[PATCH 02/12] sched_ext: Avoid NULL scx_root deref through SCX_HAS_OP()

[Tejun Heo <tj@kernel.org>]

SCX_HAS_OP() tests scx_root->has_op bitmap. The bitmap is currently in a
statically allocated struct scx_sched and initialized while loading the BPF
scheduler and cleared while unloading, and thus can be tested anytime.
However, scx_root will be switched to dynamic allocation and thus won't
always be deferenceable.

Most usages of SCX_HAS_OP() are already protected by scx_enabled() either
directly or indirectly (e.g. through a task which is on SCX). However, there
are a couple places that could try to dereference NULL scx_root. Update them
so that scx_root is guaranteed to be valid before SCX_HAS_OP() is called.

- In handle_hotplug(), test whether scx_root is NULL before doing anything
  else. This is safe because scx_root updates will be protected by
  cpus_read_lock().

- In scx_tg_offline(), test scx_cgroup_enabled before invoking SCX_HAS_OP(),
  which should guarnatee that scx_root won't turn NULL. This is also in line
  with other cgroup operations. As the code path is synchronized against
  scx_cgroup_init/exit() through scx_cgroup_rwsem, this shouldn't cause any
  behavior differences.

Signed-off-by: Tejun Heo <tj@kernel.org>
---
 kernel/sched/ext.c | 11 ++++++++++-
 1 file changed, 10 insertions(+), 1 deletion(-)

diff --git a/kernel/sched/ext.c b/kernel/sched/ext.c
index 975f6963a01b..ad392890d2dd 100644
--- a/kernel/sched/ext.c
+++ b/kernel/sched/ext.c
@@ -3498,6 +3498,14 @@ static void handle_hotplug(struct rq *rq, bool online)
 
 	atomic_long_inc(&scx_hotplug_seq);
 
+	/*
+	 * scx_root updates are protected by cpus_read_lock() and will stay
+	 * stable here. Note that we can't depend on scx_enabled() test as the
+	 * hotplug ops need to be enabled before __scx_enabled is set.
+	 */
+	if (!scx_root)
+		return;
+
 	if (scx_enabled())
 		scx_idle_update_selcpu_topology(&scx_root->ops);
 
@@ -3994,7 +4002,8 @@ void scx_tg_offline(struct task_group *tg)
 
 	percpu_down_read(&scx_cgroup_rwsem);
 
-	if (SCX_HAS_OP(scx_root, cgroup_exit) && (tg->scx_flags & SCX_TG_INITED))
+	if (scx_cgroup_enabled && SCX_HAS_OP(scx_root, cgroup_exit) &&
+	    (tg->scx_flags & SCX_TG_INITED))
 		SCX_CALL_OP(SCX_KF_UNLOCKED, cgroup_exit, NULL, tg->css.cgroup);
 	tg->scx_flags &= ~(SCX_TG_ONLINE | SCX_TG_INITED);
 
-- 
2.49.0