Re: [PATCH v5.5 01/30] KVM: Ensure local memslot copies operate on up-to-date arch-specific data

On Thu, Nov 04, 2021, Ben Gardon wrote:
> > @@ -1597,6 +1596,26 @@ static int kvm_set_memslot(struct kvm *kvm,
> >                 kvm_copy_memslots(slots, __kvm_memslots(kvm, as_id));
> >         }
> >
> > +       /*
> > +        * Make a full copy of the old memslot, the pointer will become stale
> > +        * when the memslots are re-sorted by update_memslots(), and the old
> > +        * memslot needs to be referenced after calling update_memslots(), e.g.
> > +        * to free its resources and for arch specific behavior.  This needs to
> > +        * happen *after* (re)acquiring slots_arch_lock.
> > +        */
> > +       slot = id_to_memslot(slots, new->id);
> > +       if (slot) {
> > +               old = *slot;
> > +       } else {
> > +               WARN_ON_ONCE(change != KVM_MR_CREATE);
> > +               memset(&old, 0, sizeof(old));
> > +               old.id = new->id;
> > +               old.as_id = as_id;
> > +       }
> > +
> > +       /* Copy the arch-specific data, again after (re)acquiring slots_arch_lock. */
> > +       memcpy(&new->arch, &old.arch, sizeof(old.arch));
> > +
> 
> Is new->arch not initialized before this function is called? Does this
> need to be here, or could it be moved above into the first branch of
> the if statement?
> Oh I see you removed the memset below and replaced it with this. I
> think this is fine, but it might be easier to reason about if we left
> the memset and moved the memcopy into the if.
> No point in doing a memcpy of zeros here.

Hmm, good point.  I wrote it like this so that the "arch" part is more identifiable
since that's what needs to be protected by the lock, but I completely agree that
it's odd when viewed without that lens.