Re: [PATCH v5 bpf-next 5/9] bpf: cpumap: add the possibility to attach an eBPF program to cpumap

[Daniel Borkmann <daniel@xxxxxxxxxxxxx>]

On 7/3/20 10:48 PM, Lorenzo Bianconi wrote:
> > On 6/30/20 2:49 PM, Lorenzo Bianconi wrote:
> > [...]
>
> [...]
>
> > >    	old_rcpu = xchg(&cmap->cpu_map[key_cpu], rcpu);
> > >    	if (old_rcpu) {
> > > +		if (old_rcpu->prog)
> > > +			bpf_prog_put(old_rcpu->prog);
> > >    		call_rcu(&old_rcpu->rcu, __cpu_map_entry_free);
> > >    		INIT_WORK(&old_rcpu->kthread_stop_wq, cpu_map_kthread_stop);
> > >    		schedule_work(&old_rcpu->kthread_stop_wq);
> >
> > Hm, not quite sure I follow the logic here. Why is the bpf_prog_put() not placed inside
> > __cpu_map_entry_free(), for example? Wouldn't this at least leave a potential small race
> > window of UAF given the rest is still live? If we already piggy-back from RCU side on
> > rcpu entry, why not having it in __cpu_map_entry_free()?
>
> ack right, thanks for spotting this issue. I guess we can even move
> "bpf_prog_put(rcpu->prog)" in put_cpu_map_entry() so the last consumer
> of bpf_cpu_map_entry will free the attached program. Agree?

Yes, that sounds reasonable to me.

Thanks,
Daniel