Re: [PATCH v5 12/19] KVM: Move memslot deletion to helper function

[Sean Christopherson]

On Thu, Feb 06, 2020 at 11:14:15AM -0500, Peter Xu wrote:
> On Tue, Jan 21, 2020 at 02:31:50PM -0800, Sean Christopherson wrote:
> > Move memslot deletion into its own routine so that the success path for
> > other memslot updates does not need to use kvm_free_memslot(), i.e. can
> > explicitly destroy the dirty bitmap when necessary.  This paves the way
> > for dropping @dont from kvm_free_memslot(), i.e. all callers now pass
> > NULL for @dont.
> > 
> > Add a comment above the code to make a copy of the existing memslot
> > prior to deletion, it is not at all obvious that the pointer will become
> > stale during sorting and/or installation of new memslots.
> 
> Could you help explain a bit on this explicit comment?  I can follow
> up with the patch itself which looks all correct to me, but I failed
> to catch what this extra comment wants to emphasize...

It's tempting to write the code like this (I know, because I did it):

	if (!mem->memory_size)
		return kvm_delete_memslot(kvm, mem, slot, as_id);

	new = *slot;

Where @slot is a pointer to the memslot to be deleted.  At first, second,
and third glances, this seems perfectly sane.

The issue is that slot was pulled from struct kvm_memslots.memslots, e.g.

	slot = &slots->memslots[index];

Note that slots->memslots holds actual "struct kvm_memory_slot" objects,
not pointers to slots.  When update_memslots() sorts the slots, it swaps
the actual slot objects, not pointers.  I.e. after update_memslots(), even
though @slot points at the same address, it's could be pointing at a
different slot.  As a result kvm_free_memslot() in kvm_delete_memslot()
will free the dirty page info and arch-specific points for some random
slot, not the intended slot, and will set npages=0 for that random slot.