linux-kernel - Re: [PATCH 11/16] KVM: x86/mmu: Explicitly disallow private accesses to emulated MMIO

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Message-ID: <ZejxqaEBi3q0TU_d@google.com>
Date: Wed, 6 Mar 2024 14:43:53 -0800
From: Sean Christopherson <seanjc@...gle.com>
To: Kai Huang <kai.huang@...el.com>
Cc: Paolo Bonzini <pbonzini@...hat.com>, kvm@...r.kernel.org, linux-kernel@...r.kernel.org, 
	Yan Zhao <yan.y.zhao@...el.com>, Isaku Yamahata <isaku.yamahata@...el.com>, 
	Michael Roth <michael.roth@....com>, Yu Zhang <yu.c.zhang@...ux.intel.com>, 
	Chao Peng <chao.p.peng@...ux.intel.com>, Fuad Tabba <tabba@...gle.com>, 
	David Matlack <dmatlack@...gle.com>
Subject: Re: [PATCH 11/16] KVM: x86/mmu: Explicitly disallow private accesses
 to emulated MMIO

On Thu, Mar 07, 2024, Kai Huang wrote:
> 
> 
> On 28/02/2024 3:41 pm, Sean Christopherson wrote:
> > Explicitly detect and disallow private accesses to emulated MMIO in
> > kvm_handle_noslot_fault() instead of relying on kvm_faultin_pfn_private()
> > to perform the check.  This will allow the page fault path to go straight
> > to kvm_handle_noslot_fault() without bouncing through __kvm_faultin_pfn().
> > 
> > Signed-off-by: Sean Christopherson <seanjc@...gle.com>
> > ---
> >   arch/x86/kvm/mmu/mmu.c | 5 +++++
> >   1 file changed, 5 insertions(+)
> > 
> > diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> > index 5c8caab64ba2..ebdb3fcce3dc 100644
> > --- a/arch/x86/kvm/mmu/mmu.c
> > +++ b/arch/x86/kvm/mmu/mmu.c
> > @@ -3314,6 +3314,11 @@ static int kvm_handle_noslot_fault(struct kvm_vcpu *vcpu,
> >   {
> >   	gva_t gva = fault->is_tdp ? 0 : fault->addr;
> > +	if (fault->is_private) {
> > +		kvm_mmu_prepare_memory_fault_exit(vcpu, fault);
> > +		return -EFAULT;
> > +	}
> > +
> 
> As mentioned in another reply in this series, unless I am mistaken, for TDX
> guest the _first_ MMIO access would still cause EPT violation with MMIO GFN
> being private.
> 
> Returning to userspace cannot really help here because the MMIO mapping is
> inside the guest.

That's a guest bug.  The guest *knows* it's a TDX VM, it *has* to know.  Accessing
emulated MMIO and thus taking a #VE before enabling paging is nonsensical.  Either
enable paging and setup MMIO regions as shared, or go straight to TDCALL.

> 
> I am hoping I am missing something here?