[CVE-2026-93211][MODERATE REGULAR] nfsd: initialize DRC hash table before registering shrinker

[CVE-2026-93211][MODERATE REGULAR] nfsd: initialize DRC hash table before registering shrinker

[AL-KERNEL]

CVE: CVE-2026-93211
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: nfsd: initialize DRC hash table before registering shrinker
Commit: f060f43a67635dbb393bf5dcbfc3e8b9a44942cb
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=f060f43a67635dbb393bf5dcbfc3e8b9a44942cb
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-93211
Analysis date: Thu, 24 Sep 2026 13:55:05 -0400
ActionableScore: 0
ActionableScore lower bound: unknown
Actionable bucket: Ordinary Moderate
Manual review required: NO

Summary:
No one-sentence report phrase was found.

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

The original Linux kernel CVE announcement for CVE-2026-93211 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-93211

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: NO

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

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

CVE-2026-93211	MODERATE	https://www.kernelcve.org/list/?q=CVE-2026-93211 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:N/I:N/A:H';CWE-362;CWE-665;**CWE-476;BEST CVSS score: '4.7';DESCR 'A race in nfsd_reply_cache_init can expose the DRC hash table to the memory shrinker before its bucket list heads are initialized. On weakly ordered architectures such as arm64 and ppc, a concurrent shrinker scan may observe a nonzero drc_hashsize and then traverse an uninitialized bucket, causing a NULL pointer dereference and kernel crash. Reliable triggering requires NFSD initialization to overlap with memory reclaim during a narrow timing window. For the CVSS the PR:L reflects that a local unprivileged process may be able to create memory pressure, while administrative privileges are normally required only to configure or start NFSD itself. The issue is not directly triggered by an NFS packet, and a reliable remote attack path is not demonstrated because the vulnerable window occurs during server initialization. Impact is denial of service with A:H. No UAF, OOB write, information disclosure, or controlled write primitive is visible, so confidentiality and integrity remain C:N and I:N even for the paranoid score.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate (with actual score 0)  SKIP CVE-2026-93211 UNKNOWN SKIP The Fixes patch not applied yet, so unlikely that actual: url YES  NO NO unknown 	MAYBE	DISK INIT SIMPLEFIX NETWORK HARDWARE LINUS  DECREASED_TO_MODERATE_REGULAR_FROM_MODERATE7_BASED_ON_GUESSCVSS_AND_CIANNH	-	-	checked

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

ActionableScore=0
PrimitiveClass=CLASS_C
RealUAF=NO
HostAdminRequired=UNKNOWN
NamespaceScopedPrivilege=UNKNOWN
ImportantCandidate=NO
ManualReview=RECOMMENDED
AutoDowngradeAllowed=YES

1. ActionableScore

* Raw conservative score: 0
* Conservative score: 0
* Raw paranoid score: 0
* Paranoid score: 0
* No floor or cap was applied.
* Final recommended bucket: **Ordinary Moderate**::

2. Signal breakdown

* +1 Reliable kernel crash or strong DoS. A shrinker callback can traverse an uninitialized DRC bucket and cause a NULL pointer dereference.
* -1 Hard or unreliable race. Triggering requires a shrinker scan to overlap the narrow NFSD reply-cache initialization window on a weakly ordered architecture.
* +0 Weak corruption concern. The established primitive is an invalid traversal and NULL dereference, not independently useful memory corruption.
* No UAF, stale post-lifetime object use, repeated release, or refcount mutation is established.
* Controlled reclaim, arbitrary write, OOB access, information disclosure, and attacker-controlled replacement are not demonstrated.
* Remote reachability is not established. NFS packets do not directly invoke the vulnerable initialization race.
* Local unprivileged reachability is not awarded. A local process may create memory pressure, but the complete trigger also depends on separately initiated NFSD setup.
* No extra Availability point is awarded because it would duplicate the same kernel fault.
* No C or I signal is awarded because the uninitialized list head is expected to produce a NULL dereference rather than a useful read or write primitive.
* No subsystem or hardware rarity penalty is applied. The weak-memory architecture condition is reflected only in the supported timing penalty.

3. Reachability analysis

NFSD must first be configured and initialized, which ordinarily involves a privileged system component. A concurrent shrinker scan must then occur before bucket initialization completes. A local process could potentially contribute memory pressure, but the evidence does not establish that an ordinary user controls the complete sequence or can repeat the initialization window reliably.

The vulnerable operation is not directly reached through NFS traffic. Remote requests might contribute general load only after suitable service setup, and no concrete packet-to-race path is supplied. The relevant namespace capability scope is unresolved, and the complete trigger is not shown to be available through an attacker-created namespace. No special peripheral hardware is required, but practical manifestation is tied to weakly ordered architectures such as arm64 and ppc.

4. Primitive and severity interpretation

The exact primitive is publication of a partially initialized DRC hash table followed by traversal of an uninitialized list head. This is an initialization-order race resulting in an invalid pointer dereference.

There is no freed object, stale post-lifetime reference, second release, or active structured-object reuse. Therefore, this is CLASS_C rather than CLASS_B. The demonstrated consequence is kernel Oops or panic and host denial of service. Privilege escalation, disclosure, controlled corruption, and other consequences beyond the crash are unsupported.

ImportantCandidate is NO because no independently useful corruption or security-boundary primitive exists. AutoDowngradeAllowed is YES because the issue is a narrow crash-oriented initialization race without unresolved lifetime, ownership, authorization, or controlled-memory leverage. Operationally, it is an ordinary Moderate or Low-like availability issue.

5. One-sentence report phrase

A weak-memory ordering race can expose an incompletely initialized NFSD DRC hash table to a concurrent shrinker scan, causing a NULL pointer dereference and local kernel denial of service without evidence of memory corruption or privilege escalation.

6. Manual review recommendation

MANUAL CHECK RECOMMENDED

Confirm architecture-specific reproducibility, whether the Oops is fatal under product configuration, and whether an unprivileged actor can reliably induce reclaim during repeated NFSD initialization.

7. Call-site confidence

Call-site confidence: medium

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

Patch: nfsd: initialize DRC hash table before registering shrinker
Commit: f060f43a67635dbb393bf5dcbfc3e8b9a44942cb
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=f060f43a67635dbb393bf5dcbfc3e8b9a44942cb

Commit description:

shrinker_register() precedes the INIT_LIST_HEAD loop and the
drc_hashsize store. On weakly-ordered architectures (arm64, ppc),
a shrinker scan can observe drc_hashsize before the bucket list
heads are initialized, causing a NULL deref in the DRC shrinker
callback.

Move bucket initialization and the drc_hashsize store before
shrinker_register() so the hash table is fully initialized before
it becomes visible to the shrinker.

Fixes: 8eea99a ("nfsd: dynamically allocate the nfsd-reply shrinker")
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/20260611-nfsd-testing-v2-18-5b90e276f2d9@kernel.org
Signed-off-by: Chuck Lever <cel@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  fs/nfsd/nfscache.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=f060f43a67635dbb393bf5dcbfc3e8b9a44942cb

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

The original Linux kernel CVE announcement for CVE-2026-93211 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-93211

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: Not available
  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: 0

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).