[CVE-2026-99253][LOW] io_uring/cancel: rearm local work wake up before sleep

[CVE-2026-99253][LOW] io_uring/cancel: rearm local work wake up before sleep

[AL-KERNEL]

CVE: CVE-2026-99253
Priority: LOW
AL-KERNEL base severity: LOW
KPANIC flag: NO
Patch: io_uring/cancel: rearm local work wake up before sleep
Commit: aaa06bb9fc03384f7cf2dbc3e43f2939cc7791b2
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=aaa06bb9fc03384f7cf2dbc3e43f2939cc7791b2
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-99253
Analysis date: Tue, 03 Nov 2026 23:08:57 -0400
ActionableScore: 1
ActionableScore lower bound: unknown
Actionable bucket: Ordinary Moderate
Manual review required: NO

Summary:
A missed io_uring local-work wakeup can race with cancellation and indefinitely stall an unprivileged process in execve or exit.

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

The original Linux kernel CVE announcement for CVE-2026-99253 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-99253

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: LOW
Manual review required: NO

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-2026-99253	LOW	https://www.kernelcve.org/list/?q=CVE-2026-99253 CHECK WITH IMPACT FROM ORIG NN IMPORTANT | AEGIS IMPACT MODERATE_KPANIC AEGIS_AS=1 LEGACY=LOW RESULT=DIFF TEMP_DISTANCE_MERGE_SKIP=AEGIS_NO_POSITIVE_SEVERITY_EVIDENCE (final Legacy and Aegis differ; FINAL: LOW, Aegis: 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:N/I:N/A:H';CWE-362;CWE-667;*CWE-833;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:L';BEST CVSS score: '4.7';DESCR 'A missed wakeup race in io_uring cancellation can occur when local work consumes cq_wait_nr before the cancelling task enters TASK_INTERRUPTIBLE. If the queued work is completed before the pending check and new work is added afterward, the task can sleep without another wakeup and remain stuck in execve or exit. For the CVSS the PR:L value is selected because a local process and its io_uring execution context are required to reach the affected cancellation path. AC:H reflects the required race window. The demonstrated primitive is a task stall with no evidence of UAF, OOB access, information disclosure, or writable memory corruption. Base availability impact is low. The paranoid availability impact is high only if repeated triggering can accumulate permanently stuck exiting tasks and retain enough resources to affect the wider host. Automated impact decision path. The original SimpleNet classifier selected IMPORTANT. The primary LLM estimated paranoid CVSS 4.7 and the production ActionableScore returned 1 with recommendation Ordinary Moderate. The production rules then evaluated CVSS thresholds and ActionableScore thresholds. Recorded rule transitions were CVSS and initial triage stage IMPORTANT to MODERATE_KPANIC because the CVSS score, vector or bug class threshold rule was applied. ActionableScore thresholds stage MODERATE_KPANIC to LOW because the ActionableScore threshold rule was applied. final reconciliation stage LOW to LOW because KP specialist did not establish a qualifying panic and the cross model evidence reconciliation rule was applied. The legacy KP specialist has no review artifact for this run. The legacy FD or DH downgrade specialist has no review artifact for this run. Exact triggered rule markers were LOWERED_FROM_HIGH_BASED_ON_GUESSCVSS and DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN4 and DECREASED_TO_LOW_FROM_MODERATE_BASED_ON_ACTIONABLESCORELESSTHAN3 and KPANIC_UNSUPPORTED_REMOVED and V64_AEGIS_UNSUPPORTED_PROMOTION_BLOCKED. The independent weaker Aegis shadow evaluated CVSS 2.7 and ActionableScore 1 with primitive class CLASS_C and manual review REQUIRED. Its independently reconciled impact was MODERATE_KPANIC. The models disagreed but the aggregation rules retained the production result. The final reported impact is LOW.';NO MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate (with actual score 1)	MAYBE	LEAK IMPROVEONLY IO_URING LOWERED_FROM_HIGH_BASED_ON_GUESSCVSS DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN4 DECREASED_TO_LOW_FROM_MODERATE_BASED_ON_ACTIONABLESCORELESSTHAN3 KPANIC_UNSUPPORTED_REMOVED V64_AEGIS_UNSUPPORTED_PROMOTION_BLOCKED	-	-	checked

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

ActionableScore=1
PrimitiveClass=CLASS_C
RealUAF=NO
HostAdminRequired=NO
NamespaceScopedPrivilege=NO
ImportantCandidate=NO
ManualReview=NO
AutoDowngradeAllowed=YES
RequiresHostUID0=NO
AuthorizationDecisionBypass=NO
ProtectedResourceImpact=NONE
B4PendingReview=NO
B4PendingConfirmed=NO

