[CVE-2026-93228][MODERATE REGULAR] svcrdma: Reject Write/Reply chunks with segcount 0 [ Upstream

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

CVE: CVE-2026-93228
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: svcrdma: Reject Write/Reply chunks with segcount 0 [ Upstream
Commit: fe533ae1bed2ef2b3d54ee7ab410da1874ed3d4f
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=fe533ae1bed2ef2b3d54ee7ab410da1874ed3d4f
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-93228
Analysis date: Thu, 24 Sep 2026 12:28:49 -0400
ActionableScore: 2
ActionableScore lower bound: unknown
Actionable bucket: Ordinary Moderate
Manual review required: NO

Summary:
No one-sentence report phrase was found.

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

The original Linux kernel CVE announcement for CVE-2026-93228 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-93228

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 REGULAR
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-2026-93228	MODERATE	https://www.kernelcve.org/list/?q=CVE-2026-93228 CHECK WITH IMPACT FROM ORIG NN MODERATE	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:A/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:N';*CWE-20;CWE-1284;CWE-703;BEST CVSS score: '0';DESCR 'A malformed Write or Reply chunk with a zero segcount can pass the svcrdma decoder and be inserted into a parsed chunk list. The current code path is described as safe because an earlier change made pcl_for_each_segment safe for zero segment chunks, and this patch is primarily defense in depth. For the CVSS the PR:N value applies because a peer that can send RPC over RDMA traffic does not require local access on the target. The attack vector is adjacent network because NFS/RDMA traffic normally requires access to the RDMA fabric. No crash, information disclosure, integrity impact, memory corruption, or privilege escalation primitive is demonstrated by this patch.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate (with actual score 2)	YES	REMOTE WRITE OOB NETWORK HARDWARE  DECREASED_TO_MODERATE_REGULAR_FROM_MODERATE7_BASED_ON_GUESSCVSS	NO	NO	checked

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

ActionableScore=2
PrimitiveClass=NONE
RealUAF=NO
HostAdminRequired=NO
NamespaceScopedPrivilege=UNKNOWN
ImportantCandidate=NO
ManualReview=RECOMMENDED
AutoDowngradeAllowed=YES

1. ActionableScore

* Raw conservative score: 2
* Conservative score: 2
* Raw paranoid score: 2
* Paranoid score: 2
* Final recommended bucket: **Ordinary Moderate**::

The score reflects direct attacker-controlled protocol input reaching the NFS/RDMA parser. It does not establish a currently exploitable memory-safety or availability consequence.

2. Signal breakdown

Positive signals:

* +2 Directly network-triggerable parser input. An NFS/RDMA peer with access to the RDMA fabric can submit a malformed Write or Reply chunk with `segcount == 0`. This is adjacent-network exposure, not public-Internet reachability.

Non-awarded material signals:

* +0 Memory corruption. No UAF, OOB access, invalid dereference, stale pointer use, refcount misuse, or write primitive is established.
* +0 Reliable crash or DoS. The commit explicitly states that an earlier patch made `pcl_for_each_segment()` safe for zero-segment chunks, so this malformed input no longer drives the previous memory walk.
* +0 Confidentiality or integrity. No disclosure, unauthorized modification, or security-boundary bypass is demonstrated.
* +0 Strong or weak exploitation leverage. A zero-segment chunk is malformed parsed state, but no consumer that currently turns this state into a useful corruption primitive is shown.
* +0 Resource exhaustion. The patch does not show unbounded allocation, reference leakage, or accumulation.
* +0 Local unprivileged trigger. The relevant trigger is a network RDMA peer, not a local userspace interface.
* No privilege penalty applied. The attacker does not need host administrative privilege merely to send protocol traffic to an already exposed NFS/RDMA service.
* No niche-subsystem penalty applied. NFS/RDMA deployment is more limited than ordinary TCP networking, but the prompt does not permit a negative score solely because a subsystem is specialized.

