linux-kernel - Re: [PATCH 2/5] KVM: TDX: Exit with MEMORY_FAULT on unexpected pending S-EPT Violation

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Message-ID: <e45806c6ae3eef2c707f0c3886cb71015341741b.camel@intel.com>
Date: Tue, 29 Jul 2025 22:58:02 +0000
From: "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>
To: "seanjc@...gle.com" <seanjc@...gle.com>
CC: "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"oliver.upton@...ux.dev" <oliver.upton@...ux.dev>, "Annapurve, Vishal"
	<vannapurve@...gle.com>, "Li, Xiaoyao" <xiaoyao.li@...el.com>,
	"kvmarm@...ts.linux.dev" <kvmarm@...ts.linux.dev>, "Hunter, Adrian"
	<adrian.hunter@...el.com>, "maz@...nel.org" <maz@...nel.org>,
	"linux-arm-kernel@...ts.infradead.org"
	<linux-arm-kernel@...ts.infradead.org>, "pbonzini@...hat.com"
	<pbonzini@...hat.com>, "nik.borisov@...e.com" <nik.borisov@...e.com>,
	"kvm@...r.kernel.org" <kvm@...r.kernel.org>
Subject: Re: [PATCH 2/5] KVM: TDX: Exit with MEMORY_FAULT on unexpected
 pending S-EPT Violation

On Tue, 2025-07-29 at 15:54 -0700, Sean Christopherson wrote:
> > The vm_dead was added because mirror EPT will KVM_BUG_ON() if there is an
> > attempt to set the mirror EPT entry when it is already present. And the
> > unaccepted memory access will trigger an EPT violation for a mirror PTE that
> > is
> > already set. I think this is a better solution irrespective of the vm_dead
> > changes.
> 
> In that case, this change will expose KVM to the KVM_BUG_ON(), because nothing
> prevents userspace from re-running the vCPU. 

If userspace runs the vCPU again then an EPT violation gets triggered again,
which again gets kicked out to userspace. The new check will prevent it from
getting into the fault handler, right?

>  Which KVM_BUG_ON() exactly gets hit?

Should be:

static int __must_check set_external_spte_present(struct kvm *kvm, tdp_ptep_t
sptep,
						 gfn_t gfn, u64 old_spte,
						 u64 new_spte, int level)
{
	bool was_present = is_shadow_present_pte(old_spte);
	bool is_present = is_shadow_present_pte(new_spte);
	bool is_leaf = is_present && is_last_spte(new_spte, level);
	kvm_pfn_t new_pfn = spte_to_pfn(new_spte);
	int ret = 0;

	KVM_BUG_ON(was_present, kvm);