[CVE-2026-53398][IMPORTANT] NFSD: Fix SECINFO_NO_NAME decode error cleanup

[CVE-2026-53398][IMPORTANT] NFSD: Fix SECINFO_NO_NAME decode error cleanup

[AL-KERNEL]

CVE: CVE-2026-53398
Priority: IMPORTANT
AL-KERNEL base severity: IMPORTANT
KPANIC flag: YES
Patch: NFSD: Fix SECINFO_NO_NAME decode error cleanup
Commit: 8836405abdc53ca3dd5fc68b2cf6f8f012fad011
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=8836405abdc53ca3dd5fc68b2cf6f8f012fad011
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-53398
Analysis date: Sun, 19 Jul 2026 09:03:03 -0400
ActionableScore: 9
ActionableScore lower bound: 7
Actionable bucket: Strong Important candidate / Actionable Moderate at minimum
Manual review required: YES

Summary:
A remote NFSv4 client can trigger stale `sin_exp` cleanup after a truncated SECINFO_NO_NAME decode, causing an erroneous `exp_put()` on reused operation storage and creating a remote DoS with plausible UAF style lifetime corruption.

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ABOUT THIS REPORT
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The original Linux kernel CVE announcement for CVE-2026-53398 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-53398

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
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CVE-2026-53398	IMPORTANT	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:N/UI:N/S:U/C:H/I:H/A:H';*CWE-416;*CWE-457;*CWE-672;Other CVSS 'AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:L/A:H';BEST CVSS score: '9.8';DESCR 'NFSD can release a stale sin_exp pointer when SECINFO_NO_NAME decoding fails before sin_exp is initialized. A remote NFSv4 client can trigger the bad XDR cleanup path with a truncated SECINFO_NO_NAME operation after previous RPC state left stale union contents in the reused inline operation array. This can cause an erroneous exp_put on a stale export pointer, leading at least to a remote denial of service and plausibly to use-after-free style memory corruption through reference count imbalance. For the CVSS the PR:N is used because the trigger is a network RPC request to the NFS server and does not require local code execution on the target. NFS servers are usually deployed on internal storage or data center networks rather than the public Internet, so practical exploitation depends on network reachability to NFSD.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate / Actionable Moderate at minimum (with actual score 9)	YES	DISK SIMPLEFIX NETWORK KPANIC  KPANIC INCREASED_TO_HIGH_BASED_ON_GUESSCVSS	YES	NO	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=9
ActionableScoreLower=7

## 1. ActionableScore

* Conservative score: 7
* Paranoid score: 9
* Final recommended bucket: **Strong Important candidate / Actionable Moderate at minimum**::

## 2. Signal breakdown

Conservative scoring:

* Remote reachable / network-triggerable: +2. The affected path is NFSD NFSv4 XDR decoding and can be reached by a remote NFSv4 client that can send a malformed or truncated SECINFO_NO_NAME operation.
* Memory corruption, weak/indirect corruption candidate: +1. The stale union field can be interpreted as a stale `sin_exp` pointer, but the patch does not demonstrate attacker-controlled reclaim or object replacement.
* Real lifetime corruption: +1. The cleanup path can call `exp_put()` on stale contents, causing an export reference count imbalance and possible stale object lifetime handling.
* Weak LPE concern: +1. Refcount misuse on a kernel export object can theoretically become more than DoS, but no concrete overwrite, callback control, or reclaim primitive is shown.
* Reliable kernel crash / strong DoS: +1. A bad `exp_put()` on stale or invalid data can plausibly crash the kernel.
* Important filesystem/storage path: +1. NFSD export lifetime handling is part of a remote filesystem service and affects trusted server-side storage state.

Paranoid scoring delta:

* Treat stale `exp_put()` as a stronger lifetime corruption primitive: +1 additional over the conservative weak corruption classification.
* Confidentiality impact plausible: +1. If the refcount imbalance leads to UAF rather than only a crash, object reuse could theoretically expose kernel or filesystem service state.
* Integrity impact plausible: +1. If the stale export object lifetime is corrupted, subsequent NFSD operations could theoretically act on corrupted export state.

Not counted:

* No Dirty-Pipe-like primitive is visible.
* No arbitrary write or attacker-controlled object replacement is demonstrated.
* No race penalty applies because this is a deterministic decode error cleanup path, not a timing race.

## 3. Reachability analysis

A remote NFSv4 client can plausibly trigger the bug by sending a truncated SECINFO_NO_NAME operation after previous RPC state has left stale union contents in the reused inline operation array. PR:N is appropriate for the scoring model because the trigger is a network RPC request to the NFS server and does not require local code execution on the target host. Practical exposure depends on whether NFSD is enabled and reachable. NFS servers are commonly deployed in internal storage, virtualization, and data center networks, but are usually not intentionally exposed to the public Internet. Namespaces do not materially reduce the server-side impact once the RPC reaches host NFSD.

Call-site confidence: high. The commit explicitly identifies the failing decode path, the stale union field, the error cleanup path, and the `nfsd4_secinfo_no_name_release()` call to `exp_put()`.

## 4. Severity interpretation

This should not be treated as an ordinary Moderate issue. The realistic minimum impact is remote DoS against an NFSD server. The security-relevant concern is stronger than a simple NULL pointer dereference because the stale field is passed to `exp_put()`, which can corrupt export object lifetime accounting. Theoretical escalation beyond DoS is not proven by the patch, but the stale pointer plus refcount release pattern is sufficient for Important-candidate manual review.

## 5. One-sentence report phrase

A remote NFSv4 client can trigger stale `sin_exp` cleanup after a truncated SECINFO_NO_NAME decode, causing an erroneous `exp_put()` on reused operation storage and creating a remote DoS with plausible UAF style lifetime corruption.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

The bug is network reachable and involves stale pointer cleanup plus refcount lifetime corruption in NFSD, so it should not be auto-closed even though a practical privilege-escalation primitive is not demonstrated.

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

Patch: NFSD: Fix SECINFO_NO_NAME decode error cleanup
Commit: 8836405abdc53ca3dd5fc68b2cf6f8f012fad011
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=8836405abdc53ca3dd5fc68b2cf6f8f012fad011

Commit description:

nfsd4_decode_secinfo_no_name() currently initializes sin_exp after
decoding sin_style. If the XDR stream is truncated, the decoder returns
nfserr_bad_xdr before sin_exp is initialized.

Since commit 3fdc546 ("NFSD: Reduce amount of struct
nfsd4_compoundargs that needs clearing"), the inline iops array is not
cleared between RPC calls. A failed SECINFO_NO_NAME decode can therefore
leave sin_exp holding stale union contents from a previous operation.

The error response path still invokes nfsd4_secinfo_no_name_release(),
which calls exp_put() on a non-NULL sin_exp.

Initialize sin_exp before the first failable decode step, matching
nfsd4_decode_secinfo().

Fixes: 3fdc546 ("NFSD: Reduce amount of struct nfsd4_compoundargs that needs clearing")
Cc: stable@vger.kernel.org
Signed-off-by: Guannan Wang <wgnbuaa@gmail.com>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  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=8836405abdc53ca3dd5fc68b2cf6f8f012fad011

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

The original Linux kernel CVE announcement for CVE-2026-53398 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-53398

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:L/PR:N/UI:N/S:U/C:L/I:L/A:H
  The Best / paranoid CVSS vector: AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H
  The Best / paranoid CVSS score: 9.8

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: 7
  Paranoid ActionableScore: 9

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).