linux-kernel - Re: [RFC PATCH v2 12/18] KVM: TDX: Bug the VM if extended the initial measurement fails

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Message-ID: <aLFiPq1smdzN3Ary@yzhao56-desk.sh.intel.com>
Date: Fri, 29 Aug 2025 16:18:06 +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>, Ira Weiny <ira.weiny@...el.com>, Kai Huang
	<kai.huang@...el.com>, Michael Roth <michael.roth@....com>, Vishal Annapurve
	<vannapurve@...gle.com>, Rick Edgecombe <rick.p.edgecombe@...el.com>,
	Ackerley Tng <ackerleytng@...gle.com>
Subject: Re: [RFC PATCH v2 12/18] KVM: TDX: Bug the VM if extended the
 initial measurement fails

On Thu, Aug 28, 2025 at 05:06:12PM -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 impossibe 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.
> 
> 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 06dd2861eba7..bc92e87a1dbb 100644
> --- a/arch/x86/kvm/vmx/tdx.c
> +++ b/arch/x86/kvm/vmx/tdx.c
> @@ -3145,14 +3145,22 @@ 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)) {
I suspect tdh_mr_extend() running on one vCPU may contend with
tdh_vp_create()/tdh_vp_addcx()/tdh_vp_init*()/tdh_vp_rd()/tdh_vp_wr()/
tdh_mng_rd()/tdh_vp_flush() on other vCPUs, if userspace invokes ioctl
KVM_TDX_INIT_MEM_REGION on one vCPU while initializing other vCPUs.

It's similar to the analysis of contention of tdh_mem_page_add() [1], as
both tdh_mr_extend() and tdh_mem_page_add() acquire exclusive lock on
resource TDR.

I'll try to write a test to verify it and come back to you.

[1] https://lore.kernel.org/kvm/20250113021050.18828-1-yan.y.zhao@intel.com/ 
> +			pr_tdx_error_2(TDH_MR_EXTEND, err, entry, level_state);
> +			ret = -EIO;
> +			goto out;
>  		}
>  	}
>  
> -- 
> 2.51.0.318.gd7df087d1a-goog
>