CVE-2026-99085
UnknownSummary
A Linux kernel vulnerability was fixed in the sched_ext scheduler that caused a CPU hotplug hang when a dying CPU's tasks remained in the BPF scheduler. Such tasks were still counted on the runqueue, but an inactive CPU no longer called ops.dispatch() and could not pull them back, blocking the hotplug thread and cpu_down() while holding cpu_hotplug_lock. The fix re-enqueues tasks on the offline rq to the local DSQ so balance_push() can move them off.
Risk Assessment
An organization may experience a hang of CPU offlining or the entire system during hotplug operations, leading to service unavailability and forced restarts. In scenarios with tasks affine only to the dying CPU, offlining may stall until the watchdog intervenes.
Recommendation
Update the Linux kernel to a version containing the fix for CVE-2026-99085. Until patched, avoid routine CPU hotplug operations on systems using sched_ext with a BPF scheduler.
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Original NVD description (English source)
In the Linux kernel, the following vulnerability has been resolved: sched_ext: Fix CPU hotplug hang when a dying CPU's tasks sit in the BPF scheduler A CPU going down has to empty its own rq. Its hotplug thread waits in sched_cpu_wait_empty() until nothing else is left, and the only thing that wakes it is balance_push(), which runs from __schedule() on the dying CPU and pushes off the migratable tasks the CPU picks. sched_ext breaks this. A task sitting on a user DSQ or held by the BPF scheduler is still counted on its rq, but an inactive CPU no longer calls ops.dispatch() and can't pull the task back. The dying CPU goes idle with the task still counted, and one of two things happens: - Another CPU consumes the task. The rq empties without a __schedule() on the dying CPU, so the hotplug thread is never woken and cpu_down() hangs holding cpu_hotplug_lock. - The task is affine only to the dying CPU. Nothing can move it, and the offline stalls until the watchdog ejects the BPF scheduler. When the rq goes offline, re-enqueue every task on it that isn't already on the local DSQ. An enqueue on an offline rq lands on the local DSQ, so the dying CPU itself picks the tasks and balance_push() pushes them off, the same as for the other sched classes. From then on no sched_ext path on another CPU can pull a task off the rq. ops.dispatch() currently stops as soon as the CPU goes inactive, and CPU hotplug then waits for an RCU grace period before taking the rq offline. A BPF-held task affine only to the dying CPU and preempted inside an RCU read-side critical section would block that grace period, and the rq would never go offline. Test SCX_RQ_ONLINE directly so that ops.dispatch() keeps running until the rq goes offline. Only the dying CPU's own hotplug thread clears the flag during teardown, and task_can_run_on_remote_rq() still keeps other rqs' tasks off the CPU.
Vulnerability data from NVD (NIST) · CISA KEV · EPSS