Re: [PATCH v5 04/13] KVM: x86: Move n_memslots_pages recalc to kvm_arch_prepare_memory_region()

[Sean Christopherson]

On Mon, Sep 20, 2021, Maciej S. Szmigiero wrote:
> From: "Maciej S. Szmigiero" <maciej.szmigiero@xxxxxxxxxx>
> 
> This allows us to return a proper error code in case we spot an underflow.
> 
> Signed-off-by: Maciej S. Szmigiero <maciej.szmigiero@xxxxxxxxxx>
> ---
>  arch/x86/kvm/x86.c | 49 ++++++++++++++++++++++++++--------------------
>  1 file changed, 28 insertions(+), 21 deletions(-)
> 
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 97d86223427d..0fffb8414009 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -11511,9 +11511,23 @@ int kvm_arch_prepare_memory_region(struct kvm *kvm,
>  				const struct kvm_userspace_memory_region *mem,
>  				enum kvm_mr_change change)
>  {
> -	if (change == KVM_MR_CREATE || change == KVM_MR_MOVE)
> -		return kvm_alloc_memslot_metadata(kvm, new,
> -						  mem->memory_size >> PAGE_SHIFT);
> +	if (change == KVM_MR_CREATE || change == KVM_MR_MOVE) {
> +		int ret;
> +
> +		ret = kvm_alloc_memslot_metadata(kvm, new,
> +						 mem->memory_size >> PAGE_SHIFT);
> +		if (ret)
> +			return ret;
> +
> +		if (change == KVM_MR_CREATE)
> +			kvm->arch.n_memslots_pages += new->npages;
> +	} else if (change == KVM_MR_DELETE) {
> +		if (WARN_ON(kvm->arch.n_memslots_pages < old->npages))
> +			return -EIO;

This is not worth the churn.  In a way, it's worse because userspace can spam
the living snot out of the kernel log by retrying the ioctl().

Since underflow can happen if and only if there's a KVM bug, and a pretty bad one
at that, just make the original WARN_ON a KVM_BUG_ON.  That will kill the VM and
also provide the WARN_ON_ONCE behavior that we probably want.

> +
> +		kvm->arch.n_memslots_pages -= old->npages;
> +	}
> +
>  	return 0;
>  }
>