Re: [PATCH v3 01/16] perf: Ensure perf_guest_cbs aren't reloaded between !NULL check and deref

[Sean Christopherson <seanjc@google.com>]

On Thu, Nov 04, 2021, Like Xu wrote:
> On 22/9/2021 8:05 am, Sean Christopherson wrote:
> > diff --git a/kernel/events/core.c b/kernel/events/core.c
> > index 464917096e73..80ff050a7b55 100644
> > --- a/kernel/events/core.c
> > +++ b/kernel/events/core.c
> > @@ -6491,14 +6491,21 @@ struct perf_guest_info_callbacks *perf_guest_cbs;
> >   int perf_register_guest_info_callbacks(struct perf_guest_info_callbacks *cbs)
> >   {
> > -	perf_guest_cbs = cbs;
> > +	if (WARN_ON_ONCE(perf_guest_cbs))
> > +		return -EBUSY;
> > +
> > +	WRITE_ONCE(perf_guest_cbs, cbs);
> 
> So per Paolo's comment [1], does it help to use
> 	smp_store_release(perf_guest_cbs, cbs)
> or
> 	rcu_assign_pointer(perf_guest_cbs, cbs)
> here?

Heh, if by "help" you mean "required to prevent bad things on weakly ordered
architectures", then yes, it helps :-)  If I'm interpeting Paolo's suggestion
correctly, he's pointing out that oustanding stores to the function pointers in
@cbs need to complete before assigning a non-NULL pointer to perf_guest_cbs,
otherwise a perf event handler may see a valid pointer with half-baked callbacks.

I think smp_store_release() with a comment would be appropriate, assuming my
above interpretation is correct.

> [1] https://lore.kernel.org/kvm/37afc465-c12f-01b9-f3b6-c2573e112d76@redhat.com/
> 
> >   	return 0;
> >   }
> >   EXPORT_SYMBOL_GPL(perf_register_guest_info_callbacks);
> >   int perf_unregister_guest_info_callbacks(struct perf_guest_info_callbacks *cbs)
> >   {
> > -	perf_guest_cbs = NULL;
> > +	if (WARN_ON_ONCE(perf_guest_cbs != cbs))
> > +		return -EINVAL;
> > +
> > +	WRITE_ONCE(perf_guest_cbs, NULL);
> > +	synchronize_rcu();
> >   	return 0;
> >   }
> >   EXPORT_SYMBOL_GPL(perf_unregister_guest_info_callbacks);
> >