* [CVE-2026-89521][MODERATE REGULAR] sched/core: Handle pick_task() releasing the rq lock
@ 2026-09-11 21:55 21% AL-KERNEL
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From: AL-KERNEL @ 2026-09-11 21:55 UTC (permalink / raw)
To: kernel-cve
CVE: CVE-2026-89521
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: sched/core: Handle pick_task() releasing the rq lock
Commit: 88ed5a66467ca2a5148b9997af9c71d8c43060ad
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=88ed5a66467ca2a5148b9997af9c71d8c43060ad
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89521
Analysis date: Fri, 11 Sep 2026 17:55:59 -0400
ActionableScore: 4
ActionableScore lower bound: 3
Actionable bucket: Borderline / manual review recommended
Manual review required: YES
Summary:
No one-sentence report phrase was found.
======================================================================
ABOUT THIS REPORT
======================================================================
The original Linux kernel CVE announcement for CVE-2026-89521 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89521
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 REGULAR
Manual review required: YES
A detailed explanation of the methodology and priority rules is included
at the end of this message.
======================================================================
AL-KERNEL CLASSIFICATION RESULT
======================================================================
CVE-2026-89521 MODERATE https://www.kernelcve.org/list/?q=CVE-2026-89521 CHECK WITH IMPACT FROM ORIG NN 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:L/I:N/A:H';CWE-362;CWE-667;CWE-693;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:N/A:L';BEST CVSS score: '5.3';DESCR 'Core scheduling selection can become inconsistent when pick_task() releases the rq lock and returns RETRY_TASK, because state derived before the release may no longer match the state committed by an interleaving selection. This can allow an uncookied task selection even after the core became cookied, weakening the expected core scheduling isolation and potentially affecting SMT side channel separation. For the CVSS the PR:L is used because a local user or workload can be enough in a deployment where sched_ext and core scheduling are already enabled, even though installing or changing the scheduler itself usually requires administrator privileges. The issue is not network reachable and requires local scheduling activity on an affected host. Impact is mainly denial of service or scheduling degradation, with limited confidentiality concern from isolation mismatch rather than a direct memory disclosure primitive.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline / manual review recommended (with actual score 4) SKIP CVE-2026-89521 UNKNOWN SKIP The Fixes patch not applied yet, so unlikely that actual: url YES NO NO unknown MAYBE HARDWARE LINUS INCREASED_FROM_LOW_BASED_ON_REQUIREMANUALCHECK DECREASED_TO_MODERATEREG_BASED_ON_FALSEPOSCHECKOFKP - - checked
======================================================================
ACTIONABLESCORE ANALYSIS
======================================================================
ActionableScore=4
ActionableScoreLower=3
PrimitiveClass=CLASS_C
RealUAF=NO
HostAdminRequired=UNKNOWN
NamespaceScopedPrivilege=UNKNOWN
ImportantCandidate=NO
ManualReview=RECOMMENDED
AutoDowngradeAllowed=NO
1. ActionableScore
* Raw conservative score: 3
* Conservative score: 3
* Raw paranoid score: 4
* Paranoid score: 4
* Final recommended bucket: **Borderline / manual review recommended**::
No floor or cap was applied.
2. Signal breakdown
Conservative signals:
* +1 Local runtime trigger surface. Once the affected sched_ext and core scheduling configuration is active, ordinary local workload scheduling can reach pick_next_task().
* +1 Active scheduler state inconsistency. pick_task() may drop the rq lock and return RETRY_TASK, leaving previously derived selection state stale.
* +1 Availability or scheduler degradation concern. The stale selection state can skew forceidle accounting and break core scheduling selection behavior.
Paranoid additional signal:
* +1 Security isolation concern. The commit explicitly states that the single CPU fast path can commit an uncookied pick after the core became cookied, which may weaken core scheduling isolation and SMT side channel separation.
Non-awarded signals:
* RealUAF not awarded. The patch does not show freed object reuse, stale lifetime access, refcount corruption, or double release.
* Memory corruption not awarded. No OOB, UAF, type confusion, arbitrary write, or controlled reclaim is established.
* Strong LPE leverage not awarded. The issue is a scheduler state race and policy consistency bug, not a demonstrated privilege escalation primitive.
* Confidentiality bonus not awarded conservatively. The isolation mismatch can matter for SMT side channel policy, but no direct information disclosure path is demonstrated.
* Remote reachability not awarded. This is not packet triggered or network reachable.
* Hard race penalty not applied. Although the bug is concurrency related, the patch does not establish a narrow one shot race that is materially hard to repeat under affected scheduling conditions.
* Host admin penalty not applied. Enabling sched_ext or core scheduling may require administrative setup, but the supplied evidence does not prove that host admin privilege is required for the complete runtime trigger once the configuration is active.
