[CVE-2026-52928][LOW] af_unix: Reject SIOCATMARK on non-stream sockets

[CVE-2026-52928][LOW] af_unix: Reject SIOCATMARK on non-stream sockets

[AL-KERNEL]

CVE: CVE-2026-52928
Priority: LOW
AL-KERNEL base severity: LOW
KPANIC flag: NO
Patch: af_unix: Reject SIOCATMARK on non-stream sockets
Commit: 645b1ed3259af38b7814242a420bc2081bdd1eb6
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=645b1ed3259af38b7814242a420bc2081bdd1eb6
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-52928
Analysis date: Wed, 24 Jun 2026 04:23:46 -0400
ActionableScore: 2
ActionableScore lower bound: unknown
Actionable bucket: Ordinary Moderate / Low-like
Manual review required: NO

Summary:
AF_UNIX SIOCATMARK incorrectly accepted non-stream sockets even though MSG_OOB is only supported for SOCK_STREAM, but the patch indicates an API validation fix with no demonstrated confidentiality, integrity, availability, or privilege escalation impact.

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

The original Linux kernel CVE announcement for CVE-2026-52928 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-52928

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: LOW
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-2026-52928	LOW	CHECK	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:N';*CWE-20;CWE-754;CWE-703;BEST CVSS score: '0';DESCR 'AF_UNIX SIOCATMARK could be called on SOCK_DGRAM or SOCK_SEQPACKET sockets even though AF_UNIX out of band data is supported only for SOCK_STREAM. This is an API consistency and missing socket type validation issue because non stream sockets already reject MSG_OOB in sendmsg and recvmsg. For the CVSS the PR:L value is used because triggering requires a local process that can create an AF_UNIX socket and call ioctl. The issue is not network reachable. No confidentiality integrity or availability impact is evident from the code path because the ioctl only reports mark state and the patch does not indicate memory corruption, crash, or privilege bypass.';NO MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate / Low-like (with actual score 2)	YES	REMOTE OOB SIMPLEFIX SKB  LOWERED_FROM_HIGH_BASED_ON_GUESSCVSS  DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN5 DECREASED_TO_LOW_FROM_MODERATE_BASED_ON_ACTIONABLESCORELESSTHAN3	YES	NO	checked

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

ActionableScore=2

## 1. ActionableScore

* Conservative score: 2
* Paranoid score: 2
* Final recommended bucket: **Ordinary Moderate / Low-like**::

## 2. Signal breakdown

* Local unprivileged trigger: +1. A local process can create an AF_UNIX SOCK_DGRAM or SOCK_SEQPACKET socket and call ioctl SIOCATMARK.
* Broad/default/common subsystem: +1. AF_UNIX sockets are a common local IPC mechanism and normally available on Linux systems.
* No memory corruption: +0. The patch only rejects an unsupported ioctl use before inspecting the receive queue.
* No reliable kernel crash / strong DoS: +0. The commit does not describe panic, Oops, WARN-based crash, stall, UAF, OOB access, or resource exhaustion.
* No confidentiality or integrity impact: +0. SIOCATMARK only reports urgent mark state, and MSG_OOB is unsupported for these socket types.

## 3. Reachability analysis

The issue is locally reachable by an unprivileged process through AF_UNIX socket creation and ioctl. It is not network reachable because AF_UNIX is local IPC. Namespaces do not materially increase severity, because the operation does not cross a meaningful security boundary and does not expose privileged objects. The path is broadly available, but the observable effect is only that an unsupported operation returned a result instead of EOPNOTSUPP.

## 4. Severity interpretation

This behaves like a validation/API consistency issue, not an actionable kernel vulnerability. There is no concrete exploit primitive, no memory lifetime issue, no write primitive, no sensitive disclosure, and no demonstrated DoS. Theoretical concern is minimal because the fixed code only adds a socket type check before receive queue inspection.

## 5. One-sentence report phrase

AF_UNIX SIOCATMARK incorrectly accepted non-stream sockets even though MSG_OOB is only supported for SOCK_STREAM, but the patch indicates an API validation fix with no demonstrated confidentiality, integrity, availability, or privilege escalation impact.

## 6. Manual review recommendation

NO MANUAL CHECK NEEDED

Reason: no memory corruption, no network reachability, no privilege escalation path, no reliable DoS, and no sensitive-object access primitive are supported by the commit or patch.

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

Patch: af_unix: Reject SIOCATMARK on non-stream sockets
Commit: 645b1ed3259af38b7814242a420bc2081bdd1eb6
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=645b1ed3259af38b7814242a420bc2081bdd1eb6

Commit description:

SIOCATMARK reports whether the receive queue is at the urgent mark for
MSG_OOB.

In AF_UNIX, MSG_OOB is supported only for SOCK_STREAM sockets.
SOCK_DGRAM and SOCK_SEQPACKET reject MSG_OOB in sendmsg() and recvmsg(),
so they should not support SIOCATMARK either.

Return -EOPNOTSUPP for non-stream sockets before checking the receive
queue.

Fixes: 314001f ("af_unix: Add OOB support")
Cc: stable@kernel.org
Reported-by: Yuan Tan <yuantan098@gmail.com>
Reported-by: Yifan Wu <yifanwucs@gmail.com>
Reported-by: Juefei Pu <tomapufckgml@gmail.com>
Reported-by: Xin Liu <bird@lzu.edu.cn>
Suggested-by: Kuniyuki Iwashima <kuniyu@google.com>
Signed-off-by: Jiexun Wang <wangjiexun2025@gmail.com>
Signed-off-by: Ren Wei <n05ec@lzu.edu.cn>
Reviewed-by: Kuniyuki Iwashima <kuniyu@google.com>
Link: https://patch.msgid.link/20260506140825.2987635-1-n05ec@lzu.edu.cn
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  net/unix/af_unix.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=645b1ed3259af38b7814242a420bc2081bdd1eb6

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

The original Linux kernel CVE announcement for CVE-2026-52928 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-52928

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:L/AC:L/PR:L/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: LOW
KPANIC detected for this report: NO
Published priority for this report (same as in Subject): LOW

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).