CVE-2025-39953 cgroup: split cgroup_destroy_wq into 3 workqueues
A race condition in Linux kernel cgroup workqueues can cause a system hang during controller mount/unmount operations, leading to denial of service.
- Exploited in the wild
- Not known
- Login needed
- Any signed-in user
- Attackable
- Local or same network only
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2025-39953?
CVE-2025-39953 is a medium-severity denial of service vulnerability in Linux kernel, published on 4 Oct 2025. It can only be exploited from the same machine or local network, with no action from the victim.
Official description
In the Linux kernel, the following vulnerability has been resolved:
cgroup: split cgroup_destroy_wq into 3 workqueues
A hung task can occur during [1] LTP cgroup testing when repeatedly mounting/unmounting perf_event and net_prio controllers with systemd.unified_cgroup_hierarchy=1. The hang manifests in cgroup_lock_and_drain_offline() during root destruction.
Related case: cgroup_fj_function_perf_event cgroup_fj_function.sh perf_event cgroup_fj_function_net_prio cgroup_fj_function.sh net_prio
Call Trace: cgroup_lock_and_drain_offline+0x14c/0x1e8 cgroup_destroy_root+0x3c/0x2c0 css_free_rwork_fn+0x248/0x338 process_one_work+0x16c/0x3b8 worker_thread+0x22c/0x3b0 kthread+0xec/0x100 ret_from_fork+0x10/0x20
Root Cause:
CPU0 CPU1 mount perf_event umount net_prio cgroup1_get_tree cgroup_kill_sb rebind_subsystems // root destruction enqueues // cgroup_destroy_wq // kill all perf_event css // one perf_event css A is dying // css A offline enqueues cgroup_destroy_wq // root destruction will be executed first css_free_rwork_fn cgroup_destroy_root cgroup_lock_and_drain_offline // some perf descendants are dying // cgroup_destroy_wq max_active = 1 // waiting for css A to die
Problem scenario: 1. CPU0 mounts perf_event (rebind_subsystems) 2. CPU1 unmounts net_prio (cgroup_kill_sb), queuing root destruction work 3. A dying perf_event CSS gets queued for offline after root destruction 4. Root destruction waits for offline completion, but offline work is blocked behind root destruction in cgroup_destroy_wq (max_active=1)
Solution: Split cgroup_destroy_wq into three dedicated workqueues: cgroup_offline_wq – Handles CSS offline operations cgroup_release_wq – Manages resource release cgroup_free_wq – Performs final memory deallocation
This separation eliminates blocking in the CSS free path while waiting for offline operations to complete.
[1] https://github.com/linux-test-project/ltp/blob/master/runtest/controllers
Which versions are affected by CVE-2025-39953?
It affects 1 product.
Products include Linux kernel.
How do I fix CVE-2025-39953?
Apply the vendor's patch.
Upgrade, then put limits on request size, rate and nesting depth so one client cannot exhaust the service. More about Denial of service.
Is CVE-2025-39953 being exploited?
There is no known exploitation of CVE-2025-39953 in the wild and no public exploit code.
Other vulnerabilities in Linux kernel
Sources
Also known as AZL-68165, AZL-75179, BELL-CVE-2025-39953, DEBIAN-CVE-2025-39953, GHSA-vgcj-37hg-gm6g, ROOT-OS-DEBIAN-11-CVE-2025-39953 and 5 more.