[CVE-2026-46274][IMPORTANT] io-wq: check that the predecessor is hashed in io_wq_remove_pending()

[CVE-2026-46274][IMPORTANT] io-wq: check that the predecessor is hashed in io_wq_remove_pending()

[AL-KERNEL]

CVE: CVE-2026-46274
Priority: IMPORTANT
AL-KERNEL base severity: IMPORTANT
KPANIC flag: YES
Patch: io-wq: check that the predecessor is hashed in io_wq_remove_pending()
Commit: d6bda9df0c0a3080804181464d5c0f4d78a4e769
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=d6bda9df0c0a3080804181464d5c0f4d78a4e769
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-46274
Analysis date: Mon, 08 Jun 2026 10:52:52 -0400
ActionableScore: 9
ActionableScore lower bound: 8
Actionable bucket: Strong Important candidate / Actionable Moderate at minimum
Manual review required: YES

Summary:
A local io_uring user can trigger an io-wq use-after-free where hash_tail retains a freed io_kiocb pointer and a later hashed enqueue writes through freed memory, causing at least a kernel crash and plausibly enabling local privilege escalation.

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ABOUT THIS REPORT
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The original Linux kernel CVE announcement for CVE-2026-46274 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-46274

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: IMPORTANT
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-2026-46274	IMPORTANT	CHECK	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H';*CWE-416;CWE-825;*CWE-787;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:H';BEST CVSS score: '7.8';DESCR 'An io-wq use-after-free can occur when io_wq_remove_pending updates hash_tail for a hashed bucket-0 work item using a predecessor that is not hashed. A non-hashed predecessor can then be dequeued without clearing hash_tail, later freed back to req_cachep, and the next hashed bucket-0 enqueue can write through the dangling pointer. For the CVSS the PR:L because a local user or local process with access to io_uring is needed to create and cancel the relevant work items. The issue is not network reachable. Impact is at least local denial of service via kernel crash and may allow privilege escalation because the primitive is a kernel use-after-free with a write through freed memory.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate / Actionable Moderate at minimum (with actual score 9)	YES	DANGER UAF IMPROVEONLY IO_URING KPANIC 	YES	NO	guess

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ACTIONABLESCORE ANALYSIS
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ActionableScore=9
ActionableScoreLower=8

## 1. ActionableScore

* Conservative score: 8
* Paranoid score: 9
* Final recommended bucket: **Strong Important candidate / Actionable Moderate at minimum**::

## 2. Signal breakdown

Conservative signals:

* Local unprivileged trigger: +1. io_uring and io-wq work submission and cancellation are reachable by a local process when io_uring is enabled.
* Strong LPE plausibility: +2. The bug creates a dangling hash_tail pointer to a freed io_kiocb and later writes through it, which is stronger than a simple stale read or NULL dereference.
* Memory corruption, strong corruption primitive: +2. The commit explicitly describes use-after-free and write through freed memory.
* Real lifetime corruption: +1. The stale pointer survives after the non-hashed work is completed and freed back to req_cachep.
* Local unprivileged high-control kernel data-plane API: +1. io_uring gives local users fine-grained control over request lifetime and async work scheduling.
* Broad/default/common subsystem exposure: +1. io_uring is a common kernel interface, although some distributions restrict or disable it.

Paranoid additional signal:

* Reliable kernel crash / strong DoS: +1. Even if privilege escalation is not demonstrated, writing through freed kernel memory is very likely to produce at least a kernel crash under realistic triggering.

No subtraction applied:

* No admin or CAP_SYS_ADMIN requirement in typical io_uring-enabled environments.
* No remote/network reachability.
* No hard race penalty because the described stale-pointer sequence is state/lifetime dependent, but not merely a narrow timing-only race.

## 3. Reachability analysis

A local user or local process can plausibly trigger the bug by creating io_uring work items with mixed hashed and non-hashed states, cancelling a hashed bucket-0 tail work item, and later causing another hashed bucket-0 enqueue. The bug is not network reachable by itself. Containers and sandboxes matter because many deployments allow unprivileged io_uring from containerized workloads, while others restrict it using sysctls, seccomp, LSM policy, or distribution hardening. The path is not a rare hardware or legacy mode, but practical exposure depends on whether io_uring is available to untrusted users.

## 4. Severity interpretation

This behaves like an Important-class local kernel memory corruption candidate, not an ordinary Moderate DoS. The patch context shows a concrete lifetime bug: a stale pointer to a freed io_kiocb remains in wq->hash_tail[0] and is later used for a write through freed memory. Realistic exploitation evidence for full privilege escalation is not provided, so the conservative interpretation should avoid claiming reliable LPE. However, the primitive is strong enough that auto-closing as a low Moderate would be unsafe.

## 5. One-sentence report phrase

A local io_uring user can trigger an io-wq use-after-free where hash_tail retains a freed io_kiocb pointer and a later hashed enqueue writes through freed memory, causing at least a kernel crash and plausibly enabling local privilege escalation.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED.

This is a local kernel use-after-free with an explicit write-through-freed-memory primitive in io_uring/io-wq, so it should be manually reviewed even if no public exploit chain is shown.

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

Patch: io-wq: check that the predecessor is hashed in io_wq_remove_pending()
Commit: d6bda9df0c0a3080804181464d5c0f4d78a4e769
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=d6bda9df0c0a3080804181464d5c0f4d78a4e769

Commit description:

io_wq_remove_pending() needs to fix up wq->hash_tail[] if the cancelled
work was the tail of its hash bucket. When doing this, it checks whether
the preceding entry in acct->work_list has the same hash value, but
never checks that the predecessor is hashed at all. io_get_work_hash()
is simply atomic_read(&work->flags) >> IO_WQ_HASH_SHIFT, and the hash
bits are never set for non-hashed work, so it returns 0. Thus, when a
hashed bucket-0 work is cancelled while a non-hashed work is its list
predecessor, the check spuriously passes and a pointer to the non-hashed
io_kiocb is stored in wq->hash_tail[0].

Because non-hashed work is dequeued via the fast path in
io_get_next_work(), which never touches hash_tail[], the stale pointer
is never cleared. Therefore, after the non-hashed io_kiocb completes and
is freed back to req_cachep, wq->hash_tail[0] is a dangling pointer. The
io_wq is per-task (tctx->io_wq) and survives ring open/close, so the
dangling pointer persists for the lifetime of the task; the next hashed
bucket-0 enqueue dereferences it in io_wq_insert_work() and
wq_list_add_after() writes through freed memory.

Add the missing io_wq_is_hashed() check so a non-hashed predecessor
never inherits a hash_tail[] slot.

Cc: stable@vger.kernel.org
Fixes: 204361a ("io-wq: fix hang after cancelling pending hashed work")
Signed-off-by: Nicholas Carlini <nicholas@carlini.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  io_uring/io-wq.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=d6bda9df0c0a3080804181464d5c0f4d78a4e769

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

The original Linux kernel CVE announcement for CVE-2026-46274 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-46274

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:H/I:H/A:H
  The Best / paranoid CVSS vector: AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H
  The Best / paranoid CVSS score: 7.8

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: 8
  Paranoid ActionableScore: 9

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: IMPORTANT
KPANIC detected for this report: YES
Published priority for this report (same as in Subject): IMPORTANT

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).