linux-kernel - Re: [RFC PATCH 01/12] KVM: TDX: Drop PROVE_MMU=y sanity check on to-be-populated mappings

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Message-ID: <aK6+TQ0r1j5j2PCx@yzhao56-desk.sh.intel.com>
Date: Wed, 27 Aug 2025 16:14:05 +0800
From: Yan Zhao <yan.y.zhao@...el.com>
To: Sean Christopherson <seanjc@...gle.com>
CC: Paolo Bonzini <pbonzini@...hat.com>, <kvm@...r.kernel.org>,
	<linux-kernel@...r.kernel.org>, Michael Roth <michael.roth@....com>, "Ira
 Weiny" <ira.weiny@...el.com>, Vishal Annapurve <vannapurve@...gle.com>, "Rick
 Edgecombe" <rick.p.edgecombe@...el.com>
Subject: Re: [RFC PATCH 01/12] KVM: TDX: Drop PROVE_MMU=y sanity check on
 to-be-populated mappings

On Tue, Aug 26, 2025 at 05:05:11PM -0700, Sean Christopherson wrote:
> Drop TDX's sanity check that an S-EPT mapping isn't zapped between creating
> said mapping and doing TDH.MEM.PAGE.ADD, as the check is simultaneously
> superfluous and incomplete.  Per commit 2608f1057601 ("KVM: x86/tdp_mmu:
> Add a helper function to walk down the TDP MMU"), the justification for
> introducing kvm_tdp_mmu_gpa_is_mapped() was to check that the target gfn
> was pre-populated, with a link that points to this snippet:
> 
>  : > One small question:
>  : >
>  : > What if the memory region passed to KVM_TDX_INIT_MEM_REGION hasn't been pre-
>  : > populated?  If we want to make KVM_TDX_INIT_MEM_REGION work with these regions,
>  : > then we still need to do the real map.  Or we can make KVM_TDX_INIT_MEM_REGION
>  : > return error when it finds the region hasn't been pre-populated?
>  :
>  : Return an error.  I don't love the idea of bleeding so many TDX details into
>  : userspace, but I'm pretty sure that ship sailed a long, long time ago.
> 
> But that justification makes little sense for the final code, as simply
> doing TDH.MEM.PAGE.ADD without a paranoid sanity check will return an error
> if the S-EPT mapping is invalid (as evidenced by the code being guarded
> with CONFIG_KVM_PROVE_MMU=y).
Checking of kvm_tdp_mmu_gpa_is_mapped() was intended to detect unexpected zaps
like kvm_zap_gfn_range() between kvm_tdp_map_page() and tdh_mem_page_add()?
In that case, TDH.MEM.PAGE.ADD would succeed without any error.

But as you said, the read mmu_lock is dropped before tdh_mem_page_add().
Moreover, it still cannot guard against atomic zaps.

As zaps between kvm_tdp_map_page() and tdh_mem_page_add() could still be
detectable through the incorrect value of nr_premapped in the end, dropping the
checks of kvm_tdp_mmu_gpa_is_mapped() looks good.

> The sanity check is also incomplete in the sense that mmu_lock is dropped
> between the check and TDH.MEM.PAGE.ADD, i.e. will only detect KVM bugs that
> zap SPTEs in a very specific window.
>
> Removing the sanity check will allow removing kvm_tdp_mmu_gpa_is_mapped(),
> which has no business being exposed to vendor code.
> 
> Signed-off-by: Sean Christopherson <seanjc@...gle.com>
> ---
>  arch/x86/kvm/vmx/tdx.c | 14 --------------
>  1 file changed, 14 deletions(-)
> 
> diff --git a/arch/x86/kvm/vmx/tdx.c b/arch/x86/kvm/vmx/tdx.c
> index 66744f5768c8..a6155f76cc6a 100644
> --- a/arch/x86/kvm/vmx/tdx.c
> +++ b/arch/x86/kvm/vmx/tdx.c
> @@ -3175,20 +3175,6 @@ static int tdx_gmem_post_populate(struct kvm *kvm, gfn_t gfn, kvm_pfn_t pfn,
>  	if (ret < 0)
>  		goto out;
>  
> -	/*
> -	 * The private mem cannot be zapped after kvm_tdp_map_page()
> -	 * because all paths are covered by slots_lock and the
> -	 * filemap invalidate lock.  Check that they are indeed enough.
> -	 */
> -	if (IS_ENABLED(CONFIG_KVM_PROVE_MMU)) {
> -		scoped_guard(read_lock, &kvm->mmu_lock) {
> -			if (KVM_BUG_ON(!kvm_tdp_mmu_gpa_is_mapped(vcpu, gpa), kvm)) {
> -				ret = -EIO;
> -				goto out;
> -			}
> -		}
> -	}
> -
>  	ret = 0;
>  	err = tdh_mem_page_add(&kvm_tdx->td, gpa, pfn_to_page(pfn),
>  			       src_page, &entry, &level_state);
> -- 
> 2.51.0.268.g9569e192d0-goog
>