1. ActionableScore

* Raw base score: 1.
* Conservative score: 1; no floor or cap applied.
* Raw paranoid score: 1.
* Paranoid score: 1; wider host resource exhaustion is not sufficiently established.
* Final recommended bucket: **Ordinary Moderate**::

2. Signal breakdown

* +1 Local unprivileged trigger: a process can exercise cancellation of its own io_uring context during exec or exit; no administrative operation is shown.
* +1 Availability impact: the missed wakeup can leave that task stuck indefinitely during exit, or leave execve blocked until a signal wakes it.
* -1 Hard race: triggering requires the documented ordering between queue insertion, cancellation processing, the ineffective wakeup while the task is running, the pending-work check, sleep, and a later queue insertion.
* No UAF, OOB access, memory overwrite, stale-object consumption, authorization bypass, or protected-resource impact is shown.
* No extra strong-DoS point: host panic or host-wide failure is not demonstrated.
* No paranoid accumulation point: repeatedly stranding enough exiting tasks to exhaust host resources is possible in the legacy hypothesis but not established by the patch evidence.
* B5-R does not apply: there is no heap OOB write or retry-accumulated index.
* B3-PENDING and B4-PENDING do not apply: there is neither shared-backing corruption nor a DENY-to-ALLOW authorization change.

3. Reachability analysis

The affected actor is an ordinary local process using its own io_uring context with deferred task running. No capability, namespace privilege, host UID 0, prior privileged setup, or special hardware is indicated. The race is attacker-invocable but requires a specific concurrent timing sequence; reliable repetition or host-wide accumulation is not proven.

4. Primitive and severity reading

The primitive is a missed-wakeup synchronization failure causing a process-local indefinite sleep, classified as CLASS_C availability behavior. It is neither memory corruption nor a lifetime violation. The proven effect is a stuck exec or exit path; host panic, persistent system-wide exhaustion, confidentiality loss, integrity loss, and privilege escalation remain unshown. ImportantCandidate is therefore NO, and ordinary downgrade is safe.

5. One-sentence report phrase

A missed io_uring local-work wakeup can race with cancellation and indefinitely stall an unprivileged process in execve or exit.

6. Manual review advice

NO MANUAL CHECK NEEDED

The patch supports a timing-dependent task stall without a corruption, authorization, or trust-boundary primitive.

7. Call-site confidence

Call-site confidence: high

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

Patch: io_uring/cancel: rearm local work wake up before sleep
Commit: aaa06bb9fc03384f7cf2dbc3e43f2939cc7791b2
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=aaa06bb9fc03384f7cf2dbc3e43f2939cc7791b2

Commit description:

io_uring_try_cancel_requests() sets ->cq_wait_nr to 1 so that the next
io_req_local_work_add() wakes the cancelling task. The add that takes the
count to zero is the only one that wakes, later ones see a count <= 0 and
return. io_uring_cancel_generic() however only becomes
TASK_INTERRUPTIBLE much later, so the one wake up can be spent on a
running task. That is fine as long as the entry of that add is still
queued, as the io_local_work_pending() check then skips the sleep. It is
not if the add was delayed after queueing its entry:

CPU0 (submitter in exec or exit)      CPU1 (io_req_local_work_add)
                                      mpscq_push(req A)
io_uring_try_cancel_requests()
  io_run_local_work()  // runs req A
io_uring_try_cancel_requests()
  atomic_set(&ctx->cq_wait_nr, 1)
                                      atomic_read(&ctx->cq_wait_nr) == 1
                                      count taken, wake_up_state()
                                        // task is running, no-op
prepare_to_wait(TASK_INTERRUPTIBLE)
io_local_work_pending() == false
schedule()
                                      mpscq_push(req B)
                                      cq_wait_nr <= 0, return

Only the sleeping task can run req B, so it is stuck in execve() until a
signal happens to wake it. An exiting task doesn't even get that.

Arm the count again once the task state is set and before the work list
is checked, for the rings whose local work this task is the one to run.

[ 7.2: the loop still has the WARN_ON_ONCE() that commit fddcf4c replaced in
  7.3; the new check makes it unreachable, so it is dropped ]
Fixes: d46ab2c ("io_uring: switch local task_work to a mpscq")
Assisted-by: LLM
Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Link: https://patch.msgid.link/20261015-b4-io-uring-fixes-v1-8-c1556f5d7bf5@toxicpanda.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>
Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  io_uring/cancel.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=aaa06bb9fc03384f7cf2dbc3e43f2939cc7791b2

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

The original Linux kernel CVE announcement for CVE-2026-99253 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-99253

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:L
  The Best / paranoid CVSS vector: AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS score: 4.7

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: 1

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: LOW
KPANIC detected for this report: NO
Published priority for this report (same as in Subject): LOW

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).