[PATCH] sched_ext: Fix NULL dereference in find_parent_sched()

find_parent_sched() dereferences cgrp->scx_sched without checking
for NULL. cgroup_get_from_id() can return cgroups from any
hierarchy, including cgroup v1 where scx_sched is never set.

A BPF program that passes a v1 cgroup ID as sub_cgroup_id would
hit parent->cgrp on a NULL pointer and crash the kernel.

Add a NULL check and return -ENODEV.

Signed-off-by: Cui Jian <[email protected]>
---
 kernel/sched/ext/sub.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/kernel/sched/ext/sub.c b/kernel/sched/ext/sub.c
index d7842a609d96..6395281bae71 100644
--- a/kernel/sched/ext/sub.c
+++ b/kernel/sched/ext/sub.c
@@ -1209,6 +1209,10 @@ static struct scx_sched *find_parent_sched(struct cgroup *cgrp)
 
 	lockdep_assert_held(&scx_sched_lock);
 
+	/* no SCX sched */
+	if (!parent)
+		return ERR_PTR(-ENODEV);
+
 	/* can't attach twice to the same cgroup */
 	if (parent->cgrp == cgrp)
 		return ERR_PTR(-EBUSY);
-- 
2.34.1