[CVE-2026-64374][MODERATE REGULAR] sched/rt: Have RT_PUSH_IPI be default off for non PREEMPT_RT

[CVE-2026-64374][MODERATE REGULAR] sched/rt: Have RT_PUSH_IPI be default off for non PREEMPT_RT

[AL-KERNEL]

CVE: CVE-2026-64374
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: sched/rt: Have RT_PUSH_IPI be default off for non PREEMPT_RT
Commit: b99f04ae3d200d2f8844aa29145bd18eccbeecde
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=b99f04ae3d200d2f8844aa29145bd18eccbeecde
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64374
Analysis date: Sat, 25 Jul 2026 10:50:40 -0400
ActionableScore: 3
ActionableScore lower bound: 1
Actionable bucket: Borderline, manual review recommended
Manual review required: YES

Summary:
RT_PUSH_IPI on non PREEMPT_RT kernels can cause a CPU livelock when heavy softirq activity overlaps with RT task wakeups and migration IPIs, leading to a local availability-only DoS without evidence of memory corruption or privilege escalation.

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ABOUT THIS REPORT
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The original Linux kernel CVE announcement for CVE-2026-64374 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64374

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.

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AL-KERNEL CLASSIFICATION RESULT
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CVE-2026-64374	MODERATE	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-835;CWE-667;Other CVSS 'AV:L/AC:H/PR:H/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '4.7';DESCR 'RT_PUSH_IPI can cause a livelock on non PREEMPT_RT kernels when heavy softirq activity overlaps with repeated RT task wakeups and RT migration IPIs. A target CPU can remain marked overloaded while softirq execution delays scheduling, causing other CPUs to keep sending IPIs and further slowing progress. For the CVSS the PR:L is used for the paranoid score because some deployments delegate realtime scheduling capability to local users, containers, or service accounts through RLIMIT_RTPRIO or CAP_SYS_NICE like privileges. The issue is not directly network reachable, although heavy network traffic can contribute to the softirq condition. Impact is denial of service through CPU livelock and severe scheduling latency, with no evidence of memory corruption, information disclosure, or privilege escalation.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline, manual review recommended (with actual score 2)  SKIP CVE-2026-64374 UNKNOWN SKIP No affected files built, so skip this CVE NO  - - unknown 	MAYBE	HARDWARE LINUS  INCREASED_FROM_LOW_BASED_ON_REQUIREMANUALCHECK DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN5	-	-	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=3
ActionableScoreLower=1

## 1. ActionableScore

* Conservative score: 1
* Paranoid score: 3
* Final recommended bucket: **Borderline, manual review recommended**

## 2. Signal breakdown

Conservative signals:

* Reliable kernel crash / strong DoS: +1. The commit describes a CPU livelock where repeated RT migration IPIs can prevent progress.
* Availability impact realistic: +1. The observed impact is severe scheduling latency or CPU livelock under heavy softirq and RT wakeup load.
* Broad/default/common subsystem: +1. This is scheduler RT migration logic, and the affected RT_PUSH_IPI feature was default enabled on non PREEMPT_RT before the patch.
* Hard or unreliable race / special timing required: -1. Triggering requires a specific combination of heavy networking softirq load, queued RT tasks, large CPU systems, and timing.
* Requires admin/root/CAP_* in typical deployments: -1 to -2. Reliable triggering usually requires RT scheduling privileges such as CAP_SYS_NICE or configured RLIMIT_RTPRIO.

Paranoid interpretation:

* Local unprivileged trigger: +1. Some systems delegate realtime scheduling to non-root users or service accounts via limits or capabilities.
* Reliable kernel crash / strong DoS: +1.
* Availability impact realistic: +1.
* Broad/default/common subsystem: +1.
* Hard timing / workload dependency: -1.

No memory corruption, UAF, OOB write, information leak, or privilege escalation primitive is supported by the patch.

## 3. Reachability analysis

The bug is locally influenced through realtime scheduling behavior plus heavy softirq activity, especially networking traffic. It is not directly network reachable, because packets alone do not appear sufficient without local RT task wakeups or a workload that creates the RT scheduling pressure.

Typical exploitation requires privileges to create or run RT tasks, commonly CAP_SYS_NICE or an administrator configured realtime limit. Containers or service accounts may reduce the practical privilege requirement if RT scheduling is delegated. The path was default enabled for non PREEMPT_RT kernels affected by RT_PUSH_IPI, but realistic triggering depends on large systems, heavy networking, and RT workloads.

Call-site confidence: medium. The commit explains the scheduler behavior and observed livelock, but the patch is a feature-default change rather than a direct memory-safety fix.

## 4. Severity interpretation

This behaves like a DoS-only scheduler livelock, not an Important-class memory corruption issue. The realistic impact is availability loss through CPU livelock or severe latency. There is no evidence of confidentiality impact, integrity impact, privilege escalation, stale pointer reuse, or attacker-controlled memory corruption.

