[CVE-2026-89713][IMPORTANT] NFSD: check truncate permission under inode lock

[CVE-2026-89713][IMPORTANT] NFSD: check truncate permission under inode lock

[AL-KERNEL]

CVE: CVE-2026-89713
Priority: IMPORTANT
AL-KERNEL base severity: IMPORTANT
KPANIC flag: YES
Patch: NFSD: check truncate permission under inode lock
Commit: 3afa17d93ba8c925f49370c816c6dae5112d8c24
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=3afa17d93ba8c925f49370c816c6dae5112d8c24
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89713
Analysis date: Sat, 12 Sep 2026 00:10:52 -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-2026-89713 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89713

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-2026-89713	IMPORTANT	https://www.kernelcve.org/list/?q=CVE-2026-89713 CHECK WITH IMPACT FROM ORIG NN MODERATE	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:N/AC:H/PR:L/UI:N/S:U/C:N/I:H/A:H';CWE-367;CWE-284;CWE-362;Other CVSS 'AV:N/AC:H/PR:L/UI:N/S:U/C:N/I:H/A:L';BEST CVSS score: '6.8';DESCR 'NFSD performs the truncate permission decision for SETATTR size changes using a file size sampled before inode_lock(), while notify_change applies ATTR_SIZE later under the inode lock. A remote NFS client can race a SETATTR that was not a shrink at the unlocked sample with a concurrent append, so the later locked update becomes a real truncation of an append-only file without the NFSD_MAY_TRUNC check that should reject IS_APPEND(inode). For the CVSS the PR:L is used because the attacker normally needs NFS credentials or export access with permission to issue the relevant SETATTR operation on the target file. The issue is network reachable through NFS server control operations and is not triggered by ordinary packet forwarding. Impact is mainly integrity loss through bypass of append-only truncation protection and possible data loss or service impact for affected files.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate / Strong Important candidate requiring manual review (with actual score 6)	YES	DISK NETWORK KPANIC  KPANIC INCREASED_TO_HIGH_BASED_ON_GUESSCVSS	NO	NO	checked

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

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

1. ActionableScore

* Raw conservative score: 5
* Conservative score: 5
* Raw paranoid score: 6
* Paranoid score: 6
* Final recommended bucket: **Actionable Moderate / Strong Important candidate requiring manual review**::

No floor or cap was applied. The difference is that the conservative score counts the established network-reachable authorization bypass and integrity impact, while the paranoid score additionally counts the concrete race-to-truncation exploit sequence as a practical chain that should not be automatically closed.

2. Signal breakdown

Conservative positive signals:

* +2 Concrete authorization decision bypass. NFSD_MAY_TRUNC is the permission gate that should reject truncation of IS_APPEND(inode), but the decision is made using an unlocked stale file size and can incorrectly allow a later real truncation.
* +2 Remote reachable NFS control operation. The vulnerable SETATTR path is reachable through an NFS client operation against an exported file, not through local-only syscalls.
* +1 Integrity plausible. Successful exploitation can truncate an append-only file contrary to the intended append-only protection, causing unauthorized data modification or data loss.

Paranoid additional signal:

* +1 Practical exploit chain strongly implied. The supplied context gives a concrete sequence: SETATTR appears non-shrinking at the unlocked sample, concurrent append extends the file, then notify_change applies a real truncation under inode_lock without the required truncate permission check.

Negative or non-awarded signals:

* 0 RealUAF not awarded. This is not a lifetime or memory-corruption bug.
* 0 Controlled reclaim or arbitrary write not demonstrated. The primitive is semantic authorization bypass, not kernel memory corruption.
* 0 Confidentiality not awarded. No read or disclosure path is established.
* 0 Strong LPE not awarded. The patch supports unauthorized truncation of append-only files, not host privilege escalation.
* 0 Race penalty not applied. Although this is a TOCTOU race, the attacker can plausibly repeat SETATTR and append operations, and the prompt requires not subtracting merely because a race exists.
* 0 Extra availability not included in conservative score. Data loss or service impact may occur, but the direct established primitive is integrity bypass. The paranoid score accounts for stronger practical consequence without separately double-counting availability.

Repeated release/refcount checklist:

* First release can end lifetime: no such operation.
* Exact same reference survives: not applicable.
* Second operation mutates lifetime/refcount: not applicable.
* Concrete repeated release/refcount +1 awarded: no.
* Paranoid repeated-refcount leverage +1 awarded: no.
* FLOOR 5 applied: no.

