[CVE-2026-89690][IMPORTANT] nfsd: defer vfree of compound ops to fix rpc_status UAF

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

CVE: CVE-2026-89690
Priority: IMPORTANT
AL-KERNEL base severity: IMPORTANT
KPANIC flag: YES
Patch: nfsd: defer vfree of compound ops to fix rpc_status UAF
Commit: 48f72aff24f7a7329209dc6cbc27c869f3e3595c
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=48f72aff24f7a7329209dc6cbc27c869f3e3595c
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89690
Analysis date: Fri, 11 Sep 2026 23:45:16 -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.

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

The original Linux kernel CVE announcement for CVE-2026-89690 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89690

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: IMPORTANT
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-2026-89690	IMPORTANT	https://www.kernelcve.org/list/?q=CVE-2026-89690 CHECK WITH IMPACT FROM ORIG NN IMPORTANT	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:H';*CWE-416;CWE-362;*CWE-672;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '5.8';DESCR 'A use-after-free in the NFSD rpc_status netlink dump path can occur because the dumpit code walks in-flight svc_rqst objects under rcu_read_lock and reads NFSv4 compound operation numbers from args ops while the separate vmalloc buffer may be freed by nfsd4_release_compoundargs. A concurrent NFSv4 compound completion can release the ops buffer while rpc_status is still reading it, so the RCU protection for the svc_rqst object is not sufficient for the auxiliary args ops allocation. For the CVSS the PR:L is used for the paranoid score because a local user or local process must be able to request the rpc_status netlink dump, while NFSv4 server traffic supplies the concurrent in-flight requests. The issue is not directly network reachable by NFS traffic alone unless an exposed management path can trigger the local netlink dump. Impact is at least local denial of service via kernel crash. In the worst still defensible case, limited confidentiality or integrity impact may be possible from reading stale vmalloc memory, so the issue should be reviewed manually.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate / Strong Important candidate requiring manual review (with actual score 6)	YES	DISK UAF NETWORK KPANIC  KPANIC LOWERED_FROM_HIGH_BASED_ON_GUESSCVSS  KPANIC INCREASED_TO_HIGH_BASED_ON_GUESSCVSS	YES	NO	checked

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

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

1. ActionableScore

* Raw conservative score: 4
* Conservative score: 5. Applied concrete RCU referenced-object lifetime mismatch calibration for active structured state: 4 -> 5.
* Raw paranoid score: 5
* Paranoid score: 6. Applied paranoid active RCU stale-read calibration because post-lifetime structured reads remain unresolved: 5 -> 6.
* Final recommended bucket: **Actionable Moderate / Strong Important candidate requiring manual review**::

2. Signal breakdown

* +2 Real UAF / active lifetime violation. `rpc_status` dumpit reads `args->ops[]` under `rcu_read_lock`, while `nfsd4_release_compoundargs()` can synchronously `vfree()` that separate vmalloc buffer.
* +1 Concrete RCU referenced-object lifetime mismatch. The RCU-visible `svc_rqst` remains protected, but its referenced auxiliary object `args->ops` was freed before the RCU grace period. The fix moves freeing to `kvfree_rcu_mightsleep()`.
* +1 Reliable kernel crash / strong DoS. Reading freed vmalloc memory can fault or produce kernel UAF symptoms during netlink dump processing.
* +1 paranoid confidentiality concern. The stale data is read by a netlink dump path, so limited user-visible stale-field disclosure is technically plausible, although not demonstrated as arbitrary kernel disclosure.

Non-awarded signals:

* No controlled reclaim or attacker-shaped replacement is demonstrated.
* No arbitrary or directed kernel write is demonstrated.
* No privilege escalation is demonstrated.
* No integrity point is awarded. The stale path reads operation data and does not modify protected state.
* No remote reachability bonus is awarded. NFS traffic can supply concurrent compound requests, but the vulnerable dump path is local netlink control-plane access.
* No race penalty is applied. A race exists, but the evidence does not prove a one-shot or externally uncontrollable timing window.
* No host-admin penalty is applied. Host-admin-equivalent privilege is not positively established for the complete dump trigger.
* No concrete repeated release/refcount signal applies. There is no first put/free followed by a second lifetime-mutating operation on the same reference.
* FLOOR 5 is applied due to the concrete RCU referenced-object lifetime mismatch involving active structured state, not due to repeated release/refcount.

