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

Overview

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.

Affected

• Linux >= 7a0aabd9ce697fc581ec691b21f748050fbcf004 < 8d88d3221204fbb1b6dc2c0f2f2181fe49126639
• Linux >= 7a0aabd9ce697fc581ec691b21f748050fbcf004 < a21c255159bfe0034740292e10d2e258feeeb10d
• Linux >= 7a0aabd9ce697fc581ec691b21f748050fbcf004 < 23b1ab15ca91f2cffaf149d9bb0fafeb700c7db2
• Linux 6.16

Remediation

Refer to the linked advisories for vendor-supplied fixes and affected version ranges.