If a BPF scheduler provides an invalid CPU (outside the nr_cpu_ids
range) as prev_cpu to scx_bpf_select_cpu_dfl() it can cause a kernel
crash.

To prevent this, validate prev_cpu in scx_bpf_select_cpu_dfl() and
trigger an scx error if an invalid CPU is specified.

Fixes: f0e1a0643a59b ("sched_ext: Implement BPF extensible scheduler class")
Cc: stable@vger.kernel.org # v6.12+
Signed-off-by: Andrea Righi <arighi@nvidia.com>
---
 kernel/sched/ext.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/kernel/sched/ext.c b/kernel/sched/ext.c
index 0f1da199cfc7c..88b2ea58ff942 100644
--- a/kernel/sched/ext.c
+++ b/kernel/sched/ext.c
@@ -6422,6 +6422,11 @@ static bool check_builtin_idle_enabled(void)
 __bpf_kfunc s32 scx_bpf_select_cpu_dfl(struct task_struct *p, s32 prev_cpu,
 				       u64 wake_flags, bool *is_idle)
 {
+	if (!ops_cpu_valid(prev_cpu, NULL)) {
+		prev_cpu = cpumask_any(p->cpus_ptr);
+		goto prev_cpu;
+	}
+
 	if (!check_builtin_idle_enabled())
 		goto prev_cpu;
 
-- 
2.48.1

Hello,

On Sun, Mar 02, 2025 at 11:09:03PM +0100, Andrea Righi wrote:
> If a BPF scheduler provides an invalid CPU (outside the nr_cpu_ids
> range) as prev_cpu to scx_bpf_select_cpu_dfl() it can cause a kernel
> crash.
> 
> To prevent this, validate prev_cpu in scx_bpf_select_cpu_dfl() and
> trigger an scx error if an invalid CPU is specified.
> 
> Fixes: f0e1a0643a59b ("sched_ext: Implement BPF extensible scheduler class")
> Cc: stable@vger.kernel.org # v6.12+
> Signed-off-by: Andrea Righi <arighi@nvidia.com>
> ---
>  kernel/sched/ext.c | 5 +++++
>  1 file changed, 5 insertions(+)
> 
> diff --git a/kernel/sched/ext.c b/kernel/sched/ext.c
> index 0f1da199cfc7c..88b2ea58ff942 100644
> --- a/kernel/sched/ext.c
> +++ b/kernel/sched/ext.c
> @@ -6422,6 +6422,11 @@ static bool check_builtin_idle_enabled(void)
>  __bpf_kfunc s32 scx_bpf_select_cpu_dfl(struct task_struct *p, s32 prev_cpu,
>  				       u64 wake_flags, bool *is_idle)
>  {
> +	if (!ops_cpu_valid(prev_cpu, NULL)) {
> +		prev_cpu = cpumask_any(p->cpus_ptr);
> +		goto prev_cpu;

I'd just return the invalid prev_cpu. The scheduler is getting aborted
anyway.

Thanks.

-- 
tejun

On Mon, Mar 03, 2025 at 06:11:15AM -1000, Tejun Heo wrote:
> Hello,
> 
> On Sun, Mar 02, 2025 at 11:09:03PM +0100, Andrea Righi wrote:
> > If a BPF scheduler provides an invalid CPU (outside the nr_cpu_ids
> > range) as prev_cpu to scx_bpf_select_cpu_dfl() it can cause a kernel
> > crash.
> > 
> > To prevent this, validate prev_cpu in scx_bpf_select_cpu_dfl() and
> > trigger an scx error if an invalid CPU is specified.
> > 
> > Fixes: f0e1a0643a59b ("sched_ext: Implement BPF extensible scheduler class")
> > Cc: stable@vger.kernel.org # v6.12+
> > Signed-off-by: Andrea Righi <arighi@nvidia.com>
> > ---
> >  kernel/sched/ext.c | 5 +++++
> >  1 file changed, 5 insertions(+)
> > 
> > diff --git a/kernel/sched/ext.c b/kernel/sched/ext.c
> > index 0f1da199cfc7c..88b2ea58ff942 100644
> > --- a/kernel/sched/ext.c
> > +++ b/kernel/sched/ext.c
> > @@ -6422,6 +6422,11 @@ static bool check_builtin_idle_enabled(void)
> >  __bpf_kfunc s32 scx_bpf_select_cpu_dfl(struct task_struct *p, s32 prev_cpu,
> >  				       u64 wake_flags, bool *is_idle)
> >  {
> > +	if (!ops_cpu_valid(prev_cpu, NULL)) {
> > +		prev_cpu = cpumask_any(p->cpus_ptr);
> > +		goto prev_cpu;
> 
> I'd just return the invalid prev_cpu. The scheduler is getting aborted
> anyway.
> 
> Thanks.

Yeah, makes sense. I'll send a new patch soon.

Thanks,
-Andrea