[CVE-2026-99144][MODERATE 7.0] sched_ext: Fix use-after-free of a destroyed user DSQ by a deferred reenqueue

[CVE-2026-99144][MODERATE 7.0] sched_ext: Fix use-after-free of a destroyed user DSQ by a deferred reenqueue

[AL-KERNEL]

CVE: CVE-2026-99144
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: sched_ext: Fix use-after-free of a destroyed user DSQ by a deferred reenqueue
Commit: 633f586329bfd9843bf8acf86c95353f4c16e22d
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=633f586329bfd9843bf8acf86c95353f4c16e22d
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-99144
Analysis date: Fri, 30 Oct 2026 11:09:59 -0400
ActionableScore: 4
ActionableScore lower bound: unknown
Actionable bucket: Borderline / manual review recommended
Manual review required: YES

Summary:
A privileged or delegated sched_ext controller can race user-DSQ destruction with deferred reenqueue processing, causing a post-RCU use-after-free read and plausible kernel crash.

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ABOUT THIS REPORT
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The original Linux kernel CVE announcement for CVE-2026-99144 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-99144

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.

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AL-KERNEL CLASSIFICATION RESULT
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CVE-2026-99144	MODERATE	https://www.kernelcve.org/list/?q=CVE-2026-99144 CHECK WITH IMPACT FROM ORIG NN LOW | AEGIS IMPACT MODERATE_KPANIC AEGIS_AS=6 LEGACY=MODERATE_KPANIC RESULT=SAME V39_AEGIS_ONE_STEP_UP=MODERATE->MODERATE_KPANIC (final reconciliation agrees; DECISION: both 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:H/I:H/A:H';*CWE-416;CWE-362;*CWE-672;Other CVSS 'AV:L/AC:H/PR:H/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '7';DESCR 'A race in sched_ext user DSQ destruction allows run_deferred to retain a deferred reenqueue request across an RCU grace period and then access the DSQ after its storage is freed. The concrete primitive is a kernel use after free read of the DSQ id, with freed per CPU request and list storage also involved. A kernel crash is directly plausible, while allocator reuse creates a reason to examine broader memory safety impact but does not prove an arbitrary write or privilege escalation. For the CVSS the PR:L value in the paranoid vector reflects deployments that delegate sched_ext management to a service or reduced capability administrator, although reliable triggering normally requires control of a sched_ext BPF scheduler and user DSQ lifecycle and the base vector therefore uses PR:H. AC:H reflects the required ordering between deferred processing, DSQ destruction, and the RCU callback. The issue is local and is not directly network reachable. The paranoid HHH impact is an upper bound for manual review due to the confirmed kernel UAF, not a claim of demonstrated code execution. Automated impact decision path. The original SimpleNet classifier selected LOW. The primary LLM estimated paranoid CVSS 7 and the production ActionableScore returned 4 with recommendation Borderline / manual review recommended. The production rules then evaluated CVSS thresholds and ActionableScore thresholds. Recorded rule transitions were CVSS and initial triage stage LOW to MODERATE_KPANIC because the ActionableScore threshold rule was applied. KP and FD specialist checks stage MODERATE_KPANIC to MODERATE because KP specialist did not establish a qualifying panic. final reconciliation stage MODERATE to MODERATE_KPANIC because the cross model evidence reconciliation rule was applied. The legacy KP specialist produced a review artifact and its decision was processed by the applicable KP rules. The legacy FD or DH downgrade specialist has no review artifact for this run. Exact triggered rule markers were INCREASED_TO_MODERATE7_FROM_LOW_BASED_ON_GUESSCVSS_AND_ACTIONABLESCOREHIGHER4 and DECREASED_TO_MODERATEREG_BASED_ON_FALSEPOSCHECKOFKP and V39_AEGIS_ONE_STEP_UP and MANUAL_REVIEW_REQUIRED. The independent weaker Aegis shadow evaluated CVSS 2.5 and ActionableScore 6 with primitive class CLASS_B and manual review REQUIRED. Its independently reconciled impact was MODERATE_KPANIC. Cross model aggregation changed the production result under an enabled evidence gated merge rule. The final reported impact is MODERATE_KPANIC.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline / manual review recommended (with actual score 4)	MAYBE	INIT LEAK UAF HARDWARE LINUS  INCREASED_TO_MODERATE7_FROM_LOW_BASED_ON_GUESSCVSS_AND_ACTIONABLESCOREHIGHER4 DECREASED_TO_MODERATEREG_BASED_ON_FALSEPOSCHECKOFKP KPANIC V39_AEGIS_ONE_STEP_UP MANUAL_REVIEW_REQUIRED	-	-	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=4
PrimitiveClass=CLASS_B
RealUAF=YES
HostAdminRequired=UNKNOWN
NamespaceScopedPrivilege=UNKNOWN
ImportantCandidate=NO
ManualReview=REQUIRED
AutoDowngradeAllowed=YES
RequiresHostUID0=UNKNOWN
AuthorizationDecisionBypass=NO
ProtectedResourceImpact=NONE
B4PendingReview=NO
B4PendingConfirmed=NO

