linux-kernel - Re: [PATCH RFC 0/5] mm/gup: Introduce exclusive GUP pinning

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Message-ID: <20240802112205.GA478300@nvidia.com>
Date: Fri, 2 Aug 2024 08:22:05 -0300
From: Jason Gunthorpe <jgg@...dia.com>
To: "Tian, Kevin" <kevin.tian@...el.com>
Cc: David Hildenbrand <david@...hat.com>,
	Mostafa Saleh <smostafa@...gle.com>,
	John Hubbard <jhubbard@...dia.com>,
	Elliot Berman <quic_eberman@...cinc.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Shuah Khan <shuah@...nel.org>, Matthew Wilcox <willy@...radead.org>,
	"maz@...nel.org" <maz@...nel.org>,
	"kvm@...r.kernel.org" <kvm@...r.kernel.org>,
	"linux-arm-msm@...r.kernel.org" <linux-arm-msm@...r.kernel.org>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-kselftest@...r.kernel.org" <linux-kselftest@...r.kernel.org>,
	"pbonzini@...hat.com" <pbonzini@...hat.com>,
	Fuad Tabba <tabba@...gle.com>, "Xu, Yilun" <yilun.xu@...el.com>,
	"Qiang, Chenyi" <chenyi.qiang@...el.com>
Subject: Re: [PATCH RFC 0/5] mm/gup: Introduce exclusive GUP pinning

On Fri, Aug 02, 2024 at 08:26:48AM +0000, Tian, Kevin wrote:
> > From: Jason Gunthorpe <jgg@...dia.com>
> > Sent: Thursday, June 20, 2024 10:34 PM
> > 
> > On Thu, Jun 20, 2024 at 04:14:23PM +0200, David Hildenbrand wrote:
> > 
> > > 1) How would the device be able to grab/access "private memory", if not
> > >    via the user page tables?
> > 
> > The approaches I'm aware of require the secure world to own the IOMMU
> > and generate the IOMMU page tables. So we will not use a GUP approach
> > with VFIO today as the kernel will not have any reason to generate a
> > page table in the first place. Instead we will say "this PCI device
> > translates through the secure world" and walk away.
> > 
> > The page table population would have to be done through the KVM path.
> 
> Sorry for noting this discussion late. Dave pointed it to me in a related
> thread [1].
> 
> I had an impression that above approach fits some trusted IO arch (e.g.
> TDX Connect which has a special secure I/O page table format and
> requires sharing it between IOMMU/KVM) but not all.
> 
> e.g. SEV-TIO spec [2] (page 8) describes to have the IOMMU walk the
> existing I/O page tables to get HPA and then verify it through a new
> permission table (RMP) for access control.

It is not possible, you cannot have the unsecure world control the
IOMMU translation and expect a secure guest.

The unsecure world can attack the guest by scrambling the mappings of
its private pages. A RMP does not protect against this.

This is why the secure world controls the CPU's GPA translation
exclusively, same reasoning for iommu.

Jason