[CVE-2026-99096][MODERATE 7.0] io_uring/register: don't exempt disabled rings from task restrictions

[CVE-2026-99096][MODERATE 7.0] io_uring/register: don't exempt disabled rings from task restrictions

[AL-KERNEL]

CVE: CVE-2026-99096
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: io_uring/register: don't exempt disabled rings from task restrictions
Commit: f55c240192ded562386de843bfa067671d2731d5
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=f55c240192ded562386de843bfa067671d2731d5
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-99096
Analysis date: Mon, 26 Oct 2026 22:12:47 -0400
ActionableScore: 5
ActionableScore lower bound: unknown
Actionable bucket: Actionable Moderate / Strong Important candidate requiring manual review
Manual review required: YES

Summary:
A local restricted task can use a disabled io_uring to execute registration operations forbidden by inherited task policy and can enable the ring despite that policy.

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ABOUT THIS REPORT
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The original Linux kernel CVE announcement for CVE-2026-99096 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-99096

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-2026-99096	MODERATE	https://www.kernelcve.org/list/?q=CVE-2026-99096 CHECK WITH IMPACT FROM ORIG NN MODERATE | 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:L/PR:L/UI:N/S:U/C:L/I:L/A:L';CWE-863;CWE-693;CWE-284;Other CVSS 'AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:L/A:L';BEST CVSS score: '5.3';DESCR 'A logic flaw in io_uring register restriction enforcement allows a locally executing task to bypass inherited per task register opcode restrictions by creating a ring with IORING_SETUP_R_DISABLED. The task can issue register operations that its policy prohibited and can enable the ring even when IORING_REGISTER_ENABLE_RINGS is not in the allowed set. For the CVSS the PR:L value is selected because exploitation requires a local task or process that can create and operate an io_uring instance. This is not directly network reachable. The demonstrated primitive is a security policy bypass rather than memory corruption, and the practical confidentiality, integrity, and availability effects depend on the prohibited opcode and resources already accessible to the task. Automated impact decision path. The original SimpleNet classifier selected MODERATE_KPANIC. The primary LLM estimated paranoid CVSS 5.3 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 MODERATE_KPANIC to MODERATE because the CVSS score, vector or bug class threshold rule was applied. ActionableScore thresholds stage MODERATE 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 DECREASED_TO_MODERATE_REGULAR_FROM_MODERATE7_BASED_ON_GUESSCVSS_AND_CIANNH and INCREASED_TO_MODERATE70_BASED_ON_ACTIONABLESCOREHIGHEREQTHAN5. The independent weaker Aegis shadow evaluated CVSS 0.0 and ActionableScore 3 with primitive class NONE 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	INIT SIMPLEFIX IO_URING  DECREASED_TO_MODERATE_REGULAR_FROM_MODERATE7_BASED_ON_GUESSCVSS_AND_CIANNH KPANIC INCREASED_TO_MODERATE70_BASED_ON_ACTIONABLESCOREHIGHEREQTHAN5	-	-	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=5
PrimitiveClass=CLASS_A
RealUAF=NO
HostAdminRequired=NO
NamespaceScopedPrivilege=NO
ImportantCandidate=YES
ManualReview=REQUIRED
AutoDowngradeAllowed=NO
RequiresHostUID0=NO
AuthorizationDecisionBypass=YES
ProtectedResourceImpact=UNRESOLVED
B4PendingReview=YES
B4PendingConfirmed=NO

1. ActionableScore

* Raw base score: 5.
* Conservative score: 5; no floor or cap applied.
* Raw paranoid score: 5.
* Paranoid score: 5; no additional downstream impact is sufficiently evidenced.
* Final recommended bucket: **Actionable Moderate / Strong Important candidate requiring manual review**::

2. Signal breakdown

* +2 Concrete authorization decision bypass: inherited register restrictions should deny disallowed opcodes, but the disabled-ring exemption skips `test_bit()` enforcement and changes DENY to ALLOW.
* +1 Local unprivileged trigger: supplied evidence grounds the complete sequence in a local task able to create and operate its own io_uring, without host-administrator powers.
* +1 Practical bypass chain: create disabled ring → inherit task restrictions → execute a prohibited register opcode, including potentially enabling the ring → avoid the intended restriction.
* +1 Integrity impact: the task can violate an enforced io_uring registration policy and alter ring registration state through an operation that policy prohibited.
* No memory corruption, UAF, crash, confidentiality disclosure, sensitive-resource access, or privilege escalation is shown.
* No extra protected-resource or broad fan-out points: the prohibited opcode and any materially sensitive downstream resource are not identified.

