[CVE-2026-99263][LOW] io_uring/poll: avoid retry wakeup loss when dropping ownership

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

CVE: CVE-2026-99263
Priority: LOW
AL-KERNEL base severity: LOW
KPANIC flag: NO
Patch: io_uring/poll: avoid retry wakeup loss when dropping ownership
Commit: d1f6850086ac6f8b0fa44fb19aa2a468c73ee071
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=d1f6850086ac6f8b0fa44fb19aa2a468c73ee071
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-99263
Analysis date: Tue, 03 Nov 2026 22:51:42 -0400
ActionableScore: 1
ActionableScore lower bound: unknown
Actionable bucket: Ordinary Moderate
Manual review required: NO

Summary:
A local io_uring poll race can lose a retry wakeup and temporarily stall a request until another wakeup occurs.

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

The original Linux kernel CVE announcement for CVE-2026-99263 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-99263

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-2026-99263	LOW	https://www.kernelcve.org/list/?q=CVE-2026-99263 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:L';CWE-362;CWE-667;BEST CVSS score: '2.5';DESCR 'A race in io_poll_check_events can lose a retry wakeup when IO_POLL_RETRY_FLAG is set after the owning task consumes the previous flag but before it completes the loop check. The affected poll request can then remain stalled until another unrelated wakeup occurs. For the CVSS the PR:L value is used because a local process must create and operate the io_uring poll request. AC:H reflects the need to hit a narrow concurrent wakeup window. The issue is not directly network reachable because remote traffic alone cannot create the required io_uring request, although traffic may provide readiness events for a socket already registered by a local process. The demonstrated impact is limited availability loss for the request or its application with no supported memory corruption, confidentiality loss, integrity loss, kernel crash, or privilege escalation primitive. Automated impact decision path. The original SimpleNet classifier selected LOW. The primary LLM estimated paranoid CVSS 2.5 and the production ActionableScore returned 1 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 3.7 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 1)	MAYBE	SIMPLEFIX LEAK IO_URING 	-	-	checked

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

ActionableScore=1
PrimitiveClass=NONE
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: 1 (+1 local unprivileged trigger, +1 limited availability impact, -1 narrow concurrent timing).
* Conservative score: 1; no floor or cap applies.
* Raw paranoid score: 1; no technically supported stronger primitive.
* Paranoid score: 1.
* Final recommended bucket: **Ordinary Moderate**::

2. Signal breakdown

* +1 Local unprivileged trigger: an ordinary local process can create and operate an io_uring poll request.
* +1 Availability impact: the lost retry wakeup can stall the affected request/application until an unrelated wakeup occurs.
* -1 Hard timing: the retry flag must be set between consumption of the prior flag and the loop’s reference check.
* No kernel crash, memory overwrite, UAF, persistent resource loss, authorization bypass, confidentiality impact, or integrity impact is shown.
* No extra availability point: only the same transient request stall is evidenced.
* NEW52/B5-R does not apply: there is no heap OOB write, accumulating retry index, or later consumption of corrupted structured state.
* NEW53 and B4-PENDING do not apply: neither shared-backing modification nor a DENY-to-ALLOW authorization change exists.

3. Reachability analysis

The trigger is a local io_uring user coordinating a poll request with concurrent readiness wakeups. No host UID 0, capability, namespace privilege, prior privileged setup, special hardware, or remote-only path is required. Remote traffic could supply readiness for an already registered socket but cannot independently create the vulnerable io_uring request. The race window is narrow and an unrelated wakeup resolves the stall.

4. Primitive and severity reading

The defect is an atomic synchronization/lost-wakeup race. `atomic_sub_return()` does not atomically verify that both the observed reference count and retry state remain unchanged; `atomic_cmpxchg()` closes that check-to-update window. This shows delayed request progress, not corrupted memory or security state. PrimitiveClass is NONE, ImportantCandidate is NO, and automatic downgrade is safe.

5. One-sentence report phrase

A local io_uring poll race can lose a retry wakeup and temporarily stall a request until another wakeup occurs.

6. Manual review advice

NO MANUAL CHECK NEEDED

The patch supports only a narrow, recoverable availability issue without a security primitive.

7. Call-site confidence

Call-site confidence: high

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

Patch: io_uring/poll: avoid retry wakeup loss when dropping ownership
Commit: d1f6850086ac6f8b0fa44fb19aa2a468c73ee071
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=d1f6850086ac6f8b0fa44fb19aa2a468c73ee071

Commit description:

If a wakeup sets IO_POLL_RETRY_FLAG between the owning task consuming
the existing instance of IO_POLL_RETRY_FLAG and the loop check
of io_poll_check_events(), then we stall on processing the request
until an unrelated wakeup comes along.

Using atomic_cmpxchg instead of atomic_sub_return to guarantee both the
count v and poll_refs are 0 and no wake-ups are missed.

Cc: stable@vger.kernel.org
Fixes: a26a35e ("io_uring: make poll refs more robust")
Signed-off-by: Juanlu Herrero <juanlu@fastmail.com>
Assisted-by: LLM
Suggested-by: Anand Natarajan <anand@meta.com>
Link: https://patch.msgid.link/20261013-pu-cmpxchg-v1-1-4ddc10fd8a03@fastmail.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  io_uring/poll.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=d1f6850086ac6f8b0fa44fb19aa2a468c73ee071

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

The original Linux kernel CVE announcement for CVE-2026-99263 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-99263

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:H/PR:L/UI:N/S:U/C:N/I:N/A:L
  The Best / paranoid CVSS score: 2.5

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: 1

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).