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    SubjectRe: [PATCH v2 01/10] kvm: Restrict non-existing slot state transitions
    On Mon, Dec 10, 2012 at 10:32:45AM -0700, Alex Williamson wrote:
    > The API documentation states:
    >
    > When changing an existing slot, it may be moved in the guest
    > physical memory space, or its flags may be modified.
    >
    > An "existing slot" requires a non-zero npages (memory_size). The only
    > transition we should therefore allow for a non-existing slot should be
    > to create the slot, which includes setting a non-zero memory_size. We
    > currently allow calls to modify non-existing slots, which is pointless,
    > confusing, and possibly wrong.
    >
    > With this we know that the invalidation path of __kvm_set_memory_region
    > is always for a delete or move and never for adding a zero size slot.
    >
    > Signed-off-by: Alex Williamson <alex.williamson@redhat.com>
    > ---
    > virt/kvm/kvm_main.c | 9 +++++++--
    > 1 file changed, 7 insertions(+), 2 deletions(-)
    >
    > diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
    > index 6e8fa7e..e426704 100644
    > --- a/virt/kvm/kvm_main.c
    > +++ b/virt/kvm/kvm_main.c
    > @@ -753,10 +753,15 @@ int __kvm_set_memory_region(struct kvm *kvm,
    > new.npages = npages;
    > new.flags = mem->flags;
    >
    > - /* Disallow changing a memory slot's size. */
    > + /*
    > + * Disallow changing a memory slot's size or changing anything about
    > + * zero sized slots that doesn't involve making them non-zero.
    > + */
    > r = -EINVAL;
    > if (npages && old.npages && npages != old.npages)
    > goto out_free;
    > + if (!npages && !old.npages)
    > + goto out_free;

    /* General sanity checks */
    if (mem->memory_size & (PAGE_SIZE - 1))
    goto out;
    if (mem->guest_phys_addr & (PAGE_SIZE - 1))
    goto out;

    if (mem->guest_phys_addr + mem->memory_size < mem->guest_phys_addr)
    goto out;

    If mem->memory_size is zero, then check above fails.
    If mem->memory_size > 0, it must be at least PAGE_SIZE, in which case
    npages == 1.

    Which means the code does not allow changes to non-existing slots.
    Yes?



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