[CVE-2026-89665][MODERATE 7.0] nfsd: reject out-of-range useconds in NFSv2 SETATTR/CREATE

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

CVE: CVE-2026-89665
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: nfsd: reject out-of-range useconds in NFSv2 SETATTR/CREATE
Commit: a937dd1aa2d92291fe0a1860d17e90d8f206cd22
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=a937dd1aa2d92291fe0a1860d17e90d8f206cd22
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89665
Analysis date: Sat, 12 Sep 2026 03:00:58 -0400
ActionableScore: 4
ActionableScore lower bound: 3
Actionable bucket: Borderline / manual review recommended
Manual review required: YES

Summary:
An NFSv2 sattr input validation flaw allows malformed useconds values in SETATTR or CREATE RPCs to wrap during nanosecond conversion on ILP32 systems, causing incorrect timestamp metadata to be accepted when NFSv2 exports are reachable.

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ABOUT THIS REPORT
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The original Linux kernel CVE announcement for CVE-2026-89665 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89665

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.

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AL-KERNEL CLASSIFICATION RESULT
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CVE-2026-89665	MODERATE	https://www.kernelcve.org/list/?q=CVE-2026-89665 CHECK WITH IMPACT FROM ORIG NN MODERATE	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:N';CWE-190;*CWE-20;CWE-682;Other CVSS 'AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:L/A:N';BEST CVSS score: '5.3';DESCR 'An input validation flaw in the NFSv2 server sattr decoder allows out-of-range useconds values to be converted to nanoseconds before the raw range is checked. On 32-bit ILP32 systems the tmp2 * NSEC_PER_USEC multiplication can wrap and produce an apparently valid tv_nsec value, so malformed SETATTR or CREATE RPCs may be accepted with incorrect timestamp data. The atime handling also missed the Sun server time convention for useconds 1000000, allowing an invalid one second nanosecond value to reach filesystem code in some requests. For the CVSS the PR:N is used for the paranoid network exposed case where an attacker can send NFSv2 RPCs to a writable or anonymous export without local privileges on the server. The issue is network reachable only when nfsd serves NFSv2 to an attacker reachable export. Impact is primarily low integrity impact to file timestamp metadata, with no clear memory corruption or privilege escalation primitive.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline / manual review recommended (with actual score 4)  SKIP CVE-2026-89665 UNKNOWN SKIP The Fixes patch not applied yet, so unlikely that actual: url YES  NO NO unknown 	MAYBE	OOB DANGER DISK NETWORK LINUS KPANIC 	-	-	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=4
ActionableScoreLower=3
PrimitiveClass=CLASS_C
RealUAF=NO
HostAdminRequired=NO
NamespaceScopedPrivilege=NO
ImportantCandidate=NO
ManualReview=RECOMMENDED
AutoDowngradeAllowed=YES

## 1. ActionableScore

* Raw conservative score: 3
* Conservative score: 3. No floor or cap applied.
* Raw paranoid score: 4
* Paranoid score: 4. No floor or cap applied.
* Final recommended bucket: **Borderline / manual review recommended**::

## 2. Signal breakdown

Conservative signals:

* +2 Remote reachable / directly network-triggerable: malformed NFSv2 SETATTR or CREATE RPC input can reach `svcxdr_decode_sattr()` when an attacker can access an NFSv2 export.
* +1 Integrity plausible: out-of-range `useconds` can be accepted as an incorrect timestamp value, causing low-impact filesystem metadata integrity corruption.
* 0 No memory corruption signal: the wrap affects timestamp conversion, not kernel heap or object memory.
* 0 No UAF / lifetime signal: no stale object, refcount, RCU, or post-free use is established.
* 0 No reliable kernel crash / DoS: the patch describes malformed metadata reaching the filesystem, not panic or host-wide availability failure.
* 0 No confidentiality signal: no disclosure path is shown.
* 0 No authorization bypass: the bug does not turn a denied NFS operation into an allowed one by bypassing credentials or export permissions.

Paranoid additional signal:

* +1 Filesystem/storage metadata concern: the corrupted value is persistent file metadata and may affect workloads or checks relying on timestamps, though this remains below an ownership/COW/security-boundary violation.

Non-awarded material signals:

* Controlled reclaim not demonstrated.
* Arbitrary write not demonstrated.
* Privilege escalation not demonstrated.
* Dirty-Pipe-like shared-page or COW ownership violation not established.
* Important filesystem path not scored as strong because this is timestamp metadata validation, not extent ownership, page cache corruption, journal corruption, or cross-file modification.
* No extra availability point because no crash, shutdown, or persistent resource exhaustion is supported.
* No legacy penalty applied conservatively because NFSv2 is legacy but the supplied evidence does not prove that the evaluated deployment cannot expose it.

## 3. Reachability analysis

The vulnerable path is in the NFS server XDR decoder for NFSv2 `sattr` arguments. A remote NFS client can supply the malformed `useconds` value in SETATTR or CREATE requests if NFSv2 service and a reachable export are available.

Host administrator privileges on the victim are not required for the attacker path itself. The attacker does need NFS reachability and normally some export-level permission or accepted NFS credential allowing SETATTR or CREATE. In a paranoid case, a writable anonymous export or otherwise weakly authenticated NFSv2 export can make this effectively PR:N in CVSS terms.

