Re: Plan for /dev/ioasid RFC v2

[Liu Yi L <yi.l.liu@linux.intel.com>]

Hi Alex,

On Wed, 16 Jun 2021 13:39:37 -0600, Alex Williamson wrote:

> On Wed, 16 Jun 2021 06:43:23 +0000
> "Tian, Kevin" <kevin.tian@intel.com> wrote:
> 
> > > From: Alex Williamson <alex.williamson@redhat.com>
> > > Sent: Wednesday, June 16, 2021 12:12 AM
> > > 
> > > On Tue, 15 Jun 2021 02:31:39 +0000
> > > "Tian, Kevin" <kevin.tian@intel.com> wrote:
> > >     
> > > > > From: Alex Williamson <alex.williamson@redhat.com>
> > > > > Sent: Tuesday, June 15, 2021 12:28 AM
> > > > >    
> > > > [...]    
> > > > > > IOASID. Today the group fd requires an IOASID before it hands out a
> > > > > > device_fd. With iommu_fd the device_fd will not allow IOCTLs until it
> > > > > > has a blocked DMA IOASID and is successefully joined to an iommu_fd.    
> > > > >
> > > > > Which is the root of my concern.  Who owns ioctls to the device fd?
> > > > > It's my understanding this is a vfio provided file descriptor and it's
> > > > > therefore vfio's responsibility.  A device-level IOASID interface
> > > > > therefore requires that vfio manage the group aspect of device access.
> > > > > AFAICT, that means that device access can therefore only begin when all
> > > > > devices for a given group are attached to the IOASID and must halt for
> > > > > all devices in the group if any device is ever detached from an IOASID,
> > > > > even temporarily.  That suggests a lot more oversight of the IOASIDs by
> > > > > vfio than I'd prefer.
> > > > >    
> > > >
> > > > This is possibly the point that is worthy of more clarification and
> > > > alignment, as it sounds like the root of controversy here.
> > > >
> > > > I feel the goal of vfio group management is more about ownership, i.e.
> > > > all devices within a group must be assigned to a single user. Following
> > > > the three rules defined by Jason, what we really care is whether a group
> > > > of devices can be isolated from the rest of the world, i.e. no access to
> > > > memory/device outside of its security context and no access to its
> > > > security context from devices outside of this group. This can be achieved
> > > > as long as every device in the group is either in block-DMA state when
> > > > it's not attached to any security context or attached to an IOASID context
> > > > in IOMMU fd.
> > > >
> > > > As long as group-level isolation is satisfied, how devices within a group
> > > > are further managed is decided by the user (unattached, all attached to
> > > > same IOASID, attached to different IOASIDs) as long as the user
> > > > understands the implication of lacking of isolation within the group. This
> > > > is what a device-centric model comes to play. Misconfiguration just hurts
> > > > the user itself.
> > > >
> > > > If this rationale can be agreed, then I didn't see the point of having VFIO
> > > > to mandate all devices in the group must be attached/detached in
> > > > lockstep.    
> > > 
> > > In theory this sounds great, but there are still too many assumptions
> > > and too much hand waving about where isolation occurs for me to feel
> > > like I really have the complete picture.  So let's walk through some
> > > examples.  Please fill in and correct where I'm wrong.    
> > 
> > Thanks for putting these examples. They are helpful for clearing the 
> > whole picture.
> > 
> > Before filling in let's first align on what is the key difference between
> > current VFIO model and this new proposal. With this comparison we'll
> > know which of following questions are answered with existing VFIO
> > mechanism and which are handled differently.
> > 
> > With Yi's help we figured out the current mechanism:
> > 
> > 1) vfio_group_viable. The code comment explains the intention clearly:
> > 
> > --
> > * A vfio group is viable for use by userspace if all devices are in
> >  * one of the following states:
> >  *  - driver-less
> >  *  - bound to a vfio driver
> >  *  - bound to an otherwise allowed driver
> >  *  - a PCI interconnect device
> > --
> > 
> > Note this check is not related to an IOMMU security context.  
> 
> Because this is a pre-requisite for imposing that IOMMU security
> context.
>  
> > 2) vfio_iommu_group_notifier. When an IOMMU_GROUP_NOTIFY_
> > BOUND_DRIVER event is notified, vfio_group_viable is re-evaluated.
> > If the affected group was previously viable but now becomes not 
> > viable, BUG_ON() as it implies that this device is bound to a non-vfio 
> > driver which breaks the group isolation.  
> 
> This notifier action is conditional on there being users of devices
> within a secure group IOMMU context.
> 
> > 3) vfio_group_get_device_fd. User can acquire a device fd only after
> > 	a) the group is viable;
> > 	b) the group is attached to a container;
> > 	c) iommu is set on the container (implying a security context
> > 	    established);  
> 
> The order is actually b) a) c) but arguably b) is a no-op until:
> 
>     d) a device fd is provided to the user

Per the code in QEMU vfio_get_group(). The order is a) b) c). In
vfio_connect_container(), group is attached to a container.

1959 VFIOGroup *vfio_get_group(int groupid, AddressSpace *as, Error **errp)
1960 {
         ...
1978     group = g_malloc0(sizeof(*group));
1979 
1980     snprintf(path, sizeof(path), "/dev/vfio/%d", groupid);
1981     group->fd = qemu_open_old(path, O_RDWR);
1982     if (group->fd < 0) {
1983         error_setg_errno(errp, errno, "failed to open %s", path);
1984         goto free_group_exit;
1985     }
1986 
1987     if (ioctl(group->fd, VFIO_GROUP_GET_STATUS, &status)) {
1988         error_setg_errno(errp, errno, "failed to get group %d status", groupid);
1989         goto close_fd_exit;
1990     }
1991 
1992     if (!(status.flags & VFIO_GROUP_FLAGS_VIABLE)) {
1993         error_setg(errp, "group %d is not viable", groupid);
1994         error_append_hint(errp,
1995                           "Please ensure all devices within the iommu_group "
1996                           "are bound to their vfio bus driver.\n");
1997         goto close_fd_exit;
1998     }
1999 
2000     group->groupid = groupid;
2001     QLIST_INIT(&group->device_list);
2002 
2003     if (vfio_connect_container(group, as, errp)) {
2004         error_prepend(errp, "failed to setup container for group %d: ",
2005                       groupid);
2006         goto close_fd_exit;
2007     }
2008 
         ...
2024 }

-- 
Regards,
Yi Liu