Re: [PATCH 1/7] KVM: Write protect the updated slot only when we start dirty logging

[Gleb Natapov <gleb@redhat.com>]

On Tue, Dec 18, 2012 at 04:26:47PM +0900, Takuya Yoshikawa wrote:
> This is needed to make kvm_mmu_slot_remove_write_access() rmap based:
> otherwise we may end up using invalid rmap's.
> 
> Signed-off-by: Takuya Yoshikawa <yoshikawa_takuya_b1@lab.ntt.co.jp>
> ---
>  arch/x86/kvm/x86.c  |    9 ++++++++-
>  virt/kvm/kvm_main.c |    1 -
>  2 files changed, 8 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 1c9c834..9451efa 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -6897,7 +6897,14 @@ void kvm_arch_commit_memory_region(struct kvm *kvm,
>  	spin_lock(&kvm->mmu_lock);
>  	if (nr_mmu_pages)
>  		kvm_mmu_change_mmu_pages(kvm, nr_mmu_pages);
> -	kvm_mmu_slot_remove_write_access(kvm, mem->slot);
> +	/*
> +	 * Write protect all pages for dirty logging.
> +	 * Existing largepage mappings are destroyed here and new ones will
> +	 * not be created until the end of the logging.
> +	 */
> +	if ((mem->flags & KVM_MEM_LOG_DIRTY_PAGES) &&
> +	    !(old.flags & KVM_MEM_LOG_DIRTY_PAGES))
> +		kvm_mmu_slot_remove_write_access(kvm, mem->slot);
We should not check old slot flags here or at least check that
old.npages is not zero. Userspace may delete a slot using old flags,
then, if new memslot is created with dirty log enabled, it will not be
protected.

>  	spin_unlock(&kvm->mmu_lock);
>  	/*
>  	 * If memory slot is created, or moved, we need to clear all
> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> index bd31096..0ef5daa 100644
> --- a/virt/kvm/kvm_main.c
> +++ b/virt/kvm/kvm_main.c
> @@ -805,7 +805,6 @@ int __kvm_set_memory_region(struct kvm *kvm,
>  	if ((new.flags & KVM_MEM_LOG_DIRTY_PAGES) && !new.dirty_bitmap) {
>  		if (kvm_create_dirty_bitmap(&new) < 0)
>  			goto out_free;
> -		/* destroy any largepage mappings for dirty tracking */
>  	}
>  
>  	if (!npages || base_gfn != old.base_gfn) {
> -- 
> 1.7.5.4

--
			Gleb.