Unlike most virtio features ACCESS_PLATFORM is considered mandatory by
QEMU, i.e. the driver must accept it if offered by the device. The
virtio specification says that the driver SHOULD accept the
ACCESS_PLATFORM feature if offered, and that the device MAY fail to
operate if ACCESS_PLATFORM was offered but not negotiated.

While a SHOULD ain't exactly a MUST, we are certainly allowed to fail
the device when the driver fences ACCESS_PLATFORM. With commit
2943b53f ("virtio: force VIRTIO_F_IOMMU_PLATFORM") we already made the
decision to do so whenever the get_dma_as() callback is implemented (by
the bus), which in practice means for the entirety of virtio-pci.

That means, if the device needs to translate I/O addresses, then
ACCESS_PLATFORM is mandatory. The aforementioned commit tells us in the
commit message that this is for security reasons. More precisely if we
were to allow a less then trusted driver (e.g. an user-space driver, or
a nested guest) to make the device bypass the IOMMU by not negotiating
ACCESS_PLATFORM, then the guest kernel would have no ability to
control/police (by programming the IOMMU) what pieces of guest memory
the driver may manipulate using the device. Which would break security
assumptions within the guest.

If ACCESS_PLATFORM is offered not because we want the device to utilize
an IOMMU and do address translation, but because the device does not
have access to the entire guest RAM, and needs the driver to grant
access to the bits it needs access to (e.g. confidential guest support),
we still require the guest to have the corresponding logic and to accept
ACCESS_PLATFORM. If the driver does not accept ACCESS_PLATFORM, then
things are bound to go wrong, and we may see failures much less graceful
than failing the device because the driver didn't negotiate
ACCESS_PLATFORM.

So let us make ACCESS_PLATFORM mandatory for the driver regardless
of whether the get_dma_as() callback is implemented or not.

Signed-off-by: Halil Pasic <pasic@linux.ibm.com>
Fixes: 2943b53f ("virtio: force VIRTIO_F_IOMMU_PLATFORM")

Message-Id: <20220307112939.2780117-1-pasic@linux.ibm.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Cornelia Huck <cohuck@redhat.com>