[CVE-2026-43502][MODERATE REGULAR] net/rds: handle zerocopy send cleanup before the message is queued

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

CVE: CVE-2026-43502
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: net/rds: handle zerocopy send cleanup before the message is queued
Commit: 21d70744e6d3bbf9293aa1ee6fba7c53ad75275e
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=21d70744e6d3bbf9293aa1ee6fba7c53ad75275e
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-43502
Analysis date: Thu, 21 May 2026 11:09:04 -0400
ActionableScore: 4
ActionableScore lower bound: 3
Actionable bucket: Borderline, manual review recommended
Manual review required: YES

Summary:
An RDS zerocopy send failure before socket queueing can use the wrong cleanup discriminator, leaving pinned page accounting or notifier state inconsistent and allowing local resource-exhaustion DoS on systems with AF_RDS zerocopy enabled.

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

The original Linux kernel CVE announcement for CVE-2026-43502 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-43502

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: 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-2026-43502	MODERATE	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:H';*CWE-772;*CWE-401;CWE-404;Other CVSS 'AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:L';BEST CVSS score: '5.5';DESCR 'An RDS zero copy send can fail after user pages were pinned but before the message is queued on a socket. The old purge logic inferred zero copy state from rm m_rs, so an unqueued message could be cleaned up with the wrong ownership model and leave pinned page accounting or notifier resources inconsistent. For the CVSS the PR:L is used because a local process may be able to reach the RDS sendmsg zero copy path when AF_RDS and zero copy support are available. The issue is not directly network reachable because the trigger is a local sendmsg failure path rather than packet receive processing. Impact is mainly denial of service through resource leak or pinned page accounting leak. No direct UAF, double free, arbitrary write, or privilege escalation primitive is shown by this patch.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline, manual review recommended (with actual score 4)  SKIP CVE-2026-43502  SKIP No affected files built, so skip this CVE NO  - - unknown 	MAYBE	SIMPLEFIX 	-	-	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=4
ActionableScoreLower=3

## 1. ActionableScore

* Conservative score: 3
* Paranoid score: 4
* Final recommended bucket: **Borderline, manual review recommended**::

## 2. Signal breakdown

* Local unprivileged trigger: +1
  A local process may reach the RDS `sendmsg()` zerocopy path if AF_RDS and zerocopy support are enabled and accessible.

* Local unprivileged high-control kernel data-plane API: +1
  The sender can influence iovecs, page pinning, and early failure paths around zerocopy transmit setup.

* Availability impact realistic: +1
  Incorrect cleanup of pinned page accounting and notifier state can lead to resource leakage or accounting inconsistency under repeated failed sends.

* Real lifetime or cleanup mismatch concern: +1 paranoid only
  The bug is in zerocopy pinned-page lifetime cleanup before queueing. It does not show a direct double free or UAF, but it is still a lifetime-sensitive path involving pinned user pages and notifier ownership.

* Rare/configuration-dependent subsystem: -1
  RDS is not commonly enabled on all systems, and practical triggering depends on AF_RDS plus zerocopy Tx availability.

* No strong memory corruption primitive: +0
  The patch does not show UAF, double free, arbitrary write, page-cache corruption, or controlled object reuse.

* No remote reachability: +0
  The trigger is local send setup failure, not packet receive processing.

## 3. Reachability analysis

The realistic trigger is a local process using RDS zerocopy send. Namespaces or containers matter only if AF_RDS sockets are exposed inside the container or the module is available. The affected path is not default exposed on many general-purpose systems, but it is reachable through ordinary local socket operations where RDS is enabled. Exploitation likely requires causing an early zerocopy send failure after pages are pinned but before the message is queued.

## 4. Severity interpretation

This behaves more like an actionable resource-lifetime cleanup bug than an Important-class memory corruption issue. The realistic impact is local denial of service through pinned page accounting or notifier resource leakage. The theoretical concern is that zerocopy pinned-page cleanup bugs are lifetime-sensitive, but this patch does not demonstrate a double put, UAF, arbitrary write, or privilege escalation primitive. It should not be auto-closed without review, but it is weaker than the related RDS double `put_page()` case.

## 5. One-sentence report phrase

An RDS zerocopy send failure before socket queueing can use the wrong cleanup discriminator, leaving pinned page accounting or notifier state inconsistent and allowing local resource-exhaustion DoS on systems with AF_RDS zerocopy enabled.

## 6. Manual review recommendation

MANUAL CHECK RECOMMENDED.

Manual review is recommended because this is a local pinned-page zerocopy cleanup bug in a networking path, but current evidence supports resource leak or accounting DoS rather than strong memory corruption or privilege escalation.

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

Patch: net/rds: handle zerocopy send cleanup before the message is queued
Commit: 21d70744e6d3bbf9293aa1ee6fba7c53ad75275e
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=21d70744e6d3bbf9293aa1ee6fba7c53ad75275e

Commit description:

A zerocopy send can fail after user pages have been pinned but before
the message is attached to the sending socket.

The purge path currently infers zerocopy state from rm->m_rs, so an
unqueued message can be cleaned up as if it owned normal payload pages.
However, zerocopy ownership is really determined by the presence of
op_mmp_znotifier, regardless of whether the message has reached the
socket queue.

Capture op_mmp_znotifier up front in rds_message_purge() and use it as
the cleanup discriminator. If the message is already associated with a
socket, keep the existing completion path. Otherwise, drop the pinned
page accounting directly and release the notifier before putting the
payload pages.

This keeps early send failure cleanup consistent with the zerocopy
lifetime rules without changing the normal queued completion path.

Fixes: 0cebacc ("rds: zerocopy Tx 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>
Co-developed-by: Xiao Liu <lx24@stu.ynu.edu.cn>
Signed-off-by: Xiao Liu <lx24@stu.ynu.edu.cn>
Signed-off-by: Nan Li <tonanli66@gmail.com>
Signed-off-by: Ren Wei <n05ec@lzu.edu.cn>
Reviewed-by: Allison Henderson <achender@kernel.org>
Link: https://patch.msgid.link/d2ea98a6313d5467bac00f7c9fef8c7acddb9258.1777550074.git.tonanli66@gmail.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  net/rds/message.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=21d70744e6d3bbf9293aa1ee6fba7c53ad75275e

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

The original Linux kernel CVE announcement for CVE-2026-43502 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-43502

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:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:L
  The Best / paranoid CVSS vector: AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS score: 5.5

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: 3
  Paranoid ActionableScore: 4

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).