linux-kernel - Re: [PATCH v3 14/25] KVM: TDX: Bug the VM if extended the initial measurement fails

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Message-ID: <442f5488e4a66f6a1517082df3d2ae47316be010.camel@intel.com>
Date: Thu, 23 Oct 2025 22:48:10 +0000
From: "Huang, Kai" <kai.huang@...el.com>
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Subject: Re: [PATCH v3 14/25] KVM: TDX: Bug the VM if extended the initial
 measurement fails

On Thu, 2025-10-16 at 17:32 -0700, Sean Christopherson wrote:
> WARN and terminate the VM if TDH_MR_EXTEND fails, as extending the
> measurement should fail if and only if there is a KVM bug, or if the S-EPT
> mapping is invalid, and it should be impossible for the S-EPT mappings to
> be removed between kvm_tdp_mmu_map_private_pfn() and tdh_mr_extend().
> 
> Holding slots_lock prevents zaps due to memslot updates,
> filemap_invalidate_lock() prevents zaps due to guest_memfd PUNCH_HOLE,
> and all usage of kvm_zap_gfn_range() is mutually exclusive with S-EPT
> entries that can be used for the initial image.  The call from sev.c is
> obviously mutually exclusive, TDX disallows KVM_X86_QUIRK_IGNORE_GUEST_PAT
> so same goes for kvm_noncoherent_dma_assignment_start_or_stop, and while
> __kvm_set_or_clear_apicv_inhibit() can likely be tripped while building the
> image, the APIC page has its own non-guest_memfd memslot and so can't be
> used for the initial image, which means that too is mutually exclusive.
> 
> Opportunistically switch to "goto" to jump around the measurement code,
> partly to make it clear that KVM needs to bail entirely if extending the
> measurement fails, partly in anticipation of reworking how and when
> TDH_MEM_PAGE_ADD is done.
> 
> Fixes: d789fa6efac9 ("KVM: TDX: Handle vCPU dissociation")

So IIUC this patch only adds a KVM_BUG_ON() when TDH.MR.EXTEND fails.  How
does this fix anything?

Looking at v2, they may have a relationship, but it's quite confusing w/o
any explanation?

> Signed-off-by: Yan Zhao <yan.y.zhao@...el.com>
> Signed-off-by: Sean Christopherson <seanjc@...gle.com>
> ---
>  arch/x86/kvm/vmx/tdx.c | 24 ++++++++++++++++--------
>  1 file changed, 16 insertions(+), 8 deletions(-)
> 
> diff --git a/arch/x86/kvm/vmx/tdx.c b/arch/x86/kvm/vmx/tdx.c
> index c37591730cc5..f4bab75d3ffb 100644
> --- a/arch/x86/kvm/vmx/tdx.c
> +++ b/arch/x86/kvm/vmx/tdx.c
> @@ -3151,14 +3151,22 @@ static int tdx_gmem_post_populate(struct kvm *kvm, gfn_t gfn, kvm_pfn_t pfn,
>  
>  	KVM_BUG_ON(atomic64_dec_return(&kvm_tdx->nr_premapped) < 0, kvm);
>  
> -	if (arg->flags & KVM_TDX_MEASURE_MEMORY_REGION) {
> -		for (i = 0; i < PAGE_SIZE; i += TDX_EXTENDMR_CHUNKSIZE) {
> -			err = tdh_mr_extend(&kvm_tdx->td, gpa + i, &entry,
> -					    &level_state);
> -			if (err) {
> -				ret = -EIO;
> -				break;
> -			}
> +	if (!(arg->flags & KVM_TDX_MEASURE_MEMORY_REGION))
> +		goto out;
> +
> +	/*
> +	 * Note, MR.EXTEND can fail if the S-EPT mapping is somehow removed
> +	 * between mapping the pfn and now, but slots_lock prevents memslot
> +	 * updates, filemap_invalidate_lock() prevents guest_memfd updates,
> +	 * mmu_notifier events can't reach S-EPT entries, and KVM's internal
> +	 * zapping flows are mutually exclusive with S-EPT mappings.
> +	 */
> +	for (i = 0; i < PAGE_SIZE; i += TDX_EXTENDMR_CHUNKSIZE) {
> +		err = tdh_mr_extend(&kvm_tdx->td, gpa + i, &entry, &level_state);
> +		if (KVM_BUG_ON(err, kvm)) {
> +			pr_tdx_error_2(TDH_MR_EXTEND, err, entry, level_state);
> +			ret = -EIO;
> +			goto out;
>  		}
>  	}
>  
> -- 
> 2.51.0.858.gf9c4a03a3a-goog