[CVE-2026-99227][MODERATE 7.0] io_uring/waitid: prevent cancel reference wrap

[CVE-2026-99227][MODERATE 7.0] io_uring/waitid: prevent cancel reference wrap

[AL-KERNEL]

CVE: CVE-2026-99227
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: io_uring/waitid: prevent cancel reference wrap
Commit: 20b67288574c7c3df6b8ea7ed3b7af90a868006f
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=20b67288574c7c3df6b8ea7ed3b7af90a868006f
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-99227
Analysis date: Mon, 02 Nov 2026 22:38:01 -0400
ActionableScore: 5
ActionableScore lower bound: unknown
Actionable bucket: Actionable Moderate / Strong Important candidate requiring manual review
Manual review required: YES

Summary:
An unprivileged local process can accumulate failed io_uring WAITID cancellations until the ownership counter wraps, allowing erroneous double completion of an active request.

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

The original Linux kernel CVE announcement for CVE-2026-99227 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-99227

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 7.0
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-2026-99227	MODERATE	https://www.kernelcve.org/list/?q=CVE-2026-99227 CHECK WITH IMPACT FROM ORIG NN LOW | AEGIS IMPACT MODERATE_KPANIC AEGIS_AS=3 LEGACY=MODERATE_KPANIC RESULT=SAME (final reconciliation agrees; DECISION: both MODERATE_KPANIC)	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:H';CWE-190;CWE-911;*CWE-664;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:H';BEST CVSS score: '7';DESCR 'io_uring WAITID cancellation can repeatedly increment the 31 bit reference field after cancellation has already been requested. About 2 power 31 failed cancellation attempts while the request remains discoverable can wrap the field to zero, after which another cancellation may claim ownership and complete the request a second time. The concrete primitive is reference counter wrap followed by double completion. This may cause a kernel crash and can plausibly enter a use after free or double release path, although a reliable privilege escalation primitive is not established. For the CVSS the PR:L value is selected because an unprivileged local process must create and operate an io_uring request. AC:H reflects the very large attempt count and the requirement to preserve the vulnerable request state. The issue is not directly network reachable. Automated impact decision path. The original SimpleNet classifier selected LOW. The primary LLM estimated paranoid CVSS 7 and the production ActionableScore returned 5 with recommendation Actionable Moderate / Strong Important candidate requiring manual review. The production rules then evaluated CVSS thresholds and ActionableScore thresholds. Recorded rule transitions were CVSS and initial triage stage LOW to MODERATE_KPANIC because the ActionableScore threshold rule was applied. The legacy KP specialist produced a review artifact and its decision was processed by the applicable KP rules. The legacy FD or DH downgrade specialist has no review artifact for this run. Exact triggered rule markers were INCREASED_TO_MODERATE7_FROM_LOW_BASED_ON_GUESSCVSS_AND_ACTIONABLESCOREHIGHER4. The independent weaker Aegis shadow evaluated CVSS 2.7 and ActionableScore 3 with primitive class CLASS_B and manual review REQUIRED. Its independently reconciled impact was MODERATE_KPANIC. The shadow agreed with the final production impact and aggregation did not change that impact. The final reported impact is MODERATE_KPANIC.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate / Strong Important candidate requiring manual review (with actual score 5)	MAYBE	LEAK IO_URING  KPANIC INCREASED_TO_MODERATE7_FROM_LOW_BASED_ON_GUESSCVSS_AND_ACTIONABLESCOREHIGHER4	-	-	checked

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

ActionableScore=5
PrimitiveClass=CLASS_B
RealUAF=NO
HostAdminRequired=NO
NamespaceScopedPrivilege=NO
ImportantCandidate=YES
ManualReview=REQUIRED
AutoDowngradeAllowed=NO
RequiresHostUID0=NO
AuthorizationDecisionBypass=NO
ProtectedResourceImpact=NONE
B4PendingReview=NO
B4PendingConfirmed=NO

1. ActionableScore

* Raw base score: 4.
* Conservative score: 5. Manual-review safety floor raises 4 -> 5 because attacker-driven wrap corrupts active completion-ownership state that is later consumed.
* Raw paranoid score: 4.
* Paranoid score: 5. No additional speculative UAF/LPE point awarded.
* Final recommended bucket: **Actionable Moderate / Strong Important candidate requiring manual review**::

