[CVE-2026-89658][MODERATE 7.0] NFSD: Prevent client use-after-free during NFSv4.0 revoked-state cleanup

[CVE-2026-89658][MODERATE 7.0] NFSD: Prevent client use-after-free during NFSv4.0 revoked-state cleanup

[AL-KERNEL]

CVE: CVE-2026-89658
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: NFSD: Prevent client use-after-free during NFSv4.0 revoked-state cleanup
Commit: 0ae0d2b5c5a1b39c0b3c15d96b32a5b0c583d519
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=0ae0d2b5c5a1b39c0b3c15d96b32a5b0c583d519
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89658
Analysis date: Sat, 12 Sep 2026 01:31:55 -0400
ActionableScore: 4
ActionableScore lower bound: unknown
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-2026-89658 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89658

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-2026-89658	MODERATE	https://www.kernelcve.org/list/?q=CVE-2026-89658 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-416;CWE-362;CWE-667;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 NFSD NFSv4.0 revoked state cleanup can cause a client use-after-free because nfs40_clean_admin_revoked takes a stateid reference but does not pin the owning client before dropping the global client lock. A concurrent client teardown can free the client while nfsd4_drop_revoked_stid still dereferences s sc_client and its cl_lock, creating a kernel memory safety issue. For the CVSS the PR:L value is used for the paranoid score because reliable triggering may require local access to NFSD control plane operations, but such access can be delegated to a reduced service account, container root, or restricted administrator rather than full host root in some deployments. The attack is not directly network reachable as a packet only trigger based on the patch context, although the affected NFSv4.0 state belongs to network clients. Impact is at least local denial of service via kernel crash and in worst case may allow confidentiality and integrity impact due to use-after-free memory corruption, so it should be reviewed manually.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline / manual review recommended (with actual score 4)	YES	DANGER DISK RACE UAF NETWORK LINUS KPANIC 	YES	NO	checked

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

ActionableScore=4
PrimitiveClass=CLASS_B
RealUAF=YES
HostAdminRequired=UNKNOWN
NamespaceScopedPrivilege=UNKNOWN
ImportantCandidate=NO
ManualReview=RECOMMENDED
AutoDowngradeAllowed=YES

1. ActionableScore

* Raw conservative score: 2
* Conservative score: 4
* Raw paranoid score: 2
* Paranoid score: 4
* Applied conservative floor/cap: Real UAF floor 4 applied because a real active-object lifetime violation is established and host-admin-equivalent reachability is not fully proven from the supplied evidence.
* Applied paranoid floor/cap: Same floor applied. No extra paranoid leverage was added because controlled reclaim, repeated refcount mutation, or a stronger exploitation primitive is not established.
* Final recommended bucket: **Borderline / manual review recommended**::

2. Signal breakdown

Positive signals:

* +2 Real UAF / active lifetime violation: the stateid reference does not pin the owning NFSD client, so client teardown can free clp while nfsd4_drop_revoked_stid still dereferences s->sc_client.
* +1 Reliable kernel crash / strong DoS: dereferencing and locking fields of a freed client object can plausibly cause kernel fault, lock corruption, or crash.

Negative signals:

* -1 Hard or unreliable race / special timing: the demonstrated path requires the laundromat revoked-state sweep to overlap with client expiration while locks are dropped.

Non-awarded material signals:

* No concrete repeated release/refcount signal awarded. The fix adds cl_rpc_users pinning, but the vulnerable stale operation shown is client dereference/use, not a second put/release/refcount mutation on the exact same dangling client reference.
* Structured stale-object leverage not awarded. The stale object is an active NFSD client object, but controlled reclaim, attacker-shaped replacement, callback use, or security-sensitive field reuse is not demonstrated.
* Weak LPE not awarded solely from UAF class.
* Extra Availability not awarded because the crash/DoS is already counted once.
* Confidentiality and integrity points not awarded because no disclosure path, arbitrary write, controlled reclaim, or concrete privilege-escalation primitive is established.
* Remote reachability not awarded. NFS clients participate in creating state, but the supplied concrete race uses NFSD cleanup and force_expire_client control-plane activity, not packet-only triggering.

Repeated release/refcount checklist:

* First release can end lifetime: not established for the exact same reference in the shown vulnerable flow.
* Exact same reference survives to a second lifetime mutation: not established.
* Second operation is a real lifetime/refcount mutation: not established.
* +1 Concrete repeated release/refcount awarded: no.
* Paranoid repeated-refcount leverage awarded: no.
* FLOOR 5 applied: no.

