[CVE-2026-99266][MODERATE 7.0] io_uring/zcrx: fix buffer offsets in areas above 4GB

[CVE-2026-99266][MODERATE 7.0] io_uring/zcrx: fix buffer offsets in areas above 4GB

[AL-KERNEL]

CVE: CVE-2026-99266
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: io_uring/zcrx: fix buffer offsets in areas above 4GB
Commit: 9bfb315e2d0907fbcaf5d0453dd6d873c9aa8d5b
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=9bfb315e2d0907fbcaf5d0453dd6d873c9aa8d5b
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-99266
Analysis date: Tue, 03 Nov 2026 22:17:45 -0400
ActionableScore: 4
ActionableScore lower bound: unknown
Actionable bucket: Borderline / manual review recommended
Manual review required: YES

Summary:
A 32-bit offset truncation in io_uring zcrx completion handling can expose the wrong receive buffer, return an unowned buffer, and permanently strand the real buffer.

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

The original Linux kernel CVE announcement for CVE-2026-99266 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-99266

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
======================================================================

CVE-2026-99266	MODERATE	https://www.kernelcve.org/list/?q=CVE-2026-99266 CHECK WITH IMPACT FROM ORIG NN LOW | AEGIS IMPACT MODERATE_KPANIC AEGIS_AS=1 LEGACY=MODERATE_KPANIC RESULT=SAME V39_AEGIS_ONE_STEP_UP=MODERATE->MODERATE_KPANIC (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-190;CWE-681;CWE-682;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:L';BEST CVSS score: '5.3';DESCR 'A 32 bit shift in io_zcrx_queue_cqe truncates buffer offsets when an io_uring zcrx receive area extends above 4GB. The CQE can identify a lower buffer instead of the buffer containing the received data, causing the application to read stale or unrelated data, return a buffer it does not own, and permanently retain the real buffer. For the CVSS the PR:L value is used because triggering the vulnerable path requires a local process with an io_uring zcrx queue and a registered area above 4GB, even when network traffic supplies the received data. The demonstrated primitive is incorrect buffer selection and ownership accounting rather than an OOB access, UAF, or arbitrary kernel read or write. Confidentiality and integrity are Low because data and completion ownership can be confused between application requests. Availability is Low because repeated failures can reduce the usable buffer pool without evidence of a host wide kernel failure. Automated impact decision path. The original SimpleNet classifier selected LOW. The primary LLM estimated paranoid CVSS 5.3 and the production ActionableScore returned 4 with recommendation Borderline / manual review recommended. 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 manual review requirement was applied. KP and FD specialist checks stage MODERATE_KPANIC to MODERATE because KP specialist did not establish a qualifying panic. final reconciliation stage MODERATE to MODERATE_KPANIC because the cross model evidence reconciliation 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_FROM_LOW_BASED_ON_REQUIREMANUALCHECK and DECREASED_TO_MODERATEREG_BASED_ON_FALSEPOSCHECKOFKP and V39_AEGIS_ONE_STEP_UP and MANUAL_REVIEW_REQUIRED. The independent weaker Aegis shadow evaluated CVSS 4.6 and ActionableScore 1 with primitive class CLASS_B and manual review REQUIRED. Its independently reconciled impact was MODERATE_KPANIC. Cross model aggregation changed the production result under an enabled evidence gated merge rule. The final reported impact is MODERATE_KPANIC.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline / manual review recommended (with actual score 4)	MAYBE	SIMPLEFIX LEAK IO_URING  INCREASED_FROM_LOW_BASED_ON_REQUIREMANUALCHECK DECREASED_TO_MODERATEREG_BASED_ON_FALSEPOSCHECKOFKP KPANIC V39_AEGIS_ONE_STEP_UP MANUAL_REVIEW_REQUIRED	-	-	checked

