[CVE-2026-99133][LOW] net/iucv: fix the recvmsg window update

[CVE-2026-99133][LOW] net/iucv: fix the recvmsg window update

[AL-KERNEL]

CVE: CVE-2026-99133
Priority: LOW
AL-KERNEL base severity: LOW
KPANIC flag: NO
Patch: net/iucv: fix the recvmsg window update
Commit: 8f2efa742f4cfe7dd756154f8bc07350dbe3ec92
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=8f2efa742f4cfe7dd756154f8bc07350dbe3ec92
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-99133
Analysis date: Thu, 29 Oct 2026 00:19:02 -0400
ActionableScore: 2
ActionableScore lower bound: unknown
Actionable bucket: Ordinary Moderate
Manual review required: NO

Summary:
An unprivileged local AF_IUCV user can disconnect its own socket or corrupt HiperSockets receive-credit accounting through an incorrect window update and a concurrent-receive race.

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

The original Linux kernel CVE announcement for CVE-2026-99133 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-99133

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.

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AL-KERNEL CLASSIFICATION RESULT
======================================================================

CVE-2026-99133	LOW	https://www.kernelcve.org/list/?q=CVE-2026-99133 CHECK WITH IMPACT FROM ORIG NN LOW | AEGIS IMPACT LOW AEGIS_AS=1 LEGACY=LOW RESULT=SAME (final reconciliation agrees; DECISION: both LOW)	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H';CWE-362;CWE-667;*CWE-20;CWE-703;Other CVSS 'AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:L';BEST CVSS score: '4.7';DESCR 'AF_IUCV recvmsg could send a HiperSockets only window update on a classic z VM transport and cause a spurious disconnect of the calling process socket. A separate race allowed concurrent recvmsg operations to advertise the same receive credit twice and drive msg_recv negative. Calling the transmit path under message_q spinlock could also reach GFP_KERNEL allocation in a context treated as atomic. For the CVSS the PR:L value is used because an attacker needs local unprivileged process execution and access to the AF_IUCV socket interface. The issue is limited to s390 systems using z VM AF_IUCV or HiperSockets and is not directly network reachable. The demonstrated effects are socket availability loss and flow control corruption with no supported UAF, OOB access, information leak, or arbitrary write primitive. Automated impact decision path. The original SimpleNet classifier selected LOW. The primary LLM estimated paranoid CVSS 4.7 and the production ActionableScore returned 2 with recommendation Ordinary Moderate. The production rules then evaluated CVSS thresholds and ActionableScore thresholds. No severity changing production threshold or specialist override was recorded. The legacy KP specialist has no review artifact for this run. The legacy FD or DH downgrade specialist has no review artifact for this run. The independent weaker Aegis shadow evaluated CVSS 4.3 and ActionableScore 1 with primitive class CLASS_C and manual review REQUIRED. Its independently reconciled impact was LOW. The shadow agreed with the final production impact and aggregation did not change that impact. The final reported impact is LOW.';NO MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate (with actual score 2)	MAYBE	WARNONLY LEAK SKB LINUS TEST 	-	-	checked

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

ActionableScore=2
PrimitiveClass=CLASS_C
RealUAF=NO
HostAdminRequired=NO
NamespaceScopedPrivilege=NO
ImportantCandidate=NO
ManualReview=NO
AutoDowngradeAllowed=YES
RequiresHostUID0=NO
AuthorizationDecisionBypass=NO
ProtectedResourceImpact=NONE
B4PendingReview=NO
B4PendingConfirmed=NO

1. ActionableScore

* Raw base score: 2.
* Conservative score: 2; no floor or cap applied.
* Raw paranoid score: 2.
* Paranoid score: 2; the atomic-context allocation does not establish a kernel crash.
* Final recommended bucket: **Ordinary Moderate**::