3. Reachability analysis

The trigger is a remote NFS client issuing SETATTR size updates against an NFS export while a concurrent append extends the file before nfsd_setattr acquires inode_lock. The attacker normally needs valid NFS credentials or export-level access sufficient to operate on the target file. This is below host-admin-equivalent privilege, so HostAdminRequired=NO.

The relevant permission is not a namespace-scoped capability path in the supplied evidence. NamespaceScopedPrivilege=NO.

A prior host setup exists in the ordinary sense that an NFS server and export must exist, but that is the normal deployment context of NFSD and is not a special exploit-only prerequisite. No special hardware, rare execution mode, or kernel memory-corruption precondition is established.

The race is timing-dependent, but it is a repeated control-plane filesystem operation rather than a one-shot teardown race. The supplied description supports a practical repeatable race model.

4. Primitive and severity interpretation

The exact primitive is a TOCTOU authorization bypass in NFSD truncate permission handling. The old code checked whether NFSD_MAY_TRUNC was needed before inode_lock using an unlocked file size. The actual ATTR_SIZE change was later applied under inode_lock by notify_change. If the file grew in between, a request that was not a truncation at the first sample became a real truncation at application time.

This is not active structured UAF and not fixed stale storage. It is a semantic security-boundary violation in filesystem permission enforcement. The bypass changes an intended DENY condition for append-only truncation into an ALLOW condition.

PrimitiveClass=CLASS_A because a concrete kernel authorization decision bypass is established. The demonstrated consequence is unauthorized truncation of append-only file contents through NFS. The speculative consequence is broader operational impact if append-only files are logs, audit data, or service-critical files, but no kernel memory corruption or privilege escalation is proven.

ImportantCandidate=YES because this is a network-reachable filesystem authorization bypass affecting integrity semantics. AutoDowngradeAllowed=NO because automatic closure would discard a concrete security-boundary bypass, even though the score remains below Strong Important due to lack of LPE, credential theft, arbitrary write, or broader protected-resource exploit evidence.

Likely operational severity is actionable Moderate with Strong Important review sensitivity, especially for environments relying on append-only files over NFS for logging, audit, or data-integrity guarantees.

5. One-sentence report phrase

A TOCTOU flaw in NFSD SETATTR size handling can let a remote authenticated NFS client race append-only file growth and bypass the NFSD_MAY_TRUNC check, causing unauthorized truncation and data integrity loss.

6. Manual review recommendation

MANUAL CHECK REQUIRED

This is network reachable through NFS and is a concrete authorization and append-only integrity bypass, not a simple crash-only bug.

7. Call-site confidence

Call-site confidence: high

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

Patch: NFSD: check truncate permission under inode lock
Commit: 3afa17d93ba8c925f49370c816c6dae5112d8c24
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=3afa17d93ba8c925f49370c816c6dae5112d8c24

Commit description:

nfsd_setattr() checks whether a size update needs NFSD_MAY_TRUNC
before it takes inode_lock(). The comparison uses the file size sampled
by that unlocked read, but the actual ATTR_SIZE update is applied later
under inode_lock() by notify_change().

This leaves a TOCTOU window for append-only files. If a client sends a
SETATTR that does not shrink the file at the time of the unlocked
sample, a concurrent append can extend the file before nfsd_setattr()
takes inode_lock(). notify_change() then applies a real truncation
without the NFSD_MAY_TRUNC check that rejects IS_APPEND(inode). The VFS
truncate syscall paths perform their own append-only checks before
calling notify_change(), so NFSD must make this decision against the
locked size it is about to change.

Split the write-count acquisition from the truncation permission check.
Keep get_write_access() before the locked setattr work, then recheck
whether the requested size is below i_size_read(inode) after inode_lock()
has been acquired and before notify_change(ATTR_SIZE). This also avoids
the plain unlocked inode->i_size load.

Fixes: 783112f ("nfsd: special case truncates some more")
Cc: stable@vger.kernel.org
Assisted-by: kres:claude-opus-4-7
Reported-by: Chris Mason <clm@meta.com>
Signed-off-by: Jeff Layton <jlayton@kernel.org>
Link: https://patch.msgid.link/20260530-nfsd-fixes-v2-6-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=3afa17d93ba8c925f49370c816c6dae5112d8c24

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

The original Linux kernel CVE announcement for CVE-2026-89713 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89713

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:H/PR:L/UI:N/S:U/C:N/I:H/A:H
  The Best / paranoid CVSS score: 6.8

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).