3. Reachability analysis

The attacker is a below-host-admin local task that can create an `IORING_SETUP_R_DISABLED` ring and invoke io_uring registration operations. No host UID 0, capability, namespace capability, prior privileged setup, special hardware, or race is shown as necessary. The patch directly supports ordinary task-local reachability, but it does not establish what protected resource, if any, a particular prohibited opcode exposes.

4. Primitive and severity reading

The primitive is a clean authorization-policy bypass, not memory corruption. The vulnerable condition exempts all disabled rings from inherited task restrictions; the fix records restriction inheritance and continues checking the allowed-opcode bitmap while disabled. This restores the mandatory guard across the disabled-to-enabled lifecycle transition.

`B4PendingReview=YES` because a credible below-host-admin DENY-to-ALLOW path is established. `B4PendingConfirmed=NO` because no meaningfully protected downstream resource or sensitive consumer is identified. Class A and ImportantCandidate reflect the concrete policy bypass, while Important severity, LPE, and high C/I/A impact remain unproven. Auto-downgrade is unsafe until the prohibited opcode set and consumers are reviewed.

5. One-sentence report phrase

A local restricted task can use a disabled io_uring to execute registration operations forbidden by inherited task policy and can enable the ring despite that policy.

6. Manual review advice

MANUAL CHECK REQUIRED

Determine which register opcodes can be prohibited by the inherited policy and whether bypassing them reaches protected files, buffers, credentials, workers, or other security-sensitive resources.

7. Call-site confidence

Call-site confidence: high

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UPSTREAM PATCH SUMMARY
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Patch: io_uring/register: don't exempt disabled rings from task restrictions
Commit: f55c240192ded562386de843bfa067671d2731d5
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=f55c240192ded562386de843bfa067671d2731d5

Commit description:

A ring created with IORING_SETUP_R_DISABLED is exempt from its register
restrictions until it is enabled. This lets the creator register the
restrictions and then enable the ring with IORING_REGISTER_ENABLE_RINGS,
whether or not that opcode is in the allowed set.

Since commit ed82f35 ("io_uring: allow registration of per-task
restrictions"), a ring also inherits restrictions from the task that
creates it, and the same exemption applies to those. A task whose
register opcodes are restricted can therefore create a disabled ring,
issue any register opcode on it, and then enable it.

Mark rings whose restrictions were inherited from the task, and keep
checking register opcodes for them while the ring is disabled. Enabling
such a ring now requires IORING_REGISTER_ENABLE_RINGS to be in the
task's allowed set. Per-ring restrictions registered on a disabled ring
work as before.

Fixes: ed82f35 ("io_uring: allow registration of per-task restrictions")
Cc: stable@vger.kernel.org
Assisted-by: Claude:claude-opus-5-5
Signed-off-by: Eugene Gvozdetsky <gvozdet@gmail.com>
Link: https://patch.msgid.link/20261009-iouring-task-restrict-fix-v1-1-a49daf55a12c@gmail.com
[axboe: amend with inheritance flag fixup]
Signed-off-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  include/linux/io_uring_types.h
  io_uring/io_uring.c
  io_uring/register.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=f55c240192ded562386de843bfa067671d2731d5

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

The original Linux kernel CVE announcement for CVE-2026-99096 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-99096

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:L/A:L
  The Best / paranoid CVSS vector: AV:L/AC:L/PR:L/UI:N/S:U/C:L/I:L/A:L
  The Best / paranoid CVSS score: 5.3

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).