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

From: Greg Kroah-Hartman <[email protected]>

Description
===========

In the Linux kernel, the following vulnerability has been resolved:

sched/rt: Have RT_PUSH_IPI be default off for non PREEMPT_RT

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.

The Linux kernel CVE team has assigned CVE-2026-64374 to this issue.


Affected and fixed versions
===========================

	Issue introduced in 4.1 with commit b6366f048e0caff28af5335b7af2031266e1b06b and fixed in 5.10.261 with commit b99f04ae3d200d2f8844aa29145bd18eccbeecde
	Issue introduced in 4.1 with commit b6366f048e0caff28af5335b7af2031266e1b06b and fixed in 5.15.212 with commit d8312a56d9a162e3ec76476aa487e7d20bc602e9
	Issue introduced in 4.1 with commit b6366f048e0caff28af5335b7af2031266e1b06b and fixed in 6.1.178 with commit 44aae426dbfd51286f7eb601cfa14bc32164812a
	Issue introduced in 4.1 with commit b6366f048e0caff28af5335b7af2031266e1b06b and fixed in 6.6.145 with commit 860aaff72c8446fed5e576249e19952883a18885
	Issue introduced in 4.1 with commit b6366f048e0caff28af5335b7af2031266e1b06b and fixed in 6.12.96 with commit 89237c8fc15d8016a194076e648ccb57d75e65ae
	Issue introduced in 4.1 with commit b6366f048e0caff28af5335b7af2031266e1b06b and fixed in 6.18.39 with commit 4bd0da48fbc1dbef6774175129107fbbdd353e26
	Issue introduced in 4.1 with commit b6366f048e0caff28af5335b7af2031266e1b06b and fixed in 7.1.4 with commit a18f80bf5359238c4f067d691b96af00286fdd89
	Issue introduced in 4.1 with commit b6366f048e0caff28af5335b7af2031266e1b06b and fixed in 7.2-rc1 with commit dd29c017aed628076e915fe4cdfb5392fd4c5cab

Please see https://www.kernel.org for a full list of currently supported
kernel versions by the kernel community.

Unaffected versions might change over time as fixes are backported to
older supported kernel versions.  The official CVE entry at
	https://cve.org/CVERecord/?id=CVE-2026-64374
will be updated if fixes are backported, please check that for the most
up to date information about this issue.


Affected files
==============

The file(s) affected by this issue are:
	kernel/sched/features.h


Mitigation
==========

The Linux kernel CVE team recommends that you update to the latest
stable kernel version for this, and many other bugfixes.  Individual
changes are never tested alone, but rather are part of a larger kernel
release.  Cherry-picking individual commits is not recommended or
supported by the Linux kernel community at all.  If however, updating to
the latest release is impossible, the individual changes to resolve this
issue can be found at these commits:
	https://git.kernel.org/stable/c/b99f04ae3d200d2f8844aa29145bd18eccbeecde
	https://git.kernel.org/stable/c/d8312a56d9a162e3ec76476aa487e7d20bc602e9
	https://git.kernel.org/stable/c/44aae426dbfd51286f7eb601cfa14bc32164812a
	https://git.kernel.org/stable/c/860aaff72c8446fed5e576249e19952883a18885
	https://git.kernel.org/stable/c/89237c8fc15d8016a194076e648ccb57d75e65ae
	https://git.kernel.org/stable/c/4bd0da48fbc1dbef6774175129107fbbdd353e26
	https://git.kernel.org/stable/c/a18f80bf5359238c4f067d691b96af00286fdd89
	https://git.kernel.org/stable/c/dd29c017aed628076e915fe4cdfb5392fd4c5cab