Re: [PATCH v2 00/15] vfio: expose virtual Shared Virtual Addressing to VMs

[Stefan Hajnoczi <stefanha@gmail.com>]

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On Tue, Jun 16, 2020 at 02:26:38AM +0000, Tian, Kevin wrote:
> > From: Stefan Hajnoczi <stefanha@gmail.com>
> > Sent: Monday, June 15, 2020 6:02 PM
> > 
> > On Thu, Jun 11, 2020 at 05:15:19AM -0700, Liu Yi L wrote:
> > > Shared Virtual Addressing (SVA), a.k.a, Shared Virtual Memory (SVM) on
> > > Intel platforms allows address space sharing between device DMA and
> > > applications. SVA can reduce programming complexity and enhance
> > security.
> > >
> > > This VFIO series is intended to expose SVA usage to VMs. i.e. Sharing
> > > guest application address space with passthru devices. This is called
> > > vSVA in this series. The whole vSVA enabling requires QEMU/VFIO/IOMMU
> > > changes. For IOMMU and QEMU changes, they are in separate series (listed
> > > in the "Related series").
> > >
> > > The high-level architecture for SVA virtualization is as below, the key
> > > design of vSVA support is to utilize the dual-stage IOMMU translation (
> > > also known as IOMMU nesting translation) capability in host IOMMU.
> > >
> > >
> > >     .-------------.  .---------------------------.
> > >     |   vIOMMU    |  | Guest process CR3, FL only|
> > >     |             |  '---------------------------'
> > >     .----------------/
> > >     | PASID Entry |--- PASID cache flush -
> > >     '-------------'                       |
> > >     |             |                       V
> > >     |             |                CR3 in GPA
> > >     '-------------'
> > > Guest
> > > ------| Shadow |--------------------------|--------
> > >       v        v                          v
> > > Host
> > >     .-------------.  .----------------------.
> > >     |   pIOMMU    |  | Bind FL for GVA-GPA  |
> > >     |             |  '----------------------'
> > >     .----------------/  |
> > >     | PASID Entry |     V (Nested xlate)
> > >     '----------------\.------------------------------.
> > >     |             |   |SL for GPA-HPA, default domain|
> > >     |             |   '------------------------------'
> > >     '-------------'
> > > Where:
> > >  - FL = First level/stage one page tables
> > >  - SL = Second level/stage two page tables
> > 
> > Hi,
> > Looks like an interesting feature!
> > 
> > To check I understand this feature: can applications now pass virtual
> > addresses to devices instead of translating to IOVAs?
> > 
> > If yes, can guest applications restrict the vSVA address space so the
> > device only has access to certain regions?
> > 
> > On one hand replacing IOVA translation with virtual addresses simplifies
> > the application programming model, but does it give up isolation if the
> > device can now access all application memory?
> > 
> 
> with SVA each application is allocated with a unique PASID to tag its
> virtual address space. The device that claims SVA support must guarantee 
> that one application can only program the device to access its own virtual
> address space (i.e. all DMAs triggered by this application are tagged with
> the application's PASID, and are translated by IOMMU's PASID-granular
> page table). So, isolation is not sacrificed in SVA.

Isolation between applications is preserved but there is no isolation
between the device and the application itself. The application needs to
trust the device.

Examples:

1. The device can snoop secret data from readable pages in the
   application's virtual memory space.

2. The device can gain arbitrary execution on the CPU by overwriting
   control flow addresses (e.g. function pointers, stack return
   addresses) in writable pages.

Stefan

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