3. Reachability analysis

The evaluated attacker is an NFS/RDMA client or other peer capable of sending RPC-over-RDMA traffic to the target server. Such a peer can reach `xdr_check_write_chunk()` with a malformed Write or Reply chunk containing a zero `segcount`.

The path is not Internet-generic. NFS/RDMA normally operates on a provisioned RDMA-capable storage fabric, and an attacker requires network access to that fabric and to an exposed NFS/RDMA server. However, this is not equivalent to requiring host root or host `CAP_SYS_ADMIN` on the target.

HostAdminRequired is `NO` because the complete malformed-input operation is performed by a remote peer and does not require local target administration. NamespaceScopedPrivilege is `UNKNOWN` because the supplied patch does not establish whether the relevant RDMA transport, NFS/RDMA listener, RDMA device, and complete peer connectivity can be provisioned by an attacker solely inside attacker-controlled namespaces.

Prior privileged setup is normally required to deploy the NFS/RDMA server and configure RDMA-capable infrastructure, but that setup is performed by the service operator and is not a privilege requirement for an attacker who already has fabric access.

4. Primitive and severity interpretation

The exact defect is acceptance of a protocol-invalid zero-segment Write or Reply chunk. The decoder previously accepted `segcount == 0` because it rejected only values above `rc_maxpages`. `xdr_inline_decode(stream, 0)` returns a valid current cursor without advancing it, and `pcl_alloc_write()` could append a `svc_rdma_chunk` with `ch_segcount == 0` to a parsed chunk list.

This is an input-validation and invalid-state issue. It is not an established UAF, OOB access, double free, refcount mutation, active-state lifetime violation, or ownership violation. `RealUAF=NO`.

The patch description explicitly states that an earlier patch already made `pcl_for_each_segment()` safe for zero-segment chunks. Therefore, the malformed chunk no longer drives the prior memory walk. Moving `list_add_tail()` after the segment-fill loop additionally prevents partially initialized chunks from becoming visible to list consumers, but the commit presents this as defense in depth against future regressions rather than remediation of a currently demonstrated corruption primitive.

The primitive class is `NONE`. There is no evidence-supported crash, disclosure, integrity impact, privilege escalation, or useful attacker-controlled post-parse behavior. ImportantCandidate is `NO`, and AutoDowngradeAllowed is `YES`.

The practical result is best treated as parser hardening or a low-priority malformed-input correctness fix. A manual source review is still recommended because the affected code is network-reachable on NFS/RDMA deployments and because older branches may lack the earlier `pcl_for_each_segment()` hardening change referenced by the commit.

5. One-sentence report phrase

A remote NFS/RDMA peer can submit a malformed zero-segment Write or Reply chunk, but the referenced earlier iterator fix makes the current path safe and this patch is defense in depth rather than a demonstrated memory-corruption vulnerability.

6. Manual review recommendation

MANUAL CHECK RECOMMENDED

Verify whether the assessed kernel branch includes the earlier `pcl_for_each_segment()` zero-segment safety fix. If that prerequisite fix is present, this patch has no demonstrated security impact and can be treated as hardening or auto-closed. If it is absent, inspect all consumers of `svc_rdma_chunk.ch_segments` and zero-segment parsed chunks before assigning a higher score.

7. Call-site confidence

Call-site confidence: medium

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

Patch: svcrdma: Reject Write/Reply chunks with segcount 0 [ Upstream
Commit: fe533ae1bed2ef2b3d54ee7ab410da1874ed3d4f
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=fe533ae1bed2ef2b3d54ee7ab410da1874ed3d4f

Changed files:
  net/sunrpc/xprtrdma/svc_rdma_pcl.c
  net/sunrpc/xprtrdma/svc_rdma_recvfrom.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=fe533ae1bed2ef2b3d54ee7ab410da1874ed3d4f

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

The original Linux kernel CVE announcement for CVE-2026-93228 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-93228

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:A/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:N
  The Best / paranoid CVSS score: 0

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: 2

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: NO
Published priority for this report (same as in Subject): MODERATE REGULAR

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).