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

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Message-ID: <707a01ee36c28863bdc6a4444a5560e9a0b19597.camel@intel.com>
Date: Tue, 21 Oct 2025 00:10:45 +0000
From: "Edgecombe, Rick P" <rick.p.edgecombe@...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")
> Signed-off-by: Yan Zhao <yan.y.zhao@...el.com>
> Signed-off-by: Sean Christopherson <seanjc@...gle.com>
> ---

Per the discussion in v2, shouldn't it go after patch 24 'KVM: TDX: Guard VM
state transitions with "all" the locks'? Otherwise it introduces a KVM_BUG_ON()
that can be triggered from userspace. not a huge deal though.