[CVE-2026-64298][MODERATE 7.0] NFSv4: include MAY_WRITE in open permission mask for O_TRUNC

[CVE-2026-64298][MODERATE 7.0] NFSv4: include MAY_WRITE in open permission mask for O_TRUNC

[AL-KERNEL]

CVE: CVE-2026-64298
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: NFSv4: include MAY_WRITE in open permission mask for O_TRUNC
Commit: 4817c8974315b666e895b7d1bb83cd3664c323b1
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=4817c8974315b666e895b7d1bb83cd3664c323b1
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64298
Analysis date: Sat, 25 Jul 2026 06:10:32 -0400
ActionableScore: 6
ActionableScore lower bound: 5
Actionable bucket: Actionable Moderate at minimum
Manual review required: YES

Summary:
An NFSv4 client permission-check bypass can allow a local user to truncate a file without write permission when `open(O_RDONLY | O_TRUNC)` is served from a cached write delegation, causing unauthorized data modification and possible data loss.

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

The original Linux kernel CVE announcement for CVE-2026-64298 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64298

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-2026-64298	MODERATE	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:N/I:H/A:H';CWE-863;CWE-284;CWE-732;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:H/A:N';BEST CVSS score: '6.3';DESCR 'NFSv4 client authorization can be bypassed when open uses O_TRUNC with an access mode that is not write. The local permission mask was derived only from O_ACCMODE, so open(O_RDONLY O_TRUNC) could request only MAY_READ and pass the client side check when the OPEN is served from a cached write delegation. The truncation can then be sent to the server as SETATTR(size 0) over the delegation stateid, allowing a local user to truncate a file that should have required write permission. For the CVSS the PR:L is used because a local user or local process on the NFS client is required to trigger the open call, but administrative privileges are not required for the file operation itself. The issue is not directly reachable over the network by packet traffic. Impact is an authorization bypass with high integrity impact on file contents and possible availability impact if important data is truncated.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate at minimum (with actual score 6)	YES	DISK SIMPLEFIX NETWORK LINUS KPANIC 	NO	NO	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=6
ActionableScoreLower=5

## 1. ActionableScore

* Conservative score: 5
* Paranoid score: 6
* Final recommended bucket: **Actionable Moderate at minimum**

## 2. Signal breakdown

* Local unprivileged trigger: +1. A local process on the NFS client can issue `open(O_RDONLY | O_TRUNC)`.
* Security-boundary bypass with practical sensitive-object access: +2. The bug bypasses the intended write-permission check for truncation.
* Practical exploit chain strongly implied: +1. The commit describes the exact path: cached write delegation via `can_open_delegated()` and `nfs4_try_open_cached()`, then server-side `SETATTR(size=0)`.
* Integrity impact plausible: +1. A file may be truncated despite the caller lacking write permission.
* Important filesystem/storage path: +1. This affects NFSv4 open and truncation permission semantics.
* Hard or configuration-dependent condition: -1. The interesting case requires NFSv4 cached write delegation behavior, not just any local file open.
* Paranoid-only availability impact: +1. Truncating important files can cause persistent data loss or service disruption.

Call-site confidence: high. The commit message names the relevant call path and the patch changes the permission mask used by that path.

## 3. Reachability analysis

The trigger is local on the NFS client. Administrative privileges are not required for the `open()` call itself, but the attacker needs access to an affected NFSv4 mount and to a file where read access is allowed but write access should be denied. The issue is not directly network-triggerable by arbitrary packets. Containers or namespaces can matter if an untrusted workload has access to the mounted NFS filesystem, because the operation is just a normal file open from that workload. The key practical precondition is that the OPEN is satisfied locally from a cached write delegation.

## 4. Severity interpretation

This is not memory corruption and does not imply kernel code execution. It is an authorization bypass in filesystem semantics with direct integrity impact. Realistic exploitation depends on NFSv4 delegation state, but the described behavior is concrete enough that it should not be treated as an ordinary low-priority Moderate. It fits Actionable Moderate handling, with paranoid scoring reaching the lower edge of Important-like attention because unauthorized truncation can cause persistent data loss.

## 5. One-sentence report phrase

An NFSv4 client permission-check bypass can allow a local user to truncate a file without write permission when `open(O_RDONLY | O_TRUNC)` is served from a cached write delegation, causing unauthorized data modification and possible data loss.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED.

Reason: this is a real access-control bypass with direct file integrity impact, and impact depends on NFSv4 delegation behavior and deployment-specific sensitive files.

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

Patch: NFSv4: include MAY_WRITE in open permission mask for O_TRUNC
Commit: 4817c8974315b666e895b7d1bb83cd3664c323b1
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=4817c8974315b666e895b7d1bb83cd3664c323b1

Commit description:

POSIX requires write permission to truncate a file, so an open() that
specifies O_TRUNC must be authorized for write access regardless of the
O_ACCMODE access mode.

nfs_open_permission_mask() builds the access mask passed to
nfs_may_open(), which is the local authorization gate for OPENs the
client serves itself from a cached write delegation via the
can_open_delegated() path in nfs4_try_open_cached().  The mask is
derived from O_ACCMODE alone, so an open(O_RDONLY | O_TRUNC) against a
file the caller cannot write requests only MAY_READ and passes the
local check.  The OPEN is then satisfied locally and the truncation is
issued to the server as a SETATTR(size=0) over the delegation stateid,
which the server accepts under standard write-delegation semantics.
POSIX requires that this open fail with EACCES.

Include MAY_WRITE in the mask whenever O_TRUNC is set so the local
check matches the access the server would have enforced.

Suggested-by: Trond Myklebust <trondmy@kernel.org>
Fixes: af22f94 ("NFSv4: Simplify _nfs4_do_access()")
Cc: stable@vger.kernel.org
Signed-off-by: Benjamin Coddington <bcodding@hammerspace.com>
Signed-off-by: Anna Schumaker <anna.schumaker@hammerspace.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  fs/nfs/dir.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=4817c8974315b666e895b7d1bb83cd3664c323b1

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

The original Linux kernel CVE announcement for CVE-2026-64298 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64298

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:H/A:N
  The Best / paranoid CVSS vector: AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:H/A:H
  The Best / paranoid CVSS score: 6.3

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: 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: 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).