* [CVE-2026-99254][LOW] io_uring: don't flood a cancelling task with task_work
@ 2026-11-04 2:56 21% AL-KERNEL
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From: AL-KERNEL @ 2026-11-04 2:56 UTC (permalink / raw)
To: kernel-cve
CVE: CVE-2026-99254
Priority: LOW
AL-KERNEL base severity: LOW
KPANIC flag: NO
Patch: io_uring: don't flood a cancelling task with task_work
Commit: a22fb44e988b01b82235ea07f0bf0fc73e256596
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=a22fb44e988b01b82235ea07f0bf0fc73e256596
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-99254
Analysis date: Tue, 03 Nov 2026 22:56:14 -0400
ActionableScore: 2
ActionableScore lower bound: unknown
Actionable bucket: Ordinary Moderate
Manual review required: NO
Summary:
An unprivileged local process can construct cyclic io_uring polling relationships that flood cancellation with task_work, temporarily pinning CPU time and leaving exit or execve stuck.
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ABOUT THIS REPORT
======================================================================
The original Linux kernel CVE announcement for CVE-2026-99254 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-99254
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.
======================================================================
AL-KERNEL CLASSIFICATION RESULT
======================================================================
CVE-2026-99254 LOW https://www.kernelcve.org/list/?q=CVE-2026-99254 CHECK WITH IMPACT FROM ORIG NN LOW | AEGIS IMPACT LOW AEGIS_AS=2 LEGACY=LOW RESULT=SAME (final reconciliation agrees; DECISION: both LOW) 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-400;CWE-834;*CWE-835;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 task running io_uring cancellation can be flooded with repeatedly queued task_work callbacks because io_tctx_exit_cb leaves the tctx node untouched while cancellation is active. task_work_run may therefore never observe an empty callback list and cannot return to the cancellation code that would remove the nodes. This can keep an exiting or execve task stuck for minutes while consuming a CPU and processing hundreds of thousands of callbacks. For the CVSS the PR:L value is selected because an unprivileged local process must create and operate the io_uring instances. AC:H reflects the required inter ring reference pattern and cancellation timing. The demonstrated primitive is a local livelock and resource consumption condition with no evidence of UAF, OOB access, information disclosure, arbitrary write, kernel panic, or privilege escalation. Automated impact decision path. The original SimpleNet classifier selected LOW. The primary LLM estimated paranoid CVSS 4.7 and the production ActionableScore returned 2 with recommendation Ordinary Moderate. The production rules then evaluated CVSS thresholds and ActionableScore thresholds. No severity changing production threshold or specialist override was recorded. 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. The independent weaker Aegis shadow evaluated CVSS 2.8 and ActionableScore 2 with primitive class CLASS_C and manual review REQUIRED. Its independently reconciled impact was LOW. The shadow agreed with the final production impact and aggregation did not change that impact. The final reported impact is LOW.';NO MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate (with actual score 2) MAYBE INIT RACE LEAK HARDWARE IO_URING - - checked
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ACTIONABLESCORE ANALYSIS
======================================================================
ActionableScore=2
PrimitiveClass=NONE
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: 2.
* Conservative score: 2; no floor or cap applied.
* Raw paranoid score: 2.
* Paranoid score: 2; no additional technically supported primitive.
* Final recommended bucket: **Ordinary Moderate**::
2. Signal breakdown
* +1 Local unprivileged trigger: the commit explicitly states that no privilege is required to create the inter-ring polling and cancellation condition.
* +1 Strong local DoS/livelock: repeated task_work prevents cancellation from progressing, can leave an exiting or execve task unkillable and busy for minutes, and consumes a CPU plus work-item activity.
* No extra availability point: CPU consumption and the stuck task are consequences of the same livelock.
* No hard-race penalty: timing contributes, but the demonstrated 60-ring-pair setup sustained 280,000 callbacks in five seconds; proof does not establish a one-shot or exceptionally uncontrollable race.
* No broad-exposure bonus: io_uring is broadly deployed, but this specific cyclic polling/cancellation pattern is not shown as commonly exercised.
