[CVE-2026-89678][MODERATE 7.0] nfsd: fix partial-write detection in nfsd_direct_write

[CVE-2026-89678][MODERATE 7.0] nfsd: fix partial-write detection in nfsd_direct_write

[AL-KERNEL]

CVE: CVE-2026-89678
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: nfsd: fix partial-write detection in nfsd_direct_write
Commit: fa6590dfd16ab55f03b658b079072ace3504825e
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=fa6590dfd16ab55f03b658b079072ace3504825e
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89678
Analysis date: Fri, 11 Sep 2026 22:09:41 -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-2026-89678 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89678

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-2026-89678	MODERATE	https://www.kernelcve.org/list/?q=CVE-2026-89678 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:L/UI:N/S:U/C:N/I:H/A:L';CWE-682;CWE-435;*CWE-664;Other CVSS 'AV:N/AC:H/PR:L/UI:N/S:U/C:N/I:H/A:L';BEST CVSS score: '7.1';DESCR 'An incorrect partial write check in nfsd_direct_write can fail to detect a short direct IO write because the segment iterator count is reduced by write_iter before the comparison occurs. An NFS client with write access can cause the next write segment to be placed at an incorrect file offset after a partial write, resulting in corruption of data in the exported file and an incorrect reported write count. For the CVSS the PR:L is used because the attacker needs access to an NFS export with permission to write the affected file. The issue is network reachable through an NFS server export, although reliable triggering requires a partial direct IO write and is likely dependent on filesystem or storage conditions. Impact includes integrity loss and limited availability impact through corruption of application data. NFS services are commonly restricted to trusted internal networks, but affected exports reachable by untrusted clients should be reviewed.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline / manual review recommended (with actual score 4)  SKIP CVE-2026-89678 UNKNOWN SKIP The Fixes patch not applied yet, so unlikely that actual: url YES  NO NO unknown 	MAYBE	WRITE OOB DISK NETWORK KPANIC 	-	-	checked

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

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

1. ActionableScore

* Raw conservative score: 4
* Conservative score: 4
* Raw paranoid score: 4
* Paranoid score: 4
* Final recommended bucket: **Borderline / manual review recommended**::

2. Signal breakdown

* +2 Remote reachable / directly network-triggerable. An NFS client with write access to the affected export can reach nfsd_direct_write through NFS WRITE requests.
* +1 Important filesystem/storage path. The defect is in NFSD direct write processing and can cause persistent corruption of file contents.
* +1 Practical exploit chain demonstrated. A partial direct IO write of 50 to 99 percent of a segment can cause the following segment to be written at an incorrect offset and can over-report the completed byte count.
* +0 Integrity plausible. The patch establishes corruption of data in the file being written, but does not establish unauthorized modification of a separately protected file, filesystem metadata, or another principals object. The client already needs write access to the affected NFS file.
* +0 Strong exploitation leverage. No arbitrary kernel write, controlled kernel object corruption, ownership violation, COW violation, callback influence, or privilege escalation path is established.
* +0 Memory corruption. No UAF, OOB access, refcount defect, stale pointer, or active-object lifetime violation is shown.
* +0 Reliable kernel crash. The demonstrated consequence is incorrect file placement and protocol accounting, not a kernel crash.
* No negative privilege penalty. The supplied evidence establishes a remote NFS client with write permissions, which is below host-admin privilege. It does not establish host-admin-equivalent access as a prerequisite.
* No hard-race penalty. The issue is a deterministic logic error after a qualifying short direct IO write, not a timing race.
* No niche-subsystem penalty. NFSD direct writes require an active NFS server and the direct IO path, but the input does not establish that this feature is unusually rare or inaccessible to an authorized NFS client.

3. Reachability analysis

The evaluated attacker is a remote NFS client that has permission to write the exported file. The vulnerable operation is reached through NFS WRITE handling when NFSD uses nfsd_direct_write and the underlying direct IO write completes partially.

The complete trigger requires an NFS server export, client write authorization, and a qualifying partial direct IO result. The patch identifies the relevant short-write range as approximately 50 to 99 percent of a segment. This may depend on filesystem, storage, quota, error, or direct IO behavior, but the supplied evidence does not establish a narrow externally uncontrollable race.

