Re: [for-5.2 v4 10/10] s390: Recognize host-trust-limitation option

[Cornelia Huck]

On Mon, 7 Sep 2020 17:22:53 +0200
Halil Pasic <pasic@linux.ibm.com> wrote:

> On Fri, 24 Jul 2020 12:57:44 +1000
> David Gibson <david@gibson.dropbear.id.au> wrote:
> 
> > At least some s390 cpu models support "Protected Virtualization" (PV),
> > a mechanism to protect guests from eavesdropping by a compromised
> > hypervisor.
> > 
> > This is similar in function to other mechanisms like AMD's SEV and
> > POWER's PEF, which are controlled bythe "host-trust-limitation"
> > machine option.  s390 is a slightly special case, because we already
> > supported PV, simply by using a CPU model with the required feature
> > (S390_FEAT_UNPACK).
> > 
> > To integrate this with the option used by other platforms, we
> > implement the following compromise:
> > 
> >  - When the host-trust-limitation option is set, s390 will recognize
> >    it, verify that the CPU can support PV (failing if not) and set
> >    virtio default options necessary for encrypted or protected guests,
> >    as on other platforms.  i.e. if host-trust-limitation is set, we
> >    will either create a guest capable of entering PV mode, or fail
> >    outright  
> 
> Shouldn't we also fail outright if the virtio features are not PV
> compatible (invalid configuration)?
> 
> I would like to see something like follows as a part of this series.
> ----------------------------8<--------------------------
> From: Halil Pasic <pasic@linux.ibm.com>
> Date: Mon, 7 Sep 2020 15:00:17 +0200
> Subject: [PATCH] virtio: handle host trust limitation
> 
> If host_trust_limitation_enabled() returns true, then emulated virtio
> devices must offer VIRTIO_F_ACCESS_PLATFORM, because the device is not
> capable of accessing all of the guest memory. Otherwise we are in
> violation of the virtio specification.
> 
> Let's fail realize if we detect that VIRTIO_F_ACCESS_PLATFORM feature is
> obligatory but missing.
> 
> Signed-off-by: Halil Pasic <pasic@linux.ibm.com>
> ---
>  hw/virtio/virtio.c | 7 +++++++
>  1 file changed, 7 insertions(+)
> 
> diff --git a/hw/virtio/virtio.c b/hw/virtio/virtio.c
> index 5bd2a2f621..19b4b0a37a 100644
> --- a/hw/virtio/virtio.c
> +++ b/hw/virtio/virtio.c
> @@ -27,6 +27,7 @@
>  #include "hw/virtio/virtio-access.h"
>  #include "sysemu/dma.h"
>  #include "sysemu/runstate.h"
> +#include "exec/host-trust-limitation.h"
>  
>  /*
>   * The alignment to use between consumer and producer parts of vring.
> @@ -3618,6 +3619,12 @@ static void virtio_device_realize(DeviceState *dev, 
> Error **errp)
>      /* Devices should either use vmsd or the load/save methods */
>      assert(!vdc->vmsd || !vdc->load);
>  
> +    if (host_trust_limitation_enabled(MACHINE(qdev_get_machine()))
> +        && !virtio_host_has_feature(vdev, VIRTIO_F_IOMMU_PLATFORM)) {
> +        error_setg(&err, "devices without VIRTIO_F_ACCESS_PLATFORM are not 
> compatible with host trust imitation");
> +        error_propagate(errp, err);
> +        return;

How can we get here? I assume only if the user explicitly turned the
feature off while turning HTL on, as otherwise patch 9 should have
taken care of it?

> +    }
>      if (vdc->realize != NULL) {
>          vdc->realize(dev, &err);
>          if (err != NULL) {