[CVE-2026-53391][MODERATE 7.0] NFSv4/pNFS: reject zero-length r_addr in nfs4_decode_mp_ds_addr

[CVE-2026-53391][MODERATE 7.0] NFSv4/pNFS: reject zero-length r_addr in nfs4_decode_mp_ds_addr

[AL-KERNEL]

CVE: CVE-2026-53391
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: NFSv4/pNFS: reject zero-length r_addr in nfs4_decode_mp_ds_addr
Commit: 76b94cbd32aacf36a641956385a852635c6802b9
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=76b94cbd32aacf36a641956385a852635c6802b9
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-53391
Analysis date: Sun, 19 Jul 2026 11:06:13 -0400
ActionableScore: 5
ActionableScore lower bound: unknown
Actionable bucket: Actionable Moderate at minimum
Manual review required: YES

Summary:
A malicious or compromised NFSv4 pNFS metadata server can return a zero-length r_addr or r_netid in GETDEVICEINFO, causing nfs4_decode_mp_ds_addr() to dereference NULL and crash the client kernel.

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

The original Linux kernel CVE announcement for CVE-2026-53391 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-53391

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-2026-53391	MODERATE	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:N/I:N/A:H';**CWE-476;*CWE-20;CWE-754;BEST CVSS score: '7.5';DESCR 'A NULL pointer dereference in the NFSv4 pNFS client path can occur because nfs4_decode_mp_ds_addr accepts zero length r_netid or r_addr values returned in a GETDEVICEINFO multipath data server response. xdr_stream_decode_string_dup can return 0 while leaving the decoded string pointer NULL, and the next parsing step may call strrchr on that NULL pointer. A malicious or compromised NFS metadata server can trigger this against a pNFS flexfile client by returning a malformed GETDEVICEINFO body, causing a kernel crash on the client. For the CVSS the PR:N because the attacker does not need local privileges on the victim host and only needs control of or network position over the metadata server endpoint used by the client. The issue is network reachable in NFS deployments, although NFS and pNFS are typically deployed inside trusted storage or data center networks rather than exposed to the public Internet. Impact is denial of service only because the patch supports a NULL pointer dereference and does not show a UAF, OOB write, information leak, or arbitrary write primitive.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate at minimum (with actual score 5)	YES	DANGER NULLPTR DISK SIMPLEFIX NETWORK LINUS IPV6 KPANIC  KPANIC INCREASED_TO_MODERATE7_BASED_ON_GUESSCVSS	NO	NO	checked

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

ActionableScore=5

## 1. ActionableScore

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

## 2. Signal breakdown

* Remote reachable / network-triggerable: +2
  A malicious or compromised NFS metadata server can return a malformed GETDEVICEINFO response to a mounted pNFS flexfile client.

* Reliable kernel crash / strong DoS: +1
  Zero-length r_addr or r_netid can leave the decoded pointer NULL, followed by strrchr(NULL, '.') in kernel context.

* Availability impact realistic: +1
  The commit explicitly describes client panic rather than a harmless validation failure.

* Important filesystem/storage path: +1
  This is in the NFSv4/pNFS client storage path and affects handling of server-provided layout/device information.

Not counted:

* Generic memory corruption: +0
  The patch supports NULL pointer dereference only, not UAF, OOB write, double free, refcount misuse, or type confusion.

* Privilege escalation plausible: +0
  No reclaim, overwrite, callback, object replacement, or arbitrary write primitive is shown.

* Confidentiality / integrity impact: +0
  No information disclosure or data modification primitive is evident.

## 3. Reachability analysis

The bug is triggerable by a malicious or compromised NFS metadata server, or by an attacker able to tamper with the server response path. The victim must be an NFSv4 pNFS flexfile client mounted against that server. No local privileges are required on the victim host, but practical exposure is usually limited to storage or data-center networks rather than the public Internet. Containers and namespaces do not materially reduce the attacker requirement unless they can cause the host to mount or communicate with an attacker-controlled pNFS server.

Call-site confidence: high, because the commit describes the exact decode path and the immediate NULL dereference sink.

## 4. Severity interpretation

This behaves like an actionable Moderate remote DoS issue, not an Important-class memory corruption issue. The network-triggerable crash makes it unsuitable for automatic low-priority closure, but the demonstrated primitive is a NULL pointer dereference only. There is no evidence of UAF, attacker-controlled memory reuse, arbitrary write, info leak, or privilege escalation.

## 5. One-sentence report phrase

A malicious or compromised NFSv4 pNFS metadata server can return a zero-length r_addr or r_netid in GETDEVICEINFO, causing nfs4_decode_mp_ds_addr() to dereference NULL and crash the client kernel.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: the issue is network-triggerable from a malicious or compromised storage server and can crash the kernel client, but review should confirm deployment exposure because impact is DoS-only and limited to pNFS-capable NFS client configurations.

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

Patch: NFSv4/pNFS: reject zero-length r_addr in nfs4_decode_mp_ds_addr
Commit: 76b94cbd32aacf36a641956385a852635c6802b9
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=76b94cbd32aacf36a641956385a852635c6802b9

Commit description:

nfs4_decode_mp_ds_addr() decodes the r_netid and r_addr opaques of a
netaddr4 from a GETDEVICEINFO multipath-DS body, then immediately
calls strrchr(buf, '.') to locate the port separator. Both decodes
use xdr_stream_decode_string_dup(), and the current code checks only
"nlen < 0" / "rlen < 0" before dereferencing the returned string.

When the on-wire opaque has length zero, xdr_stream_decode_opaque_inline()
returns 0 and xdr_stream_decode_string_dup() falls through to its
"*str = NULL; return ret" tail, leaving buf NULL with a return value
of 0. The "< 0" check does not catch this, and the next line is
strrchr(NULL, '.'), a kernel NULL pointer dereference reachable from
any pNFS-flexfile client mounted against a malicious or compromised
metadata server.

Reject the zero-length cases explicitly so the decoder fails with
-EBADMSG (treated as a malformed GETDEVICEINFO body) instead of
panicking the client.

Cc: stable@vger.kernel.org
Fixes: 6b7f3cf ("nfs41: pull decode_ds_addr from file layout to generic pnfs")
Assisted-by: Claude:claude-opus-4-7
Signed-off-by: Michael Bommarito <michael.bommarito@gmail.com>
Signed-off-by: Anna Schumaker <anna.schumaker@hammerspace.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  fs/nfs/pnfs_nfs.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=76b94cbd32aacf36a641956385a852635c6802b9

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

The original Linux kernel CVE announcement for CVE-2026-53391 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-53391

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: Not available
  The Best / paranoid CVSS vector: AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS score: 7.5

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: unknown
  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).