Received-SPF: pass (google.com: domain of linux-kernel+bounces-94682-linux.lists.archive=gmail.com@vger.kernel.org designates 2604:1380:45d1:ec00::1 as permitted sender) client-ip=2604:1380:45d1:ec00::1;
Date: Wed, 6 Mar 2024 15:01:04 -0800
In-Reply-To: <5f230626-2738-41cc-875d-ab1a7ef19f64@intel.com>
Precedence: bulk
Mime-Version: 1.0
References: <20240228024147.41573-1-seanjc@google.com> <20240228024147.41573-12-seanjc@google.com>
 <0a05d5ec-5352-4bab-96ae-2fa35235477c@intel.com> <ZejxqaEBi3q0TU_d@google.com>
 <5f230626-2738-41cc-875d-ab1a7ef19f64@intel.com>
Message-ID: <Zej1sBTPLXIhfemS@google.com>
Subject: Re: [PATCH 11/16] KVM: x86/mmu: Explicitly disallow private accesses
 to emulated MMIO
From: Sean Christopherson <seanjc@google.com>
To: Kai Huang <kai.huang@intel.com>
Cc: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>, Paolo Bonzini <pbonzini@redhat.com>, 
	kvm@vger.kernel.org, linux-kernel@vger.kernel.org, 
	Yan Zhao <yan.y.zhao@intel.com>, Isaku Yamahata <isaku.yamahata@intel.com>, 
	Michael Roth <michael.roth@amd.com>, Yu Zhang <yu.c.zhang@linux.intel.com>, 
	Chao Peng <chao.p.peng@linux.intel.com>, Fuad Tabba <tabba@google.com>, 
	David Matlack <dmatlack@google.com>
Content-Type: text/plain; charset="utf-8"
Content-Transfer-Encoding: quoted-printable

On Thu, Mar 07, 2024, Kai Huang wrote:
>=20
>=20
> On 7/03/2024 11:43 am, Sean Christopherson wrote:
> > On Thu, Mar 07, 2024, Kai Huang wrote:
> > >=20
> > >=20
> > > 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_pri=
vate()
> > > > to perform the check.  This will allow the page fault path to go st=
raight
> > > > to kvm_handle_noslot_fault() without bouncing through __kvm_faultin=
_pfn().
> > > >=20
> > > > Signed-off-by: Sean Christopherson <seanjc@google.com>
> > > > ---
> > > >    arch/x86/kvm/mmu/mmu.c | 5 +++++
> > > >    1 file changed, 5 insertions(+)
> > > >=20
> > > > 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 kv=
m_vcpu *vcpu,
> > > >    {
> > > >    	gva_t gva =3D fault->is_tdp ? 0 : fault->addr;
> > > > +	if (fault->is_private) {
> > > > +		kvm_mmu_prepare_memory_fault_exit(vcpu, fault);
> > > > +		return -EFAULT;
> > > > +	}
> > > > +
> > >=20
> > > As mentioned in another reply in this series, unless I am mistaken, f=
or TDX
> > > guest the _first_ MMIO access would still cause EPT violation with MM=
IO GFN
> > > being private.
> > >=20
> > > Returning to userspace cannot really help here because the MMIO mappi=
ng is
> > > inside the guest.
> >=20
> > 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 nonsensic=
al.  Either
> > enable paging and setup MMIO regions as shared, or go straight to TDCAL=
L.
>=20
> +Kirill,
>=20
> I kinda forgot the detail, but what I am afraid is there might be bunch o=
f
> existing TDX guests (since TDX guest code is upstream-ed) using unmodifie=
d
> drivers, which doesn't map MMIO regions as shared I suppose.
>=20
> Kirill,
>=20
> Could you clarify whether TDX guest code maps MMIO regions as shared sinc=
e
> beginning?

Y'all get the same answer we gave the SNP folks: KVM does not yet support T=
DX,
so as far is KVM is concerned, there is no existing functionality to suppor=
t.

s/firmware/Linux if this is a Linux kernel problem.

  On Thu, Feb 08, 2024, Paolo Bonzini wrote:
  > On Thu, Feb 8, 2024 at 6:27=E2=80=AFPM Sean Christopherson <seanjc@goog=
le.com> wrote:
  > > No.  KVM does not yet support SNP, so as far as KVM's ABI goes, there=
 are no
  > > existing guests.  Yes, I realize that I am burying my head in the san=
d to some
  > > extent, but it is simply not sustainable for KVM to keep trying to pi=
ck up the
  > > pieces of poorly defined hardware specs and broken guest firmware.
  >=20
  > 101% agreed. There are cases in which we have to and should bend
  > together backwards for guests (e.g. older Linux kernels), but not for
  > code that---according to current practices---is chosen by the host
  > admin.
  >=20
  > (I am of the opinion that "bring your own firmware" is the only sane
  > way to handle attestation/measurement, but that's not how things are
  > done currently).