Re: [PATCH 3/5] KVM: x86/mmu: Harden TDP MMU iteration against root w/o shadow page

[Yu Zhang <yu.c.zhang@linux.intel.com>]

On Fri, Jul 21, 2023 at 06:23:48PM -0700, Sean Christopherson wrote:
> Explicitly check that tdp_iter_start() is handed a valid shadow page
> to harden KVM against bugs where

Sorry, where? 

It's not about guest using an invisible GFN, it's about a KVM bug, right?

> 
> Opportunistically stop the TDP MMU iteration instead of continuing on
> with garbage if the incoming root is bogus.  Attempting to walk a garbage
> root is more likely to caused major problems than doing nothing.
> 
> Signed-off-by: Sean Christopherson <seanjc@google.com>
> ---
>  arch/x86/kvm/mmu/tdp_iter.c | 11 ++++++-----
>  1 file changed, 6 insertions(+), 5 deletions(-)
> 
> diff --git a/arch/x86/kvm/mmu/tdp_iter.c b/arch/x86/kvm/mmu/tdp_iter.c
> index d2eb0d4f8710..bd30ebfb2f2c 100644
> --- a/arch/x86/kvm/mmu/tdp_iter.c
> +++ b/arch/x86/kvm/mmu/tdp_iter.c
> @@ -39,13 +39,14 @@ void tdp_iter_restart(struct tdp_iter *iter)
>  void tdp_iter_start(struct tdp_iter *iter, struct kvm_mmu_page *root,
>  		    int min_level, gfn_t next_last_level_gfn)
>  {
> -	int root_level = root->role.level;
> -
> -	WARN_ON(root_level < 1);
> -	WARN_ON(root_level > PT64_ROOT_MAX_LEVEL);
> +	if (WARN_ON_ONCE(!root || (root->role.level < 1) ||
> +			 (root->role.level > PT64_ROOT_MAX_LEVEL))) {
> +		iter->valid = false;
> +		return;
> +	}
>  

I saw many usages of WARN_ON_ONCE() and WARN_ON() in KVM. And just wonder,
is there any criteria for KVM when to use which?

B.R.
Yu