Re: [PATCH v2 01/10] kvm: Restrict non-existing slot statetransitions

[Marcelo Tosatti]

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@xxxxxxxxxx>
> ---
>  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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