Re: [PATCH] sched_ext: Validate prev_cpu in scx_bpf_select_cpu_dfl()

[Andrea Righi]

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@xxxxxxxxxxxxxxx # v6.12+
> > Signed-off-by: Andrea Righi <arighi@xxxxxxxxxx>
> > ---
> >  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