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

ActionableScore=4
PrimitiveClass=CLASS_B
RealUAF=NO
HostAdminRequired=NO
NamespaceScopedPrivilege=UNKNOWN
ImportantCandidate=NO
ManualReview=RECOMMENDED
AutoDowngradeAllowed=YES
RequiresHostUID0=NO
AuthorizationDecisionBypass=NO
ProtectedResourceImpact=NONE
B4PendingReview=NO
B4PendingConfirmed=NO

1. ActionableScore

* Raw base score: 4.
* Conservative score: 4; no floor or cap applied.
* Raw paranoid score: 4.
* Paranoid score: 4; no additional technically supported primitive.
* Final recommended bucket: **Borderline / manual review recommended**::

2. Signal breakdown

* +1 Local below-host-admin trigger: supplied evidence identifies a local process using an io_uring zcrx queue and large registered area; host-admin powers or UID 0 are not required.
* +1 Weak/indirect ownership corruption: 32-bit shift truncation makes the CQE identify a different buffer, which can then be returned through the refill queue despite not being owned by that completion.
* +1 Confidentiality plausible: userspace is directed to stale or unrelated packet data. This is limited cross-request disclosure, not arbitrary kernel memory disclosure.
* +1 Concrete persistent resource loss: the actual receive buffer is never returned, reducing the usable pool.
* No OOB access, UAF, arbitrary write, controlled reclaim, protected cross-principal object, kernel crash, or LPE is shown.
* No extra availability point: pool degradation is shown, but host-wide exhaustion or a reliable kernel failure is not.
* NEW52/B5-R does not apply: there is no retry-accumulated heap OOB write.
* NEW53/B3-PENDING and NEW54/B4-PENDING do not apply.

3. Reachability analysis

The evidenced actor is a local process able to create the zcrx queue, register an area extending above 4GB, receive traffic, and process/refill buffers. The supplied summary supports below-host-admin reachability, but does not establish namespace-scoped capabilities or any namespace boundary. No special hardware, race, retry sequence, or prior host-admin configuration is shown.

4. Primitive and severity reading

The primitive is arithmetic truncation producing incorrect buffer identity and ownership accounting. The malformed CQE is consumed by userspace and the wrongly identified buffer can subsequently enter the kernel refill path. This is CLASS_B semantic/ownership confusion, not memory corruption or a real UAF. Effects shown are wrong-data exposure and buffer-pool loss; cross-user disclosure, kernel overwrite, host crash, and privilege escalation remain unsupported. ImportantCandidate is therefore NO, while manual review remains appropriate because active buffer ownership is confused.

5. One-sentence report phrase

A 32-bit offset truncation in io_uring zcrx completion handling can expose the wrong receive buffer, return an unowned buffer, and permanently strand the real buffer.

6. Manual review advice

MANUAL CHECK RECOMMENDED

Confirm zcrx setup permissions and whether buffers can cross protection domains or only requests within the same owning process.

7. Call-site confidence

Call-site confidence: medium

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

Patch: io_uring/zcrx: fix buffer offsets in areas above 4GB
Commit: 9bfb315e2d0907fbcaf5d0453dd6d873c9aa8d5b
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=9bfb315e2d0907fbcaf5d0453dd6d873c9aa8d5b

Commit description:

io_zcrx_queue_cqe() computes the offset of the buffer in its area from
net_iov_idx(), which is an unsigned int, so the shift is done in 32 bits
and only the result is widened to the u64 that goes into the CQE. An area
can be up to 1TB in size. For every niov beyond the first 4GB the CQE
points the application at a buffer 4GB, or a multiple of that, below the
one which has the data. It reads the wrong data, and then returns a
buffer it doesn't own via the refill queue while the real one is never
given back.

Fixes: 11ed914 ("io_uring/zcrx: add io_recvzc request")
Assisted-by: LLM
Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Link: https://patch.msgid.link/20261015-b4-io-uring-fixes-v1-35-c1556f5d7bf5@toxicpanda.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  io_uring/zcrx.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=9bfb315e2d0907fbcaf5d0453dd6d873c9aa8d5b

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

The original Linux kernel CVE announcement for CVE-2026-99266 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-99266

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: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: 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: 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).