* No persistent resource-leak signal: callbacks eventually stop when senders are simultaneously late.
* No UAF, OOB access, memory write, authorization bypass, disclosure, kernel panic, or privilege escalation is shown.
* B5-R and B3-PENDING do not apply: there is no heap OOB write or shared-backing write.
3. Reachability analysis
An ordinary local process can create and operate the required io_uring instances without UID 0, capabilities, namespace privileges, prior privileged setup, special hardware, or boot-time configuration. The attacker must construct mutually polling rings and enter cancellation through exit or execve. This is a higher-complexity workload, but the supplied test demonstrates repeatable prolonged livelock rather than speculative reachability.
4. Primitive and severity reading
The exact defect is callback requeue flooding: `io_tctx_exit_cb()` completes without removing the node while cancellation is active, allowing `io_ring_exit_work()` to immediately submit more task_work before cancellation can resume and remove the nodes. The fix records `in_cancel` and delays retry.
This is resource-consumption/livelock behavior, not memory corruption or a lifetime violation. The shown impact is localized task stalling and CPU/workqueue activity. Host-wide panic, persistent exhaustion, protected-state modification, and cross-boundary effects are unproven. PrimitiveClass is therefore NONE, and automatic Moderate handling is appropriate.
5. One-sentence report phrase
An unprivileged local process can construct cyclic io_uring polling relationships that flood cancellation with task_work, temporarily pinning CPU time and leaving exit or execve stuck.
6. Manual review advice
NO MANUAL CHECK NEEDED
The patch and commit establish a bounded local livelock without a corruption, authorization, ownership, or unresolved trust-boundary primitive.
7. Call-site confidence
Call-site confidence: high
======================================================================
UPSTREAM PATCH SUMMARY
======================================================================
Patch: io_uring: don't flood a cancelling task with task_work
Commit: a22fb44e988b01b82235ea07f0bf0fc73e256596
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=a22fb44e988b01b82235ea07f0bf0fc73e256596
Commit description:
io_ring_exit_work() asks each task which has used the ring to remove its
tctx node, by task_work, and repeats that until the list is empty.
io_tctx_exit_cb() doesn't touch the node while the task is in
io_uring_cancel_generic(), which removes all the nodes at its end. So the
work item queues the callback again as soon as it has completed, for
nothing.
That is more than a waste of time. task_work_run() only returns when it
finds the list empty. If the rings are released from inside the
cancellation, because requests of other rings held the last references to
their files, up to 64 work items keep sending callbacks to the one task
which is running them. It never finds the list empty, never gets back to
the cancellation, the nodes stay, and so it goes on: a task with 60 pairs
of rings which poll each other ran 280000 callbacks in 5 seconds, 20
seconds after exit_group(). It ends when all the senders happen to be late
at the same time. Until then the task keeps a CPU and the work items busy.
An exiting task can't be killed meanwhile. execve() cancels the requests in
the same way, so a live task can get stuck there as well, for minutes. If
it takes more than five, the work items run into the
WARN_ON_ONCE(time_after(jiffies, timeout)) of io_ring_exit_work(). No
privilege is needed for any of this.
If the callback has met a task in cancellation, wait a little before it is
sent again. Nobody has to wake the work item up for that: either the node
is gone by then, or it is the turn of the next round.
Fixes: d56d938 ("io_uring: do ctx initiated file note removal")
Assisted-by: LLM
Signed-off-by: Josef Bacik <[email protected]>
Link: https://patch.msgid.link/[email protected]
Signed-off-by: Jens Axboe <[email protected]>
Signed-off-by: Greg Kroah-Hartman <[email protected]>
Changed files:
io_uring/io_uring.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=a22fb44e988b01b82235ea07f0bf0fc73e256596
======================================================================
DETAILED REPORT METHODOLOGY
======================================================================
The original Linux kernel CVE announcement for CVE-2026-99254 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-99254
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: 2
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 [email protected] (and both
send reply to CVE record itself too and see "reply" button below for howto reply).
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