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    SubjectRe: Plan for /dev/ioasid RFC v2
    On Thu, Jun 17, 2021 at 07:31:03AM +0000, Tian, Kevin wrote:
    > > > Yes. function 1 is block-DMA while function 0 still attached to IOASID.
    > > > Actually unbind from IOMMU fd doesn't change the security context.
    > > > the change is conducted when attaching/detaching device to/from an
    > > > IOASID.
    > >
    > > But I think you're suggesting that the IOMMU context is simply the
    > > device's default domain, so vfio is left in the position where the user
    > > gained access to the device by binding it to an iommu_fd, but now the
    > > device exists outside of the iommu_fd.

    I don't think unbind should be allowed. Close the fd and re-open it if
    you want to attach to a different iommu_fd.

    > > to gate device access on binding the device to the iommu_fd? The user
    > > can get an accessible device_fd unbound from an iommu_fd on the reverse
    > > path.
    >
    > yes, binding to iommu_fd is not the appropriate point of gating
    > device access.

    Binding is the only point we have enough information to make a
    full security decision. Device FDs that are not bound must be
    inoperable until bound.

    The complexities with revoking mmap/etc are what lead me to conclude
    that unbind is not worth doing - we can't go back to an inoperable
    state very easially.

    > Yes, that was the original impression. But after figuring out the new
    > block-DMA behavior, I'm not sure whether /dev/iommu must maintain
    > its own group integrity check. If it trusts vfio, I feel it's fine to avoid
    > such check which even allows a group of devices bound to different
    > IOMMU fd's if user likes. Also if we want to sustain the current vfio
    > semantics which doesn't require all devices in the group bound to
    > vfio driver, seems it's pointless to enforce such integrity check in
    > /dev/iommu.
    >
    > Jason, what's your opinion?

    I think the iommu code should do all of this, I don't see why vfio
    should be dealing with *iommu* isolation.

    The rest of this email got a bit long for me to catch up on, sorry :\

    Jason

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