[PATCHv2] sched_ext: Mark scx_bpf_cpu_rq as NULL returnable

[Christian Loehle <christian.loehle@arm.com>]

scx_bpf_cpu_rq() obviously returns NULL on invalid cpu.
Mark it as such.
While kf_cpu_valid() will trigger scx_ops_error() that leads
to the BPF scheduler exiting, this isn't guaranteed to be immediate,
allowing for a dereference of a NULL scx_bpf_cpu_rq() return value.

Cc: stable@vger.kernel.org
Fixes: 6203ef73fa5c ("sched/ext: Add BPF function to fetch rq")
Signed-off-by: Christian Loehle <christian.loehle@arm.com>
Acked-by: Andrea Righi <arighi@nvidia.com>
---
v2: Extended commit message and picked up Andrea's ACK.

 kernel/sched/ext.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/sched/ext.c b/kernel/sched/ext.c
index 7dd5cbcb7a06..b734f55f3318 100644
--- a/kernel/sched/ext.c
+++ b/kernel/sched/ext.c
@@ -7599,7 +7599,7 @@ BTF_ID_FLAGS(func, scx_bpf_get_online_cpumask, KF_ACQUIRE)
 BTF_ID_FLAGS(func, scx_bpf_put_cpumask, KF_RELEASE)
 BTF_ID_FLAGS(func, scx_bpf_task_running, KF_RCU)
 BTF_ID_FLAGS(func, scx_bpf_task_cpu, KF_RCU)
-BTF_ID_FLAGS(func, scx_bpf_cpu_rq)
+BTF_ID_FLAGS(func, scx_bpf_cpu_rq, KF_RET_NULL)
 #ifdef CONFIG_CGROUP_SCHED
 BTF_ID_FLAGS(func, scx_bpf_task_cgroup, KF_RCU | KF_ACQUIRE)
 #endif
-- 
2.34.1