* [CVE-2026-89756][MODERATE REGULAR] mm/migrate: report RCU-tasks quiescent states in migrate_pages_batch()
@ 2026-09-12 4:20 21% AL-KERNEL
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From: AL-KERNEL @ 2026-09-12 4:20 UTC (permalink / raw)
To: kernel-cve
CVE: CVE-2026-89756
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: mm/migrate: report RCU-tasks quiescent states in migrate_pages_batch()
Commit: 4996a7bc01ef35570664854dac2530c604981039
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=4996a7bc01ef35570664854dac2530c604981039
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89756
Analysis date: Sat, 12 Sep 2026 00:20:57 -0400
ActionableScore: 3
ActionableScore lower bound: 2
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-2026-89756 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89756
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-2026-89756 MODERATE https://www.kernelcve.org/list/?q=CVE-2026-89756 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:N/I:N/A:H';**CWE-400;CWE-834;CWE-667;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:L';BEST CVSS score: '4.7';DESCR 'migrate_pages_batch can keep a CPU busy for a long time while unmapping and migrating a large folio batch. On KVM hosts each unmap can run mmu_notifier callbacks such as tdp_mmu_zap_leafs, and on PREEMPTION kernels cond_resched() may not report a Tasks RCU quiescent state. This can make kcompactd or another migrating task a Tasks RCU holdout and can stall a Tasks RCU grace period for minutes. For the CVSS the PR:L is used because reliable triggering requires a local process or workload that can create memory pressure, compaction, or page migration conditions on the host. The issue is not network reachable and does not provide an evident memory corruption primitive. Impact is denial of service or host responsiveness degradation due to RCU stalls, with no direct confidentiality or integrity impact.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline / manual review recommended (with actual score 3) YES HARDWARE MEMORY DECREASED_TO_MODERATE_REGULAR_FROM_MODERATE7_BASED_ON_GUESSCVSS_AND_CIANNH NO NO checked
======================================================================
ACTIONABLESCORE ANALYSIS
======================================================================
ActionableScore=3
ActionableScoreLower=2
PrimitiveClass=CLASS_C
RealUAF=NO
HostAdminRequired=UNKNOWN
NamespaceScopedPrivilege=NO
ImportantCandidate=NO
ManualReview=RECOMMENDED
AutoDowngradeAllowed=YES
1. ActionableScore
* Raw conservative score: 2
* Conservative score: 2
* Raw paranoid score: 3
* Paranoid score: 3
* Final recommended bucket: **Borderline / manual review recommended**::
No floor or cap was applied.
2. Signal breakdown
Conservative positive signals:
* +1 Availability impact realistic. The patch and report show Tasks RCU grace period stalls lasting minutes, with kcompactd as a holdout task.
* +1 Broad realistic operational exposure. The affected path is ordinary page migration and compaction, and the supplied context says this is common on KVM hosts in a large fleet.
Paranoid additional signal:
* +1 Possible local workload influence. A local workload may be able to increase memory pressure, compaction, or migration activity enough to make the stall more likely, but the complete reliable attacker-controlled trigger is not fully established.
Non-awarded signals:
* Real UAF not awarded. No stale freed object use is established.
* Memory corruption not awarded. The patch only changes quiescent state reporting from cond_resched() to cond_resched_tasks_rcu_qs().
* Confidentiality and integrity not awarded. There is no disclosure, overwrite, ownership violation, or authorization bypass.
* Reliable kernel crash not awarded. The demonstrated effect is an RCU Tasks stall and availability degradation, not a panic or fatal fault.
* Remote reachability not awarded. The issue is not triggered by network packets directly.
* KVM guest-to-host exploitation leverage not awarded. KVM appears because mmu_notifier callbacks are expensive on KVM hosts, but no guest-controlled host corruption primitive is shown.
* Race or timing penalty not applied. This is not a narrow race window, but a long-running loop failing to report Tasks RCU quiescent states on PREEMPTION kernels.
* Host-admin penalty not applied. Host-admin requirement is not positively established, but below-host-admin reachability is also not fully proven.
3. Reachability analysis
The vulnerable behavior is reached during migrate_pages_batch(), especially when large folio batches are unmapped and migrated. On KVM hosts each unmap can invoke mmu_notifier invalidation callbacks such as kvm_mmu_notifier_invalidate_range_start() and tdp_mmu_zap_leafs(), which can keep the CPU busy for a long time.
The most plausible trigger is local workload activity that causes memory pressure, compaction, or page migration. Ordinary local reachability is plausible but not fully established from the supplied patch alone, because the input does not prove that an unprivileged user can reliably force the complete long migration sequence on the target. Therefore HostAdminRequired is UNKNOWN rather than YES or NO.
