sched: Fix pick_next_task_fair() vs try_to_wake_up() race

Syzkaller robot reported KCSAN tripping over the
ASSERT_EXCLUSIVE_WRITER(p->on_rq) in __block_task().

The report noted that both pick_next_task_fair() and try_to_wake_up()
were concurrently trying to write to the same p->on_rq, violating the
assertion -- even though both paths hold rq->__lock.

The logical consequence is that both code paths end up holding a
different rq->__lock. And looking through ttwu(), this is possible
when the __block_task() 'p->on_rq = 0' store is visible to the ttwu()
'p->on_rq' load, which then assumes the task is not queued and
continues to migrate it.

Rearrange things such that __block_task() releases @p with the store
and no code thereafter will use @p again.

Fixes: 152e11f6df29 ("sched/fair: Implement delayed dequeue")
Reported-by: syzbot+0ec1e96c2cdf5c0e512a@syzkaller.appspotmail.com
Reported-by: Kent Overstreet <kent.overstreet@linux.dev>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Tested-by: Marco Elver <elver@google.com>
Link: https://lkml.kernel.org/r/20241023093641.GE16066@noisy.programming.kicks-ass.net

1 files changed, 32 insertions, 2 deletions

diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index 081519ffab46..9f9d1cc390b1 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -2769,8 +2769,6 @@ static inline void sub_nr_running(struct rq *rq, unsigned count)
 
 static inline void __block_task(struct rq *rq, struct task_struct *p)
 {
-	WRITE_ONCE(p->on_rq, 0);
-	ASSERT_EXCLUSIVE_WRITER(p->on_rq);
 	if (p->sched_contributes_to_load)
 		rq->nr_uninterruptible++;
 
@@ -2778,6 +2776,38 @@ static inline void __block_task(struct rq *rq, struct task_struct *p)
 		atomic_inc(&rq->nr_iowait);
 		delayacct_blkio_start();
 	}
+
+	ASSERT_EXCLUSIVE_WRITER(p->on_rq);
+
+	/*
+	 * The moment this write goes through, ttwu() can swoop in and migrate
+	 * this task, rendering our rq->__lock ineffective.
+	 *
+	 * __schedule()				try_to_wake_up()
+	 *   LOCK rq->__lock			  LOCK p->pi_lock
+	 *   pick_next_task()
+	 *     pick_next_task_fair()
+	 *       pick_next_entity()
+	 *         dequeue_entities()
+	 *           __block_task()
+	 *             RELEASE p->on_rq = 0	  if (p->on_rq && ...)
+	 *					    break;
+	 *
+	 *					  ACQUIRE (after ctrl-dep)
+	 *
+	 *					  cpu = select_task_rq();
+	 *					  set_task_cpu(p, cpu);
+	 *					  ttwu_queue()
+	 *					    ttwu_do_activate()
+	 *					      LOCK rq->__lock
+	 *					      activate_task()
+	 *					        STORE p->on_rq = 1
+	 *   UNLOCK rq->__lock
+	 *
+	 * Callers must ensure to not reference @p after this -- we no longer
+	 * own it.
+	 */
+	smp_store_release(&p->on_rq, 0);
 }
 
 extern void activate_task(struct rq *rq, struct task_struct *p, int flags);