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

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Message-ID: <aQo_spywQuek7fUi@google.com>
Date: Tue, 4 Nov 2025 10:02:26 -0800
From: Sean Christopherson <seanjc@...gle.com>
To: Binbin Wu <binbin.wu@...ux.intel.com>
Cc: Marc Zyngier <maz@...nel.org>, Oliver Upton <oliver.upton@...ux.dev>, 
	Tianrui Zhao <zhaotianrui@...ngson.cn>, Bibo Mao <maobibo@...ngson.cn>, 
	Huacai Chen <chenhuacai@...nel.org>, Madhavan Srinivasan <maddy@...ux.ibm.com>, 
	Anup Patel <anup@...infault.org>, Paul Walmsley <pjw@...nel.org>, 
	Palmer Dabbelt <palmer@...belt.com>, Albert Ou <aou@...s.berkeley.edu>, 
	Christian Borntraeger <borntraeger@...ux.ibm.com>, Janosch Frank <frankja@...ux.ibm.com>, 
	Claudio Imbrenda <imbrenda@...ux.ibm.com>, Paolo Bonzini <pbonzini@...hat.com>, 
	"Kirill A. Shutemov" <kas@...nel.org>, linux-arm-kernel@...ts.infradead.org, 
	kvmarm@...ts.linux.dev, kvm@...r.kernel.org, loongarch@...ts.linux.dev, 
	linux-mips@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org, 
	kvm-riscv@...ts.infradead.org, linux-riscv@...ts.infradead.org, 
	x86@...nel.org, linux-coco@...ts.linux.dev, linux-kernel@...r.kernel.org, 
	Ira Weiny <ira.weiny@...el.com>, Kai Huang <kai.huang@...el.com>, 
	Michael Roth <michael.roth@....com>, Yan Zhao <yan.y.zhao@...el.com>, 
	Vishal Annapurve <vannapurve@...gle.com>, Rick Edgecombe <rick.p.edgecombe@...el.com>, 
	Ackerley Tng <ackerleytng@...gle.com>
Subject: Re: [PATCH v4 27/28] KVM: TDX: Bug the VM if extending the initial
 measurement fails

On Tue, Nov 04, 2025, Binbin Wu wrote:
> 
> 
> On 10/31/2025 4:09 AM, 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
> 
> Nit:
> It sounds like TDX is using the memslot for the APIC page, but for a TD, the
> memslot for the APIC page is never initialized or used?

Oh!  Good point.  I'll tweak that snippet when applying.