This is not namespace-scoped. No relevant user namespace or network namespace capability path is involved. No special hardware is required, but the most affected deployments are KVM hosts with PREEMPTION kernels and workloads that cause large migration batches.
4. Primitive and severity interpretation
The exact primitive is a scheduler and RCU progress accounting failure: migrate_pages_batch() can spend a long time in migration and mmu_notifier invalidation work without reporting a Tasks RCU quiescent state. This can make kcompactd or another migrating task a Tasks RCU holdout and stall a Tasks RCU grace period.
This is CLASS_C because the demonstrated behavior is availability degradation and RCU stall behavior without a kernel memory corruption, UAF, ownership violation, or authorization primitive. The demonstrated consequence is DoS-like operational degradation for minutes. The speculative consequence is broader host responsiveness impact under heavy migration load, but there is no patch-supported path to privilege escalation, information disclosure, or integrity compromise.
ImportantCandidate is NO because no Class A or meaningful unresolved Class B primitive is established. AutoDowngradeAllowed is YES because the issue is DoS-only and lacks unresolved corruption or security-boundary leverage, although manual review is still recommended due to KVM-host operational impact.
5. One-sentence report phrase
migrate_pages_batch() can stall Tasks RCU grace periods on KVM hosts because long folio migration batches may not report quiescent states on PREEMPTION kernels, leading to minutes-long host availability degradation without evidence of memory corruption or privilege escalation.
6. Manual review recommendation
MANUAL CHECK RECOMMENDED
The issue is not an Important candidate from the supplied evidence, but it affects RCU progress on KVM hosts and the real operational impact depends on workload, PREEMPTION behavior, and memory migration intensity.
7. Call-site confidence
Call-site confidence: high
======================================================================
UPSTREAM PATCH SUMMARY
======================================================================
Patch: mm/migrate: report RCU-tasks quiescent states in migrate_pages_batch()
Commit: 4996a7bc01ef35570664854dac2530c604981039
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=4996a7bc01ef35570664854dac2530c604981039
Commit description:
migrate_pages_batch() unmaps each folio before moving it, and every
unmap runs the mmu_notifier invalidate callbacks. On KVM hosts
try_to_migrate() ends up in kvm_mmu_notifier_invalidate_range_start() ->
tdp_mmu_zap_leafs(), which is expensive, so unmapping a large batch keeps
the CPU busy for a long time.
The loop already calls cond_resched(), but on PREEMPTION kernels that is
a no-op, and involuntary preemption is not a Tasks-RCU quiescent state.
A long batch therefore never reports a quiescent state, and the
migrating task (e.g. kcompactd) becomes a Tasks-RCU holdout, stalling the
Tasks-RCU grace period for minutes, which is common at Meta fleet:
INFO: rcu_tasks detected stalls on tasks:
0000000055349ecc: .. nvcsw: 1157401/1157401 holdout: 1 idle_cpu: -1/56 task:kcompactd0 state:R running task
Call Trace:
tdp_mmu_zap_leafs
tdp_mmu_next_root
gfn_to_pfn_cache_invalidate_start
kvm_mmu_notifier_invalidate_range_start
__mmu_notifier_invalidate_range_start
try_to_migrate_one
try_to_migrate
migrate_pages_batch
migrate_pages
compact_zone
compact_node
kcompactd
kthread
Use cond_resched_tasks_rcu_qs() so a quiescent state is reported even
when cond_resched() does nothing.
This has also been discussed at [1]
Link: https://lore.kernel.org/[email protected]
Link: https://lore.kernel.org/all/[email protected]/ [1]
Signed-off-by: Breno Leitao <[email protected]>
Acked-by: Zi Yan <[email protected]>
Reviewed-by: Gregory Price <[email protected]>
Reviewed-by: Paul E. McKenney <[email protected]>
Acked-by: David Hildenbrand (Arm) <[email protected]>
Cc: Alistair Popple <[email protected]>
Cc: Byungchul Park <[email protected]>
Cc: "Huang, Ying" <[email protected]>
Cc: Joshua Hahn <[email protected]>
Cc: Matthew Brost <[email protected]>
Cc: Rakie Kim <[email protected]>
Cc: <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Greg Kroah-Hartman <[email protected]>
Changed files:
mm/migrate.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=4996a7bc01ef35570664854dac2530c604981039
======================================================================
DETAILED REPORT METHODOLOGY
======================================================================
The original Linux kernel CVE announcement for CVE-2026-89756 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89756
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: 2
Paranoid ActionableScore: 3
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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