3. Reachability analysis

The affected path is NFSD NFSv4.0 revoked-state cleanup in the laundromat. The concrete race described is between this periodic cleanup and force_expire_client driven by a write to clients/<id>/ctl. That suggests an NFSD administrative control-plane trigger, but the supplied input does not fully resolve whether every required stage is host-admin-only in all deployments or whether another teardown path can participate.

HostAdminRequired is therefore UNKNOWN rather than YES or NO. NamespaceScopedPrivilege is UNKNOWN because the complete operation is not shown to be performable with only attacker-obtained namespace-scoped capabilities.

This is not directly network reachable as a packet-only attack based on the supplied patch. A remote NFS client may influence the existence of NFSv4 state, but the demonstrated trigger requires server-side revoked-state cleanup and client expiration behavior. Prior NFSD setup is required, but that is normal service deployment rather than by itself an attacker-controlled prerequisite. No special hardware is involved.

The race has a real timing component because the client must be freed while the cleanup path has dropped nn->client_lock and still uses the client through the stateid.

4. Primitive and severity interpretation

The exact primitive is a real use-after-free / lifetime mismatch: a stateid is pinned, but its owning client is not pinned, and later code dereferences the client through s->sc_client after a racing teardown may have freed it.

This is an active structured object, not fixed stale storage. However, the demonstrated stale use is in revoked-state cleanup and the available evidence mainly supports crash/DoS. There is no concrete repeated release/refcount operation, no controlled reclaim, no attacker-controlled replacement object, no arbitrary or directed kernel read/write, and no demonstrated privilege escalation.

PrimitiveClass is CLASS_B because the patch directly establishes a real kernel lifetime violation. ImportantCandidate is NO because the supplied evidence does not establish meaningful exploitation leverage beyond crash sufficient for Important-vs-Moderate preservation. ManualReview is RECOMMENDED because real kernel UAFs in NFSD state management deserve human review even when the demonstrated consequence is DoS. AutoDowngradeAllowed is YES because this currently looks like a constrained cleanup/control-plane UAF without a proven repeated lifetime mutation or boundary-bypass primitive.

Likely operational severity is borderline to actionable Moderate depending on product exposure and whether the control-plane trigger is reachable below full host administration.

5. One-sentence report phrase

A race in NFSD NFSv4.0 revoked-state cleanup can leave a stateid referencing a freed client object, allowing nfsd4_drop_revoked_stid to dereference stale client state and potentially crash the kernel.

6. Manual review recommendation

MANUAL CHECK RECOMMENDED

Reason: a real kernel UAF is established, but the current evidence supports mainly crash/DoS through a cleanup/control-plane race and does not establish controlled reclaim, repeated refcount mutation, or privilege escalation.

7. Call-site confidence

Call-site confidence: high

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

Patch: NFSD: Prevent client use-after-free during NFSv4.0 revoked-state cleanup
Commit: 0ae0d2b5c5a1b39c0b3c15d96b32a5b0c583d519
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=0ae0d2b5c5a1b39c0b3c15d96b32a5b0c583d519

Commit description:

nfs40_clean_admin_revoked() takes a stateid reference under
clp->cl_lock, drops nn->client_lock, and calls
nfsd4_drop_revoked_stid(), which dereferences the stateid's client
through s->sc_client->cl_lock.  The stateid reference does not pin the
client, so a teardown racing the dropped lock can free the client
while nfsd4_drop_revoked_stid() is still using it.

This cleanup runs from the laundromat, so a periodic sweep can race
force_expire_client() driven by a write to the clients/<id>/ctl file.

Skip a client that is already expiring and otherwise pin it with
cl_rpc_users under client_lock before dropping the lock, matching
nfsd4_revoke_states().

Fixes: d688d85 ("nfsd: allow admin-revoked NFSv4.0 state to be freed.")
Cc: stable@vger.kernel.org
Reviewed-by: NeilBrown <neil@brown.name>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Link: https://patch.msgid.link/20260709-cel-v4-5-1d519d9be0cb@kernel.org
Signed-off-by: Chuck Lever <cel@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  fs/nfsd/nfs4state.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=0ae0d2b5c5a1b39c0b3c15d96b32a5b0c583d519

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

The original Linux kernel CVE announcement for CVE-2026-89658 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89658

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: unknown
  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: 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).