This is more than an ordinary cosmetic scheduler fix because it affected real workloads and can livelock CPUs, but it remains below Actionable Moderate in the conservative case. The paranoid score reaches borderline manual-review territory mainly because RT scheduling may be delegated to non-root workloads in some environments.

## 5. One-sentence report phrase

RT_PUSH_IPI on non PREEMPT_RT kernels can cause a CPU livelock when heavy softirq activity overlaps with RT task wakeups and migration IPIs, leading to a local availability-only DoS without evidence of memory corruption or privilege escalation.

## 6. Manual review recommendation

MANUAL CHECK RECOMMENDED

Review is recommended for products with large CPU systems, heavy networking, realtime workloads, or delegated RT scheduling. It is not a strong Important candidate because the patch supports DoS-only impact and does not show memory corruption or a privilege-escalation primitive.

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

Patch: sched/rt: Have RT_PUSH_IPI be default off for non PREEMPT_RT
Commit: b99f04ae3d200d2f8844aa29145bd18eccbeecde
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=b99f04ae3d200d2f8844aa29145bd18eccbeecde

Commit description:

RT migration is done aggressively. When a CPU schedules out a high
priority RT task for a lower priority task, it will look to see if there's
any RT tasks that are waiting to run on another CPU that is of higher
priority than the task this CPU is about to run. If it finds one, it will
pull that task over to the CPU and allow it to run there instead.

Normally, this pulling is done by looking at the RT overloaded mask (rto)
which contains all the CPUs in the scheduler domain with RT tasks that are
waiting to run due to a higher priority RT task currently running on their
CPU. The CPU that is about to schedule a lower priority task will grab the
rq lock of the overloaded CPU and move the RT task from that CPU's runqueue
to the local one and schedule the higher priority RT task.

This caused issues when a lot of CPUs would schedule a lower priority task
at the same time. They would all try to grab the same runqueue lock of
the CPU with the overloaded RT tasks. Only the first CPU that got in will
get that task. All the others would wait until they got the runqueue lock
and see there's nothing to pull and do nothing. On systems with lots of
CPUs, this caused a large latency (up to 500us) which is beyond what
PREEMPT_RT is to allow.

The solution to that was to create an RT_PUSH_IPI logic. When any CPU
wanted to pull a task, instead of grabbing the runqueue lock of the
overloaded CPU, it would start by sending an IPI to the overloaded CPU,
and that IPI handler would have the CPU with the waiting RT task do a push
instead. Then that handler would send an IPI to the next CPU with
overloaded RT tasks, and so on. Note, after the first CPU starts this
process, if another CPU wanted to do a pull, it would see that the process
has already begun and would only increment a counter to have the IPIs
continue again.

The RT_PUSH_IPI solved the latency problem with PREEMPT_RT but could cause
a new issue with non PREEMPT_RT. Namely, softirqs run in a threaded
context on PREEMPT_RT but they can run in an interrupt context in non-RT.

If an IPI lands on a CPU that has just woken up multiple RT tasks and the
current CPU is running a non RT or a low priority RT task, instead of
doing a push, it would simply do a schedule on that CPU. But if a softirq
was also executing on this CPU, the schedule would need to wait until the
softirq finished. Until then, the CPU would still be considered overloaded
as there are RT tasks still waiting to run on it.

A live lock occurred on a workload that was doing heavy networking traffic
on a large machine where the softirqs would run 500us out of 750us. And it
would also be waking up RT tasks, causing the RT pull logic to be
constantly executed.

When a softirq triggered on a CPU with RT tasks queued but not running
yet, and the other CPUs would see this CPU as being overloaded, they would
send an IPI over to it. The CPU would notice that the waiting RT tasks are
of higher priority than the currently running task and simply schedule
that CPU instead. But because the softirq was executing, before it could
schedule, it would receive another IPI to do the same. The amount of IPIs
would slow down the currently running softirq so much that before it could
return back to task context, it would execute another softirq never
allowing the CPU to schedule. This live locked that CPU.

As RT_PUSH_IPI was created to help PREEMPT_RT, make it default off if
PREEMPT_RT is not enabled.

Fixes: b6366f0 ("sched/rt: Use IPI to trigger RT task push migration instead of pulling")
Closes: https://lore.kernel.org/all/20260506235716.2530720-1-tj@kernel.org/
Reported-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: stable@vger.kernel.org
Link: https://patch.msgid.link/20260515103740.25ccbed8@gandalf.local.home
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  kernel/sched/features.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=b99f04ae3d200d2f8844aa29145bd18eccbeecde

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

The original Linux kernel CVE announcement for CVE-2026-64374 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64374

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: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: 1
  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 alexanjelausa@gmail.com (and both
send reply to CVE record itself too and see "reply" button below for howto reply).