* [CVE-2026-99252][LOW] io_uring/mpscq: disable preemption while pushing
@ 2026-11-04 2:02 21% AL-KERNEL
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From: AL-KERNEL @ 2026-11-04 2:02 UTC (permalink / raw)
To: kernel-cve
CVE: CVE-2026-99252
Priority: LOW
AL-KERNEL base severity: LOW
KPANIC flag: NO
Patch: io_uring/mpscq: disable preemption while pushing
Commit: 8c94677e2b6145393d4c129fe0f5364b0e5000a5
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=8c94677e2b6145393d4c129fe0f5364b0e5000a5
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-99252
Analysis date: Tue, 03 Nov 2026 22:02:18 -0400
ActionableScore: 1
ActionableScore lower bound: unknown
Actionable bucket: Ordinary Moderate
Manual review required: NO
Summary:
A narrow same-CPU real-time scheduling race can leave an io_uring MPSC node temporarily unreachable and make the consumer spin while holding `uring_lock`.
======================================================================
ABOUT THIS REPORT
======================================================================
The original Linux kernel CVE announcement for CVE-2026-99252 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-99252
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-2026-99252 LOW https://www.kernelcve.org/list/?q=CVE-2026-99252 CHECK WITH IMPACT FROM ORIG NN LOW | AEGIS IMPACT LOW AEGIS_AS=0 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-362;CWE-667;*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 race in mpscq_push() allows a producer to be preempted after replacing the queue tail but before linking the new node to its predecessor. A higher priority SCHED_FIFO consumer on the same CPU can then repeatedly receive NULL from mpscq_pop() and spin while holding uring_lock because cond_resched() does not let the lower priority producer run. For the CVSS the PR:L value reflects that reliable triggering requires a local process or ring participant able to queue task_work for the affected io_uring. AC:H reflects the required priority relationship and scheduling window. The demonstrated impact is availability loss without a UAF, OOB access, information leak, or write primitive. The issue is not directly network reachable. Automated impact decision path. The original SimpleNet classifier selected LOW. 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. 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 1.9 and ActionableScore 0 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 1) MAYBE LEAK HARDWARE IO_URING - - checked
======================================================================
ACTIONABLESCORE ANALYSIS
======================================================================
ActionableScore=1
PrimitiveClass=CLASS_C
RealUAF=NO
HostAdminRequired=UNKNOWN
NamespaceScopedPrivilege=UNKNOWN
ImportantCandidate=NO
ManualReview=NO
AutoDowngradeAllowed=YES
RequiresHostUID0=UNKNOWN
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; no additional technically supported primitive.
* Final recommended bucket: **Ordinary Moderate**::
2. Signal breakdown
* +1 Reliable availability loss: a higher-priority consumer can spin indefinitely while holding `uring_lock`, preventing the preempted producer from completing the queue link.
* +1 Broadly deployed exposure: the defect is in io_uring’s task-work MPSC queue, although the vulnerable scheduling condition is specialized.
* -1 Hard timing/scheduling condition: preemption must occur in the two-instruction interval between `xchg(&q->tail, node)` and `WRITE_ONCE(prev->next, node)`, with a higher-priority same-CPU `SCHED_FIFO` consumer.
* No memory-corruption signal: the patch shows neither OOB access, UAF, stale-object consumption, nor overwrite.
* No extra availability point: indefinite spinning and lock retention are the same demonstrated DoS.
* No confidentiality, integrity, authorization-bypass, B5-R, or B3-PENDING signal is shown.
3. Reachability analysis
A producer may be an io-wq worker or a task using `IORING_OP_MSG_RING`. Reliable triggering additionally requires the producer to be preempted in the narrow queue-link window and a higher-priority `SCHED_FIFO` consumer on the same CPU. The evidence does not establish whether the complete scheduling setup is available to an ordinary user, through delegated real-time limits/capabilities, or only with host-administrator powers. No namespace-scoped complete trigger is shown. There is no network reachability or special hardware requirement.
4. Primitive and severity reading
The primitive is a transiently unreachable queue node causing priority-inversion livelock. It is CLASS_C invalid-state/DoS behavior, not heap corruption or a lifetime violation. The demonstrated effect is consumer spinning while holding `uring_lock`; host panic, arbitrary write, information disclosure, and privilege escalation are unsupported. ImportantCandidate is NO because no material non-DoS primitive or trust-boundary violation remains. Auto-downgrade is allowed.
5. One-sentence report phrase
A narrow same-CPU real-time scheduling race can leave an io_uring MPSC node temporarily unreachable and make the consumer spin while holding `uring_lock`.
6. Manual review advice
NO MANUAL CHECK NEEDED
The patch establishes a constrained scheduling livelock without memory corruption, authorization bypass, or unresolved Important-class primitive.
7. Call-site confidence
Call-site confidence: high
======================================================================
UPSTREAM PATCH SUMMARY
======================================================================
Patch: io_uring/mpscq: disable preemption while pushing
Commit: 8c94677e2b6145393d4c129fe0f5364b0e5000a5
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=8c94677e2b6145393d4c129fe0f5364b0e5000a5
Commit description:
A push makes the node the new tail, and then links it to its predecessor.
In between the queue isn't empty, but mpscq_pop() can't get at the node.
The consumers deal with that by trying again, with a cond_resched() in
the loop to give a preempted producer the chance to finish.
cond_resched() doesn't do that if the consumer has a higher priority than
the producer and both have to use the same CPU:
producer (SCHED_OTHER) consumer (SCHED_FIFO), same CPU
io_req_local_work_add()
mpscq_push()
prev = xchg(&q->tail, node)
<preempted> io_cqring_wait()
__io_run_local_work()
again:
cond_resched() // nothing to do
mpscq_pop() // NULL
// work pending, no events yet
goto again
The producer can be any task that is able to queue task_work for the
ring: an io-wq worker, or somebody who sends a message with
IORING_OP_MSG_RING. The consumer spins, holding ->uring_lock, until the
scheduler takes pity on the fair class, which takes about a second with
the default settings of the fair server, and forever without it. The
loops in tctx_task_work_run() and io_cancel_local_task_work() have the
same problem.
Don't allow preemption between the two steps. That bounds the time the
window is open on the CPU of the producer by what interrupts take, and a
consumer that waits for that is on another CPU. The cost is a
preempt_disable()/preempt_enable() pair per push.
Fixes: 50cb44b ("io_uring/mpscq: add lockless multi-producer, single-consumer FIFO queue")
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/mpscq.h
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=8c94677e2b6145393d4c129fe0f5364b0e5000a5
======================================================================
DETAILED REPORT METHODOLOGY
======================================================================
The original Linux kernel CVE announcement for CVE-2026-99252 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-99252
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 [email protected] (and both
send reply to CVE record itself too and see "reply" button below for howto reply).
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