[CVE-2026-52933][MODERATE 7.0] io_uring/poll: fix signed comparison in io_poll_get_ownership()

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

CVE: CVE-2026-52933
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: io_uring/poll: fix signed comparison in io_poll_get_ownership()
Commit: 81bf96b0abbfa4cd47ea32e12596aed3855fb2f3
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=81bf96b0abbfa4cd47ea32e12596aed3855fb2f3
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-52933
Analysis date: Wed, 24 Jun 2026 04:14:47 -0400
ActionableScore: 5
ActionableScore lower bound: 4
Actionable bucket: Actionable Moderate at minimum
Manual review required: YES

Summary:
A signed comparison bug in `io_poll_get_ownership()` can cause `IO_POLL_CANCEL_FLAG` to bypass the slowpath, creating a local `io_uring` poll cancellation race with request lifetime corruption risk and possible kernel crash.

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

The original Linux kernel CVE announcement for CVE-2026-52933 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-52933

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.

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

CVE-2026-52933	MODERATE	CHECK	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-681;CWE-362;*CWE-416;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '7';DESCR 'A signed comparison bug in io_poll_get_ownership can make poll_refs look negative when IO_POLL_CANCEL_FLAG is set, so the fast threshold check does not enter the slowpath. This can let the io_uring poll fastpath run while cancellation state is present, creating a race around poll request ownership and lifetime handling. For the CVSS the PR:L is used because a local unprivileged process may be able to create io_uring poll requests and trigger cancellation or completion races when io_uring is available. The issue is not network reachable and requires local code execution. Impact is at least a local denial of service via kernel crash and in worst case may allow confidentiality or integrity impact due to request lifetime corruption and should be reviewed manually.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate at minimum (with actual score 5)	YES	SIMPLEFIX IMPROVEONLY IO_URING KPANIC  KPANIC DECREASED_TO_MODERATE70_BASED_ON_ACTIONABLESCORELOWERTHAN7	YES	NO	checked

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

ActionableScore=5
ActionableScoreLower=4

## 1. ActionableScore

* Conservative score: **4**
* Paranoid score: **5**
* Final recommended bucket: **Actionable Moderate at minimum**

## 2. Signal breakdown

Conservative scoring:

* **Local unprivileged trigger: +1**
  `io_uring` is commonly reachable by local users unless disabled by sysctl, seccomp, or container policy.

* **Local unprivileged high-control kernel data-plane API: +1**
  `io_uring` gives users fine-grained control over request creation, polling, cancellation, and completion timing.

* **Weak or indirect corruption candidate: +1**
  The signed comparison can skip the slowpath when `IO_POLL_CANCEL_FLAG` is set, creating inconsistent ownership/refcount behavior. The patch suggests a request-state correctness bug, but not a demonstrated arbitrary write or reclaim primitive.

* **Real lifetime corruption concern: +1**
  The affected logic is in poll request ownership and cancellation/ref handling. Skipping the slowpath while a cancel flag is present can plausibly affect request lifetime handling.

* **Hard race / timing required: -1**
  Practical triggering likely depends on racing poll ownership, cancellation, and completion paths.

* **Availability impact realistic: +1**
  A broken request lifetime or ownership state in `io_uring` can plausibly lead to a kernel crash or hang, though the commit does not show a specific Oops trace.

Paranoid delta:

* **Weak LPE concern: +1**
  Because this is `io_uring` request lifetime and cancellation ownership logic, a local privilege escalation concern is technically plausible, but not demonstrated. This is not scored as strong LPE because the patch does not show attacker-controlled reclaim, overwrite, callback hijack, or type confusion.

## 3. Reachability analysis

A local user or process can potentially trigger the affected path by creating `io_uring` poll requests and racing cancellation or completion behavior. The bug is **not network reachable** by itself.

In typical deployments, `io_uring` may be available to unprivileged users, so **PR:L** is the practical assumption. Some hardened systems disable or restrict `io_uring` through `kernel.io_uring_disabled`, seccomp, container policy, or service sandboxing, which reduces exposure. Namespaces do not inherently remove the risk if the process can still use `io_uring`.

The realistic exploitation condition is a local timing race in a high-control kernel API. This supports manual review, but not an automatic Important classification without a concrete UAF reclaim or write primitive.

## 4. Severity interpretation

This behaves like an **actionable Moderate** issue, with an Important-class concern only under a paranoid interpretation.

Theoretical risk is higher because the bug affects `io_uring` poll ownership and request lifetime handling, which is historically sensitive. Realistic evidence from the patch supports a race/lifetime correctness issue and possible DoS. It does **not** directly demonstrate arbitrary write, controlled UAF reuse, type confusion, callback corruption, or a reliable privilege escalation primitive.

## 5. One-sentence report phrase

A signed comparison bug in `io_poll_get_ownership()` can cause `IO_POLL_CANCEL_FLAG` to bypass the slowpath, creating a local `io_uring` poll cancellation race with request lifetime corruption risk and possible kernel crash.

## 6. Manual review recommendation

**MANUAL CHECK REQUIRED**

Reason: this is local, non-network, and race-dependent, but it affects `io_uring` poll request ownership and cancellation lifetime logic. The patch does not prove strong exploitation, yet the subsystem and bug class are sensitive enough that auto-closing would be risky.

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

Patch: io_uring/poll: fix signed comparison in io_poll_get_ownership()
Commit: 81bf96b0abbfa4cd47ea32e12596aed3855fb2f3
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=81bf96b0abbfa4cd47ea32e12596aed3855fb2f3

Commit description:

io_poll_get_ownership() uses a signed comparison to check whether
poll_refs has reached the threshold for the slowpath:

    if (unlikely(atomic_read(&req->poll_refs) >= IO_POLL_REF_BIAS))

atomic_read() returns int (signed). When IO_POLL_CANCEL_FLAG
(BIT(31)) is set in poll_refs, the value becomes negative in
signed arithmetic, so the >= 128 comparison always evaluates to
false and the slowpath is never taken.

Fix this by casting the atomic_read() result to unsigned int
before the comparison, so that the cancel flag is treated as a
large positive value and correctly triggers the slowpath.

Fixes: a26a35e ("io_uring: make poll refs more robust")
Cc: stable@vger.kernel.org
Reported-by: Yifan Wu <yifanwucs@gmail.com>
Reported-by: Juefei Pu <tomapufckgml@gmail.com>
Co-developed-by: Yuan Tan <yuantan098@gmail.com>
Signed-off-by: Yuan Tan <yuantan098@gmail.com>
Suggested-by: Xin Liu <bird@lzu.edu.cn>
Tested-by: Zhengchuan Liang <zcliangcn@gmail.com>
Signed-off-by: Longxuan Yu <ylong030@ucr.edu>
Signed-off-by: Ren Wei <n05ec@lzu.edu.cn>
Reviewed-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://patch.msgid.link/3a3508b08bcd7f1bc3beff848ae6e1d73d355043.1775965597.git.ylong030@ucr.edu
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=81bf96b0abbfa4cd47ea32e12596aed3855fb2f3

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

The original Linux kernel CVE announcement for CVE-2026-52933 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-52933

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:N/I:N/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: 4
  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).