[CVE-2026-45983][MODERATE 7.0] nfsd: never defer requests during idmap lookup [ Upstream

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

CVE: CVE-2026-45983
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: nfsd: never defer requests during idmap lookup [ Upstream
Commit: b9abb760db20504240a7147f27934d900cd80b23
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=b9abb760db20504240a7147f27934d900cd80b23
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-45983
Analysis date: Wed, 27 May 2026 18:16:05 -0400
ActionableScore: 5
ActionableScore lower bound: 4
Actionable bucket: Actionable Moderate at minimum
Manual review required: NO

Summary:
No one-sentence report phrase was found.

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

The original Linux kernel CVE announcement for CVE-2026-45983 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-45983

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: NO

A detailed explanation of the methodology and priority rules is included
at the end of this message.

======================================================================
AL-KERNEL CLASSIFICATION RESULT
======================================================================

CVE-2026-45983	MODERATE	CHECK	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H';CWE-703;CWE-755;**CWE-400;CWE-667;Other CVSS 'AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:L';BEST CVSS score: '5.9';DESCR 'NFSv4 compound decoding can trigger idmap upcalls while RQ_USEDEFERRAL is still enabled. If the upcall response is delayed, cache_check() can mark the request for deferral and drop it before response encoding, leaving an NFSv4.1 session slot marked INUSE. For the CVSS the PR:N is used because a remote NFSv4 client that can reach the export can send requests that exercise idmap lookup paths, although reliable triggering depends on delayed idmap upcall handling. The issue is network reachable in NFSv4 server deployments. Impact is denial of service through leaked session slots and NFSERR_JUKEBOX for subsequent client requests, with no supported memory corruption or privilege escalation primitive.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate at minimum (with actual score 5) 	MAYBE	DISK LEAK NETWORK KPANIC 	-	-	checked

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

ActionableScore=5
ActionableScoreLower=4

1. ActionableScore
* Conservative score: 4
* Paranoid score: 5
* Final recommended bucket: **Actionable Moderate at minimum**::

2. Signal breakdown
* Remote reachable / network-triggerable: +2. A remote NFSv4 client can send compound operations that trigger idmap lookup paths.
* Reliable service DoS: +1. A deferred and dropped request can leave an NFSv4.1 session slot permanently marked `INUSE`, causing later requests to fail with `NFSERR_JUKEBOX`.
* Availability impact realistic: +1. The impact is client-visible denial of service through leaked session slots.
* Broad/common server exposure: +1. NFSv4 server deployments are common enough to warrant attention.
* Hard condition / special timing: -1. Reliable triggering depends on delayed idmap upcall responses beyond the allowed cache timeout.
* No memory corruption signal. The patch fixes request deferral/session-slot state handling, not UAF, OOB, arbitrary write, or info leak.

3. Reachability analysis
The issue is reachable by a remote NFSv4 client that can access the server and send operations such as `SETATTR` that require idmap lookup during compound argument decoding. No local privileges are required on the server. Practical exploitation depends on the ability to cause or coincide with delayed idmap upcalls, which may require environmental pressure, idmap service latency, or crafted identities that miss cache. Namespaces do not materially affect the remote server-side trigger.

4. Severity interpretation
This is a network-reachable availability issue, not a memory-corruption vulnerability. Conservative severity is borderline because the trigger depends on idmap delay. Paranoid severity is Actionable Moderate because the result can permanently consume NFSv4.1 session slots for a client and cause persistent service disruption without requiring local access.

5. One-sentence report phrase
A remote NFSv4 client can trigger idmap upcall deferral during compound decoding, causing dropped requests to leak NFSv4.1 session slots and produce persistent `NFSERR_JUKEBOX` denial of service.

6. Manual review recommendation
MANUAL CHECK RECOMMENDED. The bug is network reachable in NFSv4 server deployments and can cause persistent session-slot DoS, but no memory corruption, information disclosure, or privilege escalation is shown.

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

Patch: nfsd: never defer requests during idmap lookup [ Upstream
Commit: b9abb760db20504240a7147f27934d900cd80b23
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=b9abb760db20504240a7147f27934d900cd80b23

Changed files:
  fs/nfsd/nfs4idmap.c
  fs/nfsd/nfs4proc.c
  fs/nfsd/nfs4xdr.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=b9abb760db20504240a7147f27934d900cd80b23

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

The original Linux kernel CVE announcement for CVE-2026-45983 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-45983

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:N/UI:N/S:U/C:N/I:N/A:L
  The Best / paranoid CVSS vector: AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS score: 5.9

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: 4
  Paranoid ActionableScore: 5

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).