RE: [PATCH V4 05/18] iommu/ioasid: Redefine IOASID set and allocation APIs

[Tian, Kevin]

> From: Tian, Kevin
> Sent: Tuesday, March 30, 2021 10:24 AM
> 
> > From: Jason Gunthorpe <jgg@xxxxxxxxxx>
> > Sent: Tuesday, March 30, 2021 12:32 AM
> > > In terms of usage for guest SVA, an ioasid_set is mostly tied to a host mm,
> > > the use case is as the following:
> >
> > From that doc:
> >
> >   It is imperative to enforce
> >   VM-IOASID ownership such that a malicious guest cannot target DMA
> >   traffic outside its own IOASIDs, or free an active IOASID that belongs
> >   to another VM.
> >
> > Huh?
> >
> > Security in a PASID world comes from the IOMMU blocking access to the
> > PASID except from approved PCI-ID's. If a VF/PF is assigned to a guest
> > then that guest can cause the device to issue any PASID by having
> > complete control and the vIOMMU is supposed to tell the real IOMMU
> > what PASID's the device is alowed to access.
> >
> > If a device is sharing a single PCI function with different security
> > contexts (eg vfio mdev) then the device itself is responsible to
> > ensure that only the secure interface can program a PASID and a less
> > secure context can never self-enroll.
> >
> > Here the mdev driver would have to consule with the vIOMMU to ensure
> > the mdev device is allowed to access the PASID - is that what this
> > set stuff is about?
> >
> > If yes, it is backwards. The MDEV is the thing doing the security, the
> > MDEV should have the list of allowed PASID's and a single PASID
> > created under /dev/ioasid should be loaded into MDEV with some 'Ok you
> > can use PASID xyz from FD abc' command.
> >
> 
> The 'set' is per-VM. Once the mdev is assigned to a VM, all valid PASID's
> in the set of that VM are considered legitimate on this mdev. The mdev
> driver will mediate guest operations which program PASID to the backend
> context and load the PASID only if it is within the 'set' (i.e. already
> allocated through /dev/ioasid). This prevents a malicious VM from attacking
> others. Though it's not mdev which directly maintaining the list of allowed
> PASID's, the effect is the same in concept.
> 

One correction. The mdev should still construct the list of allowed PASID's as
you said (by listening to IOASID_BIND/UNBIND event), in addition to the ioasid 
set maintained per VM (updated when a PASID is allocated/freed). The per-VM
set is required for inter-VM isolation (verified when a pgtable is bound to the 
mdev/PASID), while the mdev's own list is necessary for intra-VM isolation when 
multiple mdevs are assigned to the same VM (verified before loading a PASID 
to the mdev). This series just handles the general part i.e. per-VM ioasid set and 
leaves the mdev's own list to be managed by specific mdev driver which listens
to various IOASID events).

Thanks
Kevin