---
id: "CVE-2026-99065"
kind: "vulnerability"
aliases: ["CVE-2026-99065"]
title: "Linux kernel: cpuset CPU hotplug race can cause a NULL pointer dereference"
severity: "unknown"
cvss: null
epss: 0.00206
exploited: false
status: "open"
category: "runtime"
products: []
ecosystems: []
tags: ["cgroups", "cpuset", "linux-kernel", "runtime"]
not_affected_if: ["kernel-without-cpuset"]
published_at: "2026-10-23T13:17:30+00:00"
first_seen_at: "2026-10-23T21:57:34+00:00"
updated_at: "2026-10-25T12:38:50+00:00"
sources: [{"source": "nvd", "url": "https://nvd.nist.gov/vuln/detail/CVE-2026-99065", "first_seen_at": "2026-10-23T21:57:34+00:00"}, {"source": "epss", "url": "https://api.first.org/data/v1/epss?cve=CVE-2026-99065", "first_seen_at": "2026-10-25T12:38:50+00:00"}]
---

# Linux kernel: cpuset CPU hotplug race can cause a NULL pointer dereference

During a CPU offline operation, a race in the cpuset code can leave the top cpuset with no active CPU, and guarantee_active_cpus() then dereferences a NULL pointer. The sources do not describe what an attacker can gain beyond a kernel crash.

_Summary, check and fix written by a model from the sources; labelled as such._

## How to check

- unknown

## Fix

- Apply the upstream Linux kernel fix that adds back the NULL cs check in guarantee_active_cpus() (cgroup/cpuset), and check your distribution's kernel advisory for the fixed package version.

## Timeline

- 2026-10-23T21:57:34+00:00 nvd https://nvd.nist.gov/vuln/detail/CVE-2026-99065
- 2026-10-25T12:38:50+00:00 epss https://api.first.org/data/v1/epss?cve=CVE-2026-99065

## From the sources

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

cgroup/cpuset: Handle cpu hotplug race in guarantee_active_cpus()

With commit 2125c0034c5d ("cgroup/cpuset: Make cpuset hotplug processing
synchronous"), the cpuset hotplug operation becomes synchronous. That
commit also removes the code that handles race between cpuset_hotplug_work
and cpu hotplug notifier with the assumption that race is now gone.
Later commit 7a0aabd9ce69 ("cgroup/cpuset: Always use cpu_active_mask")
updates the cpuset code to always use cpu_active_mask instead of
cpu_online_mask in some places including guarantee_online_cpus() which
is renamed to guarantee_active_cpus() in that commit.

In the case of CPU offline operation, cpu_active_mask is updated
first in sched_cpu_deactivate() to remove the offline CPU before
cpuset_handle_hotplug() is called to update the effective_cpus of the
affected cpusets. The cpu_online_mask is updated after that near the end
of the offline operation to remove the offline CPU. As a result, the race
comes back and the top cpuset may not have any active CPU leading to NULL
pointer dereference during the race window when guarantee_active_cpus()
is called after cpu_active_mask is updated to remove the CPU to be torn
down but before cpuset_handle_hotplug() is able to properly update the
effective_cpus of the top cpuset.

Fix this by adding back the NULL cs check to avoid this problem.

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