linux-kernel - Re: [PATCH v4 27/28] KVM: TDX: Bug the VM if extending the initial measurement fails

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Message-ID: <0adcd41c6c9356bb9bbc5e7dd2f7eb650ca90c9f.camel@intel.com>
Date: Thu, 30 Oct 2025 23:09:58 +0000
From: "Huang, Kai" <kai.huang@...el.com>
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Subject: Re: [PATCH v4 27/28] KVM: TDX: Bug the VM if extending the initial
 measurement fails

On Thu, 2025-10-30 at 13:09 -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.  Now that KVM makes all state transitions mutually
> exclusive via tdx_vm_state_guard, it should be impossible for 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,
> vcpu->mutex locks prevents updates from other vCPUs, kvm->lock prevents
> VM-scoped ioctls from creating havoc (e.g. by creating new vCPUs), and all
> usage of kvm_zap_gfn_range() is mutually exclusive with S-EPT entries that
> can be used for the initial image.
> 
> For kvm_zap_gfn_range(), the call from sev.c is obviously mutually
> exclusive, TDX disallows KVM_X86_QUIRK_IGNORE_GUEST_PAT so the same goes
> for kvm_noncoherent_dma_assignment_start_or_stop(), and
> __kvm_set_or_clear_apicv_inhibit() is blocked by virtue of holding all
> VM and vCPU mutexes (and 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 irrespective of locking).
> 
> Opportunistically return early if the region doesn't need to be measured
> in order to reduce line lengths and avoid wraps.  Similarly, immediately
> and explicitly return if TDH_MR_EXTEND fails to make it clear that KVM
> needs to bail entirely if extending the measurement fails.
> 
> Signed-off-by: Sean Christopherson <seanjc@...gle.com>

Reviewed-by: Kai Huang <kai.huang@...el.com>