Host-admin privilege is not required by the attacker once a writable NFS export is available. Namespace-scoped reachability is unresolved and not material to the primary remote NFS-client model. No special hardware requirement is established.

4. Primitive and severity interpretation

The exact primitive is an incorrect partial-write comparison in nfsd_direct_write. After vfs_iocb_iter_write advances the iterator, the code compares the returned byte count with the residual iterator count instead of the original segment length. A short write can therefore be treated as complete, and the next segment can be placed at an incorrect offset because kiocb->ki_pos advanced only by the actual short-write length.

This is a constrained filesystem data-placement and accounting logic defect. It is not a UAF, double free, stale-reference issue, OOB access, arbitrary kernel write, or active structured-object lifetime violation. The affected write remains within the file that the NFS client is already authorized to modify. The patch does not establish cross-file corruption, file-permission bypass, shared-page ownership violation, protected metadata corruption, confidentiality disclosure, or privilege escalation.

PrimitiveClass is CLASS_C because the demonstrated effect is invalid write-state handling and file-data corruption without an independently useful kernel-corruption primitive. ImportantCandidate is NO because the supplied patch does not show meaningful exploitation leverage beyond corruption of the writable target file. AutoDowngradeAllowed is YES because no unresolved UAF, memory-corruption, authorization-bypass, COW-ownership, or cross-security-boundary primitive is established.

Operationally, this should still be reviewed when affected NFS exports carry important application data, because the bug can silently corrupt contents and report an incorrect successful write length to the client. It is not a strong Important candidate from the supplied evidence alone.

5. One-sentence report phrase

An NFS client with write access can trigger incorrect file-offset handling after a partial NFSD direct IO write, causing subsequent request data to overwrite an unintended region of the same exported file and resulting in persistent data corruption.

6. Manual review recommendation

MANUAL CHECK RECOMMENDED

The issue is network reachable by an authorized NFS client and can cause persistent application-data corruption, but the patch does not establish kernel memory corruption, cross-file writes, permission bypass, or privilege escalation.

7. Call-site confidence

Call-site confidence: medium

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

Patch: nfsd: fix partial-write detection in nfsd_direct_write
Commit: fa6590dfd16ab55f03b658b079072ace3504825e
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=fa6590dfd16ab55f03b658b079072ace3504825e

Commit description:

nfsd_direct_write() walks a list of write segments and, after each
vfs_iocb_iter_write(), tries to detect a short write so the loop can
stop before placing the next segment at a wrong file offset:

    host_err = vfs_iocb_iter_write(file, kiocb, &segments[i].iter);
    if (host_err < 0)
            return host_err;
    *cnt += host_err;
    if (host_err < segments[i].iter.count)
            break;	/* partial write */

vfs_iocb_iter_write() runs the iter through ->write_iter(), which
advances the iter by the number of bytes written. By the time the
check runs, segments[i].iter.count is the residual, not the original
request length:

    before write_iter: iter.count == original_len
    after  write_iter: iter.count == original_len - host_err

The condition then reduces to host_err < original_len - host_err, so
the break fires only when less than half of the segment was written.
Any short write completing between 50% and 99% of the segment slips
through; the loop advances to the next segment with kiocb->ki_pos
only bumped by the short amount, writing the next segment's payload
at the wrong offset and over-reporting *cnt to the NFS client.

Snapshot the segment's byte count before the write and compare
host_err against that snapshot so any short write breaks the loop.

Fixes: 06c5c97 ("NFSD: Implement NFSD_IO_DIRECT for NFS WRITE")
Cc: stable@vger.kernel.org
Assisted-by: kres:claude-opus-4-7
Reported-by: Chris Mason <clm@meta.com>
Signed-off-by: Chris Mason <clm@meta.com>
Link: https://patch.msgid.link/20260530-nfsd-fixes-v2-7-f27e8eb4d974@kernel.org
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  fs/nfsd/vfs.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=fa6590dfd16ab55f03b658b079072ace3504825e

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

The original Linux kernel CVE announcement for CVE-2026-89678 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89678

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:H/PR:L/UI:N/S:U/C:N/I:H/A:L
  The Best / paranoid CVSS vector: AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:H/A:L
  The Best / paranoid CVSS score: 7.1

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).