CVE-2026-99065 cgroup/cpuset: Handle cpu hotplug race in guarantee_active_cpus()
- Exploited in the wild
- Not known
- Fix
- Available
What is CVE-2026-99065?
CVE-2026-99065 is a vulnerability in Linux, published on 23 Oct 2026.
Official description
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.
Which versions are affected by CVE-2026-99065?
How do I fix CVE-2026-99065?
Upgrade to a fixed version.
Is CVE-2026-99065 being exploited?
There is no known exploitation of CVE-2026-99065 in the wild and no public exploit code.
Sources
Also known as BELL-CVE-2026-99065, DEBIAN-CVE-2026-99065, GHSA-px5v-f8qg-w5f8, UBUNTU-CVE-2026-99065.