3. Reachability analysis
The vulnerable path is local scheduler execution in pick_next_task() with core scheduling enabled. The problematic condition depends on a pick_task() implementation that can release the rq lock and return RETRY_TASK, which is tied to sched_ext behavior from the fixed commit.
A pure remote attacker cannot trigger this directly. A local workload may contribute by causing scheduling activity, but the complete privilege boundary is not fully resolved because enabling or installing sched_ext scheduler behavior and core scheduling policy may require host administrator action. Therefore HostAdminRequired is UNKNOWN rather than YES or NO.
NamespaceScopedPrivilege is UNKNOWN. The patch does not show a namespace scoped capability path that lets an attacker set up the complete sched_ext plus core scheduling configuration.
Prior privileged setup is plausible because sched_ext and core scheduling are not ordinary always on scheduling behavior, but this is treated as reachability uncertainty rather than a hard host admin requirement. No special hardware beyond SMT capable CPUs is established as a strict prerequisite.
4. Primitive and severity interpretation
The exact primitive is stale scheduler selection state after rq lock release, not stale memory. The bug is best classified as a race or logic error in active scheduler state validation.
This is not active structured object UAF and not fixed stale storage UAF. There is no concrete repeated release or refcount mutation. There is no first lifetime operation that may free an object and no second lifetime mutation through a dangling reference.
PrimitiveClass is CLASS_C because the demonstrated behavior is invalid scheduler state, possible scheduling degradation, and possible core scheduling isolation mismatch without an independently useful corruption primitive.
The demonstrated consequence is inconsistent task selection and forceidle accounting after interleaving scheduler activity. The speculative consequence is weakened SMT side channel isolation when an uncookied task is selected despite the core becoming cookied. ImportantCandidate is NO because no memory corruption, privilege escalation, direct disclosure, or strong security boundary compromise is demonstrated. AutoDowngradeAllowed is NO because the core scheduling isolation mismatch is security relevant enough that automatic closure could hide a policy bypass concern.
Operational severity is borderline to actionable Moderate depending on whether the affected system actually uses sched_ext together with core scheduling for workload isolation.
5. One-sentence report phrase
A race in core scheduling can leave pick_next_task() using stale selection state after pick_task() drops the rq lock, potentially causing scheduler degradation and weakening core scheduling cookie isolation without evidence of memory corruption or privilege escalation.
6. Manual review recommendation
MANUAL CHECK RECOMMENDED
The issue is not a strong Important candidate from the patch alone, but it affects scheduler isolation semantics and should not be automatically closed without checking whether the product uses sched_ext and core scheduling as a security boundary.
7. Call-site confidence
Call-site confidence: high
======================================================================
UPSTREAM PATCH SUMMARY
======================================================================
Patch: sched/core: Handle pick_task() releasing the rq lock
Commit: 88ed5a66467ca2a5148b9997af9c71d8c43060ad
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=88ed5a66467ca2a5148b9997af9c71d8c43060ad
Commit description:
Core scheduling's pick_next_task() breaks when a ->pick_task()
implementation can release the rq lock. The selection state derived on entry
is only valid while the lock is held continuously. Once a pick can drop the
lock, an interleaving selection can invalidate all of it: the single-CPU
fast path can commit an uncookied pick although the core went cookied during
the release, and forceidle committed by the interleaving selection skews the
restarted pass's accounting.
Fix it by restarting the whole selection when a pick returns RETRY_TASK
after releasing the lock: a single restart point above the state derivation
replaces the per-loop restart labels, so a retry picks up state committed by
interleaving selections and accounts and resets forceidle like a fresh
selection would.
need_sync and fi_before latch across retries. Clock validity can't be
re-derived - there is no program-ordered way to tell whether the own and
core rq clocks are still updated after the lock was released, as other
lockers' pin cycles may or may not have invalidated them. When restarting,
clear core_clock_updated so that the sibling loop re-updates the core rq,
and update the own rq clock if invalidated.
Fixes: 4c95380 ("sched/ext: Fold balance_scx() into pick_task_scx()")
Cc: [email protected] # v6.19+
Signed-off-by: Tejun Heo <[email protected]>
Acked-by: Peter Zijlstra (Intel) <[email protected]>
Signed-off-by: Greg Kroah-Hartman <[email protected]>
Changed files:
kernel/sched/core.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=88ed5a66467ca2a5148b9997af9c71d8c43060ad
======================================================================
DETAILED REPORT METHODOLOGY
======================================================================
The original Linux kernel CVE announcement for CVE-2026-89521 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89521
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:L/I:N/A:L
The Best / paranoid CVSS vector: AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:N/A:H
The Best / paranoid CVSS score: 5.3
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: 3
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: NO
Published priority for this report (same as in Subject): MODERATE REGULAR
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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