1. ActionableScore

* Raw base score: 2.
* Conservative score: 4. REAL UAF FLOOR 4 raises 2 -> 4.
* Raw paranoid score: 2.
* Paranoid score: 4. The same floor applies; no additional exploit primitive is supported.
* Final recommended bucket: **Borderline / manual review recommended**::

2. Signal breakdown

* +2 Real UAF: a deferred reenqueue request can survive the RCU grace period and later read `dsq->id` after the DSQ is freed.
* +1 Reliable kernel crash/DoS: dereferencing reclaimed DSQ storage plausibly causes an Oops or host failure.
* -1 Hard ordering/race requirement: exploitation requires deferred processing, DSQ destruction, and the RCU callback to interleave across the grace period.
* Floor 4: this is active DSQ state rather than fixed asynchronous storage, and host-admin-equivalent privilege is not positively established.
* No structured-stale leverage: only the freed DSQ ID read is established; attacker-shaped reclaim, replacement contents, callbacks, or a later write are not shown.
* No arbitrary write, disclosure, LPE, repeated release/refcount mutation, or separate C/I impact is shown. The crash is not counted again as Availability.
* B5-R does not apply: there is no retry-accumulated heap OOB write or later consumption of OOB-corrupted state.
* No authorization decision is changed, so B4-PENDING is inapplicable.

3. Reachability analysis

The actor must control a sched_ext BPF scheduler, user-DSQ lifecycle, and deferred reenqueue timing. The supplied evidence characterizes this as normally privileged and potentially delegated, but does not establish whether the complete operation requires host-administrator-equivalent powers or works with namespace-scoped capabilities. No remote path, ordinary unprivileged trigger, special hardware, or namespace-complete path is shown. Repeated attempts may be possible, but the required race ordering remains high complexity.

4. Primitive and severity reading

The primitive is a CLASS_B active-structured-object UAF: pending deferred work retains a DSQ reference beyond its valid lifetime and reads its ID after the RCU callback frees it. This is broader than fixed stale storage but does not show useful post-reclaim field control or a write through the stale object. Crash/DoS is plausible; stronger memory corruption or privilege escalation remains theoretical. ImportantCandidate is therefore NO, while manual review remains required due to the confirmed scheduler UAF and unresolved privilege boundary. Auto-downgrade is allowed because no material non-crash exploit primitive is established.

5. One-sentence report phrase

A privileged or delegated sched_ext controller can race user-DSQ destruction with deferred reenqueue processing, causing a post-RCU use-after-free read and plausible kernel crash.

6. Manual review advice

MANUAL CHECK REQUIRED

Confirm the complete sched_ext privilege boundary and whether the stale ID reaches any material consumer beyond the documented read/crash path.

7. Call-site confidence

Call-site confidence: medium

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

Patch: sched_ext: Fix use-after-free of a destroyed user DSQ by a deferred reenqueue
Commit: 633f586329bfd9843bf8acf86c95353f4c16e22d
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=633f586329bfd9843bf8acf86c95353f4c16e22d

Commit description:

A deferred reenqueue can read a freed user DSQ. The request stays on its
CPU's deferred list until run_deferred() takes it, and the RCU callback that
frees the DSQ assumes that the grace period has flushed it, so exit_dsq()
checks the per-CPU node without the deferred list lock. A grace period waits
for readers in flight, not for a pending run_deferred(). A run_deferred()
which starts after the grace period began can detach the request right
before the callback looks and then read the freed DSQ's id.

Cancel the pending requests before the grace period instead. The irq work
that frees the DSQ unlinks every CPU's request under the per-CPU deferred
list lock, and schedule_dsq_reenq() ignores a new request under the same
lock once the id is invalid. Only a run_deferred() that started before the
grace period can hold a request, and it runs with IRQs disabled, so the
grace period waits for it and nothing touches the DSQ after the callback.

Fixes: 84b1a0e ("sched_ext: Implement scx_bpf_dsq_reenq() for user DSQs")
Cc: stable@vger.kernel.org # v7.1+
Reported-and-tested-by: Hui Su <sh_def@163.com>
Link: https://lore.kernel.org/all/20261008024753.4096008-1-sh_def@163.com/
Signed-off-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  kernel/sched/ext/ext.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=633f586329bfd9843bf8acf86c95353f4c16e22d

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

The original Linux kernel CVE announcement for CVE-2026-99144 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-99144

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:H/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: unknown
  Paranoid ActionableScore: 4

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).