3. Reachability analysis

The complete trigger requires two conditions: active NFSD/NFSv4 compound requests and a concurrent `rpc_status` netlink dump. Remote NFS clients may help create in-flight request state, but remote NFS traffic alone does not directly invoke the vulnerable netlink dumpit path.

The local privilege requirement for the `rpc_status` netlink interface is not fully established from the supplied patch. Therefore `HostAdminRequired=UNKNOWN`, not YES and not NO. No evidence establishes that the complete operation is namespace-scoped, so `NamespaceScopedPrivilege=UNKNOWN`.

A prior administrator must have enabled or deployed NFSD/NFSv4 for the affected path to exist, but that is treated as target configuration rather than proof that the attacker needs host-admin privilege. No special hardware is required. The race can likely be retried with repeated dumps and ongoing NFSv4 traffic, but exact overlap is still required.

4. Primitive and severity interpretation

The exact primitive is a real use-after-free read of a vmalloc-allocated `args->ops` buffer. The stale object is active structured storage containing NFSv4 compound operation data, referenced from an RCU-visible `svc_rqst`. This is not fixed DMA storage and not merely a cleanup-only stale dereference, because the stale read occurs in an externally visible netlink dump path.

There is no concrete repeated release/refcount operation. The first lifetime operation is `vfree(args->ops)`, but the later stale operation is a read, not another `put`, `free`, or refcount mutation. Therefore the repeated-release exception does not apply.

Primitive class is CLASS_B: a real RCU/lifetime UAF with unresolved exploitation leverage, but without controlled reclaim, arbitrary write, callback control, or demonstrated LPE. Demonstrated consequence is at least local DoS via kernel UAF/crash. Speculative consequence is limited stale data exposure through netlink output. ImportantCandidate is YES because this is an active RCU referenced-object lifetime mismatch and should not be reduced to ordinary crash-only cleanup without reviewing the dump output and reuse behavior. AutoDowngradeAllowed is NO because automatic downgrade would discard a concrete kernel UAF with unresolved stale-read exposure.

5. One-sentence report phrase

An NFSD rpc_status netlink dump can read NFSv4 compound operation data from an args ops vmalloc buffer after it has been freed by concurrent compound completion, causing a real RCU lifetime UAF with at least local DoS impact and possible limited stale data exposure.

6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: real UAF on vmalloc memory in an RCU dump path, with user-visible netlink output and unresolved stale-read exposure beyond a simple private crash.

7. Call-site confidence

Call-site confidence: high

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

Patch: nfsd: defer vfree of compound ops to fix rpc_status UAF
Commit: 48f72aff24f7a7329209dc6cbc27c869f3e3595c
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=48f72aff24f7a7329209dc6cbc27c869f3e3595c

Commit description:

The rpc_status netlink dumpit walks every in-flight svc_rqst under
rcu_read_lock and, for NFSv4 requests, reads opnums out of
args->ops[]. But args->ops is a separate vmalloc buffer freed
synchronously by vfree() in nfsd4_release_compoundargs() at the end
of every compound. The dumpit's rcu_read_lock pins the svc_rqst
struct itself (freed via kfree_rcu), but nothing defers the vfree
of the ops buffer across the RCU grace period. A concurrent compound
completion can therefore free the buffer while the dumpit is reading
it — a use-after-free on vmalloc memory.

The trailing seqcount recheck (smp_load_acquire of rq_status_counter)
cannot undo a load that already retired against freed memory.

Fix by replacing vfree(args->ops) with kvfree_rcu_mightsleep(), which
defers the free until after an RCU grace period. This makes the
existing rcu_read_lock in the dumpit sufficient to protect the read.
The tradeoff is that completed compound ops buffers (up to
200 * sizeof(struct nfsd4_op)) persist in memory slightly longer,
across one grace period, before being reclaimed.

Fixes: bd9d6a3 ("NFSD: add rpc_status netlink support")
Cc: stable@vger.kernel.org
Assisted-by: Claude:claude-opus-4-6
Signed-off-by: Jeff Layton <jlayton@kernel.org>
Link: https://patch.msgid.link/20260602-nfsd-testing-v2-1-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/nfs4xdr.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=48f72aff24f7a7329209dc6cbc27c869f3e3595c

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

The original Linux kernel CVE announcement for CVE-2026-89690 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89690

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:L/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:L/I:L/A:H
  The Best / paranoid CVSS score: 5.8

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: IMPORTANT
KPANIC detected for this report: YES
Published priority for this report (same as in Subject): IMPORTANT

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).