[PATCH 14/13] sched_ext: Reject NULL-sch callers in scx_bpf_task_set_slice/dsq_vtime

[Tejun Heo <tj@kernel.org>]

scx_prog_sched(aux) returns NULL for TRACING / SYSCALL BPF progs that
have no struct_ops association when the root scheduler has sub_attach
set. scx_bpf_task_set_slice() and scx_bpf_task_set_dsq_vtime() pass
that NULL into scx_task_on_sched(sch, p), which under
CONFIG_EXT_SUB_SCHED is rcu_access_pointer(p->scx.sched) == sch. For
any non-scx task p->scx.sched is NULL, so NULL == NULL returns true
and the authority gate is bypassed - a privileged but
non-struct_ops-associated prog can poke p->scx.slice /
p->scx.dsq_vtime on arbitrary tasks.

Reject !sch up front so the gate only admits callers with a resolved
scheduler.

Fixes: 245d09c594ea ("sched_ext: Enforce scheduler ownership when updating slice and dsq_vtime")
Reported-by: Chris Mason <clm@meta.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
---
 kernel/sched/ext.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/kernel/sched/ext.c b/kernel/sched/ext.c
index 9a31b8f064e9..52b63266e647 100644
--- a/kernel/sched/ext.c
+++ b/kernel/sched/ext.c
@@ -8619,7 +8619,7 @@ __bpf_kfunc bool scx_bpf_task_set_slice(struct task_struct *p, u64 slice,
 
 	guard(rcu)();
 	sch = scx_prog_sched(aux);
-	if (unlikely(!scx_task_on_sched(sch, p)))
+	if (unlikely(!sch || !scx_task_on_sched(sch, p)))
 		return false;
 
 	p->scx.slice = slice;
@@ -8642,7 +8642,7 @@ __bpf_kfunc bool scx_bpf_task_set_dsq_vtime(struct task_struct *p, u64 vtime,
 
 	guard(rcu)();
 	sch = scx_prog_sched(aux);
-	if (unlikely(!scx_task_on_sched(sch, p)))
+	if (unlikely(!sch || !scx_task_on_sched(sch, p)))
 		return false;
 
 	p->scx.dsq_vtime = vtime;
-- 
2.53.0