[CVE-2026-53393][MODERATE 7.0] nfsd: reset write verifier on deferred writeback errors

[CVE-2026-53393][MODERATE 7.0] nfsd: reset write verifier on deferred writeback errors

[AL-KERNEL]

CVE: CVE-2026-53393
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: nfsd: reset write verifier on deferred writeback errors
Commit: 1dd664b39774a9c89b72de8e59bf9ef4b3aaff2e
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=1dd664b39774a9c89b72de8e59bf9ef4b3aaff2e
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-53393
Analysis date: Sun, 19 Jul 2026 08:55:12 -0400
ActionableScore: 6
ActionableScore lower bound: 5
Actionable bucket: Actionable Moderate at minimum
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-2026-53393 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-53393

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-2026-53393	MODERATE	CHECK WITH IMPACT FROM ORIG NN IMPORTANT	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-703;CWE-754;CWE-345;Other CVSS 'AV:N/AC:H/PR:L/UI:N/S:U/C:N/I:H/A:L';BEST CVSS score: '6.8';DESCR 'An NFS server can fail to rotate its write verifier after a deferred writeback error detected by filemap_check_wb_err. A remote NFS client with UNSTABLE write data may later issue COMMIT, receive the unchanged verifier, and incorrectly conclude that data which failed writeback is durable. This can cause silent data loss and violates the UNSTABLE/COMMIT durability contract. For the CVSS the PR:L is used because reliable triggering normally requires write access to an exported NFS filesystem, not just packet reachability. The issue is network reachable through NFS service access, but practical exploitation usually depends on an internal or trusted storage network and on backend writeback errors such as I/O failure, ENOSPC, or similar deferred failures. Impact is primarily integrity loss and possible availability impact due to silent data loss, not kernel memory corruption or privilege escalation.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate at minimum (with actual score 6)	YES	WRITE OOB DISK SIMPLEFIX ERRORPATH NETWORK KPANIC  KPANIC DECREASED_TO_MODERATE70_BASED_ON_ACTIONABLESCORELOWERTHAN7	YES	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
* Remote reachable / network-triggerable: +2. The affected path is nfsd write and COMMIT handling, reachable by remote NFS clients with access to the export.
* Integrity impact plausible: +1. The bug can make clients believe failed writeback data is durable, causing silent data loss.
* Availability impact realistic: +1. Silent data loss can have persistent operational impact beyond a transient error.
* Important filesystem/storage path: +1. This affects NFS server writeback durability semantics and the UNSTABLE plus COMMIT contract.
* Broad/common storage exposure, paranoid only: +1. NFS servers are common in storage and data center environments, even if not exposed to the public Internet.
* No memory corruption signal. The patch fixes verifier state handling, not UAF, OOB, double free, arbitrary write, or type confusion.
* No privilege escalation signal. The issue is data durability and integrity, not LPE.

3. Reachability analysis
A remote NFS client can reach the affected logic if it has write access to an exported filesystem and uses UNSTABLE writes followed by COMMIT. The practical trigger also depends on deferred writeback errors such as backend I/O failure, ENOSPC, or similar storage-side failure conditions, so exploitation is not just packet reachability. Namespaces do not materially lower the needed privilege on the target kernel, because the relevant actor is an NFS client authorized to write to the export. NFS server deployment is common in storage environments, but usually internal or trusted-network exposed rather than Internet exposed.

Call-site confidence: high. The patch shows both affected call sites in nfsd_vfs_write and nfsd_commit.

4. Severity interpretation
This is not a memory corruption or privilege escalation bug. It behaves more like an actionable storage integrity vulnerability because it can silently violate NFS durability guarantees and cause clients to drop data that failed writeback. Conservative handling is Actionable Moderate. The paranoid score reaches the lower Important-candidate boundary only because the impact is persistent data integrity loss in a network reachable storage service, not because of kernel compromise.

5. One-sentence report phrase
An NFS server may fail to rotate the write verifier after deferred writeback errors, allowing authorized remote clients using UNSTABLE writes and COMMIT to incorrectly treat failed data as durable, causing silent data loss.

6. Manual review recommendation
MANUAL CHECK REQUIRED. The issue is network reachable in NFS deployments and affects persistent storage integrity, so it should not be auto-closed even though it is not memory corruption.

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

Patch: nfsd: reset write verifier on deferred writeback errors
Commit: 1dd664b39774a9c89b72de8e59bf9ef4b3aaff2e
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=1dd664b39774a9c89b72de8e59bf9ef4b3aaff2e

Commit description:

nfsd_vfs_write() and nfsd_commit() both call filemap_check_wb_err() to
detect deferred writeback errors, but neither rotates the server's write
verifier (nn->writeverf) when this check fails. Every other
durable-storage-failure path in these functions calls
commit_reset_write_verifier() before returning an error.

The missing rotation means clients holding UNSTABLE write data under the
current verifier will COMMIT, receive the unchanged verifier back, and
conclude their data is durable — silently dropping data that failed
writeback. This violates the UNSTABLE+COMMIT durability contract
(RFC 1813 §3.3.7, RFC 8881 §18.32).

Add commit_reset_write_verifier() calls at both filemap_check_wb_err()
error sites, matching the pattern used by adjacent error paths in the
same functions. The helper already filters -EAGAIN and -ESTALE
internally, so the calls are unconditionally safe.

Reported-by: Chris Mason <clm@meta.com>
Fixes: 555dbf1 ("nfsd: Replace use of rwsem with errseq_t")
Cc: stable@vger.kernel.org
Assisted-by: kres:claude-opus-4-6
Signed-off-by: Jeff Layton <jlayton@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=1dd664b39774a9c89b72de8e59bf9ef4b3aaff2e

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

The original Linux kernel CVE announcement for CVE-2026-53393 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-53393

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: 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).