2. Signal breakdown

* +2 — Attacker-influenced active ownership corruption: repeated cancellation increments the 31-bit ownership/reference field until wrap permits erroneous ownership acquisition and double completion.
* +1 — Local unprivileged trigger: supplied evidence grounds creation and cancellation of the io_uring request in an unprivileged local process.
* +1 — Repeatable state manipulation: the same actor can repeatedly invoke cancellation while the request remains discoverable.
* +1 — Plausible kernel crash/strong DoS from double completion.
* -1 — Materially difficult trigger: roughly 2^31 failed cancellations while preserving the request state.
* Controlled reclaim, arbitrary write, disclosure, and LPE are not shown. The possible UAF/double release is not sufficient for RealUAF=YES.
* No extra Availability point is awarded for the same crash.
* B5-R does not apply: there is no confirmed heap OOB write. B3-PENDING and B4-PENDING are also inapplicable.

3. Reachability analysis

The supported actor is an ordinary local process operating its own io_uring request; host UID 0, host-admin capability, namespace capability, prior privileged setup, and special hardware are not required. The operation is local, not remotely reachable. The enormous retry count and need to keep the request discoverable materially increase complexity but do not negate the demonstrated wrap sequence.

4. Primitive and severity reading

The exact primitive is wraparound of active cancellation/completion ownership state followed by a second completion. This is CLASS_B lifetime/ownership corruption, not a confirmed UAF and not CLASS_A refcount takeover because controlled post-lifetime reuse or a directed corruption target is absent. The patch supports later kernel consumption of the corrupted state; crash is plausible, while UAF, double release, and privilege escalation remain unresolved. ImportantCandidate is YES and auto-downgrade is disallowed because a concrete attacker-driven active-state corruption reaches a privileged completion consumer.

5. One-sentence report phrase

An unprivileged local process can accumulate failed io_uring WAITID cancellations until the ownership counter wraps, allowing erroneous double completion of an active request.

6. Manual review advice

MANUAL CHECK REQUIRED

Review downstream completion and resource-release paths for concrete UAF, double release, or exploitable post-completion reuse.

7. Call-site confidence

Call-site confidence: medium

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

Patch: io_uring/waitid: prevent cancel reference wrap
Commit: 20b67288574c7c3df6b8ea7ed3b7af90a868006f
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=20b67288574c7c3df6b8ea7ed3b7af90a868006f

Commit description:

io_waitid packs the cancellation state into bit 31 of ->refs and uses
bits 30:0 as a 31-bit reference counter:

  #define IO_WAITID_CANCEL_FLAG BIT(31)
  #define IO_WAITID_REF_MASK    GENMASK(30, 0)

__io_waitid_cancel() marks a request canceled, then increments the low
bits of ->refs to claim completion ownership. When another path owns the
request, the increment remains even though cancellation returns false.
The request remains discoverable by cancellation, so repeated failed
attempts can wrap the 31-bit reference field to zero. A later
cancellation can claim ownership and complete the request a second time.

Use atomic_fetch_or() when setting the cancel flag and inspect the old
flag value. The first cancel attempt continues to the existing ownership
increment, preserving its wake and issuer handoff semantics. Later
attempts see the cancel flag and return without incrementing the
reference field.

This makes repeated failed cancellation attempts idempotent while
leaving the first-cancel state transitions unchanged.

Fixes: f31ecf6 ("io_uring: add IORING_OP_WAITID support")
Cc: stable@vger.kernel.org
Assisted-by: LLM
Signed-off-by: Junyeong Lee <jylee5498@gmail.com>
Link: https://patch.msgid.link/20261019152100.63565-1-jylee5498@gmail.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  io_uring/waitid.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=20b67288574c7c3df6b8ea7ed3b7af90a868006f

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

The original Linux kernel CVE announcement for CVE-2026-99227 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-99227

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:H/PR:L/UI:N/S:U/C:L/I:L/A:H
  The Best / paranoid CVSS vector: AV:L/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:H
  The Best / paranoid CVSS score: 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: 5

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: YES
Published priority for this report (same as in Subject): MODERATE 7.0

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).