This is not namespace-scoped. No local namespace capability is relevant. Prior administrative setup is required only in the ordinary sense that the server must be running nfsd with NFSv2 enabled and an export exposed. No special hardware or race timing is involved.

## 4. Primitive and severity interpretation

The primitive is an input-validation and integer-overflow logic bug. `tmp2` is a wire-provided 32-bit `useconds` value, and on ILP32 the multiplication by `NSEC_PER_USEC` can wrap before later code can reject the invalid value. This can make malformed input produce an apparently valid but incorrect `tv_nsec`.

This is not an active structured UAF, fixed stale storage, double release, RCU lifetime issue, OOB access, or controlled kernel memory corruption. The demonstrated consequence is incorrect file timestamp metadata reaching filesystem operations. The likely operational severity is low-to-moderate integrity impact in environments exposing NFSv2.

PrimitiveClass is CLASS_C because the demonstrated behavior is invalid-state / metadata corruption without an independently useful memory corruption, lifetime, ownership, or authorization primitive.

ImportantCandidate is NO because there is no strong kernel corruption, LPE, protected-resource theft, or security-boundary bypass. AutoDowngradeAllowed is YES because the issue can be handled as an ordinary/borderline Moderate after confirming deployment exposure, although network reachability makes manual review advisable.

## 5. One-sentence report phrase

An NFSv2 sattr input validation flaw allows malformed useconds values in SETATTR or CREATE RPCs to wrap during nanosecond conversion on ILP32 systems, causing incorrect timestamp metadata to be accepted when NFSv2 exports are reachable.

## 6. Manual review recommendation

MANUAL CHECK RECOMMENDED

Reason: the primitive is low impact and not memory-corruption based, but the affected code is network reachable through nfsd when NFSv2 is enabled and an export is accessible.

## 7. Call-site confidence

Call-site confidence: high

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

Patch: nfsd: reject out-of-range useconds in NFSv2 SETATTR/CREATE
Commit: a937dd1aa2d92291fe0a1860d17e90d8f206cd22
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=a937dd1aa2d92291fe0a1860d17e90d8f206cd22

Commit description:

The NFSv2 sattr decoder converts the wire useconds to nanoseconds in
svcxdr_decode_sattr():

	iap->ia_atime.tv_nsec = tmp2 * NSEC_PER_USEC;

tmp2 is a u32 and NSEC_PER_USEC is 1000, so the product is computed in
unsigned long. On ILP32 that is 32 bits, and an out-of-range useconds
value such as 4294968 wraps to tv_nsec == 704. The corruption therefore
happens during decode, before any proc function can inspect the value,
and a later range check on tv_nsec would see an in-range result and
accept it. Rejecting in the decoder yields an RPC GARBAGE_ARGS reply.
NFSv2 defines no NFSERR_INVAL, so there is no NFS-level status to return
for a malformed time argument, and the check cannot move to the proc
function the way the v3/v4 nsec range checks do.

Guard the raw useconds before the multiplication and reject values
greater than 1000000. useconds == 1000000 is kept: it is the Sun
convention for "set to the current server time", and the in-tree Linux
NFSv2 client emits it in both the atime and the mtime field for a plain
touch / utimes(file, NULL) (see encode_sattr() and
xdr_encode_current_server_time() in fs/nfs/nfs2xdr.c). Rejecting 1000000
would turn that common operation into a hard decode failure for both
SETATTR and CREATE. 1000000 * NSEC_PER_USEC is 10^9, which does not wrap
on ILP32, so the Sun convention value passes through safely. Only
genuinely out-of-range values (> 1000000) are rejected. The atime and
mtime guards are therefore symmetric.

The decoder only applied the Sun convention in the mtime block, which
clears ATTR_ATIME_SET|ATTR_MTIME_SET when mtime useconds == 1000000. If a
client puts 1000000 in the atime field but not in the mtime field, the
atime block stored an out-of-range tv_nsec (10^9) and left ATTR_ATIME_SET
set, so the bogus value reached the filesystem. Apply the convention in
the atime block as well, clearing ATTR_ATIME_SET so the server uses its
current time and ignores the value. Only ATTR_ATIME_SET is cleared there.
The mtime block keeps its existing behavior, where 1000000 means "set
both atime and mtime to now".

Fixes: 1da177e ("Linux-2.6.12-rc2")
Cc: stable@vger.kernel.org
Signed-off-by: Robbie Ko <robbieko@synology.com>
[ cel: various tweaks, addenda, and clean-ups ]
Link: https://patch.msgid.link/20260616054027.2360930-1-robbieko@synology.com
Signed-off-by: Chuck Lever <cel@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  fs/nfs/nfs2xdr.c
  fs/nfsd/nfsxdr.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=a937dd1aa2d92291fe0a1860d17e90d8f206cd22

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

The original Linux kernel CVE announcement for CVE-2026-89665 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89665

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:L/PR:L/UI:N/S:U/C:N/I:L/A:N
  The Best / paranoid CVSS vector: AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:N
  The Best / paranoid CVSS score: 5.3

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: 3
  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).