Wind River Support Network

Description

In the Linux kernel, the following vulnerability has been resolved:  sched/core: Call wakeup_preempt() for queued tasks during class change  Paul reported recent RCU changes exposed a lost resched where CPU idles with a rcutorture thread waiting while being completely runnable.  Dumping the rq state at the time of idling with runnable tasks showed the following:    rq->next_class = &fair_sched_class   rq->nr_running = 1   rq->cfs.h_nr_queued = 0   rq->rt.rt_nr_running = 1  Although the waiting task was an RT task, "rq->next_class" did not point to rt_sched_class and was stuck at the lower priority fair_sched_class.  Inspecting sched_change_end() revealed the following race with __sched_setscheduler() for queued, but non-running tasks:      CPU0                                   CPU1     ====                                   ====    __schedule()      /*      * rq->donor is a fair task that blocks.      * rq->next_class = &fair_sched_class      */      rq_lock(rq0)      pick_next_task()       /* nr_running = 0 */       pick_task_fair()         sched_balance_newidle()           raw_spin_rq_unlock(rq0)                                     rq_lock(rq0)                                      activate_task(rq0, p)                                       /* p is a fair task */                                       rq0->nr_running++;                                       rq0->cfs.h_nr_queued++;                                       wakeup_preempt(rq, p)                                         rq->next_class = &fair_sched_class                                         resched_curr()                                      rq_unlock(rq0)                                      __sched_setscheduler(p)                                       /* p is moved to SCHED_RR */                                       task_rq_lock(p)                                       sched_change_begin()  				      rq0->cfs.h_nr_queued--; 				      rq0->rt.rt_nr_running++;                                        sched_change_end()                                         /*                                          * p->on_rq = 1 (ctx->queued)                                          * but p is not rq->donor                                          * (!ctx->running) hence                                          * resched_curr() is and                                          * rq->next_class upgrade is                                          * skipped.                                          */                                         task_rq_unlock()            raw_spin_rq_lock(rq0)            /*            * rq->nr_running = 1            * rq->cfs.h_nr_queued = 0            * rq->rt.rt_nr_running = 1            * rq->next_class = &fair_sched_class            */             if (rq->cfs.h_nr_queued && ...)              /* Above is false */             if (rq_modified_above(this_rq, &fair_sched_class))              /* Also false since rq->next_class is still fair */        /*        * sched_balance_newidle() is considered to        * have failed to pull any fair tasks and no        * other wakeups were detected.        */        return NULL; /* pick_task_fair() */      pick_task_idle()       return rq->idle;      next = rq->idle;      /* Preemption indicators are lost */     clear_tsk_need_resched(prev);     clear_preempt_need_resched();      rq->curr = rq->idle;    !!! rq->idle switches in with rq->running == 1 but !need_resched() !!!  In simple words, enqueue, followed by a class change of the newly queued task goes unnoticed in rq->next_class indicator and sched_balance_newidle() sees no fair task, nor any higher priority task enqueued.  The above sequence will lead to CPU idling with a runnable task on the CPU.  Call wakeup_preempt() for ctx->queued cases in sched_change_end() to re-evaluate rq->next_class after class change (ENQUEUE_CLASS) and set the NEED_RESCHED indicator accordingly.  Newidle balance will correctly see an enqueue of a higher priority t ---truncated---
Data source: kernel.org (416baaa9-dc9f-4396-8d5f-8c081fb06d67)

CVEs

• CVE-2026-99205