2. Signal breakdown

* +1 Local unprivileged trigger: the supplied evidence establishes that an unprivileged process with AF_IUCV access can use `SO_MSGLIMIT=1` and call `recvmsg()` on its own socket.
* +1 Availability impact: the wrong-transport control send can spuriously disconnect that socket; concurrent receives can also duplicate advertised credit and corrupt flow-control accounting.
* +0 Race penalty: concurrent `recvmsg()` calls are attacker-drivable; no narrow or one-shot timing is shown.
* +0 Hardware/configuration penalty: s390/z/VM and AF_IUCV materially limit deployment exposure, but no qualifying rare physical-access requirement or attacker-inaccessible configuration is established.
* No UAF, OOB access, arbitrary write, disclosure, authorization bypass, kernel panic, or protected-state integrity impact is shown.
* No extra availability point: socket disconnection and flow-control disruption are the same limited availability consequence.

3. Reachability analysis

A local unprivileged process can create and operate its own AF_IUCV socket according to the supplied evidence. The issue is not directly remotely reachable. No capability, namespace privilege, host UID 0, prior host-admin setup, or special race beyond concurrent calls is shown. Deployment requires s390/z/VM AF_IUCV or HiperSockets support.

4. Primitive and severity reading

The demonstrated primitive is invalid protocol/flow-control state: a transport-inappropriate window update causes socket disconnection, while concurrent sends can subtract the same receive credit and drive `msg_recv` negative. This is CLASS_C availability behavior, not memory corruption. Moving transmission outside the spinlock also corrects unsafe GFP_KERNEL allocation context, but the patch does not prove a host crash. No B5-R, shared-backing write, or authorization-state preservation profile applies. ImportantCandidate is therefore NO and automatic Moderate handling remains allowed.

5. One-sentence report phrase

An unprivileged local AF_IUCV user can disconnect its own socket or corrupt HiperSockets receive-credit accounting through an incorrect window update and a concurrent-receive race.

6. Manual review advice

NO MANUAL CHECK NEEDED

The patch supports limited socket availability and accounting corruption without a material memory-corruption or security-boundary primitive.

7. Call-site confidence

Call-site confidence: high

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

Patch: net/iucv: fix the recvmsg window update
Commit: 8f2efa742f4cfe7dd756154f8bc07350dbe3ec92
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=8f2efa742f4cfe7dd756154f8bc07350dbe3ec92

Commit description:

iucv_sock_recvmsg() sends the HiperSockets-only AF_IUCV_FLAG_WIN without
testing the transport, so on a classic z/VM socket iucv_send_ctrl() sizes
the skb through a NULL iucv->hs_dev. SO_MSGLIMIT accepts 1, so msglimit / 2
is zero and one recvmsg() on its own socket is enough for an unprivileged
process to take a spurious disconnect.

It also calls iucv_send_ctrl() under spin_lock_bh(&message_q.lock), which
allocates GFP_KERNEL inside a section the code treats as atomic. Sending
outside that lock lets two recvmsg() reach afiucv_hs_send() at once, where
msg_recv is sampled for the advertised window and subtracted after
dev_queue_xmit() -- and sendmsg reaches that counter under lock_sock()
while recvmsg holds no socket lock, so both can subtract the same value,
the counter goes negative and the credit reaches the peer twice.

Test the transport, claim the credit with atomic_xchg() after the last
error exit and hand it back if the transmit fails, and send once the lock
is dropped.

Fixes: 3881ac4 ("af_iucv: add HiperSockets transport")
Fixes: 238965b ("net/af_iucv: build proper skbs for HiperTransport")
Cc: stable@vger.kernel.org
Tested-by: Aswin Karuvally <aswin@linux.ibm.com>
Signed-off-by: Bryam Vargas <hexlabsecurity@proton.me>
Reviewed-by: Alexandra Winter <wintera@linux.ibm.com>
Link: https://patch.msgid.link/20260828-b4-disp-33fac0ed-v3-1-e6d061880ee0@proton.me
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  net/iucv/af_iucv.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=8f2efa742f4cfe7dd756154f8bc07350dbe3ec92

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

The original Linux kernel CVE announcement for CVE-2026-99133 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-99133

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:H/PR:L/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS score: 4.7

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).