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

[Cornelia Huck <cohuck@redhat.com>]

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
> 
>  - If host-trust-limitation is not set, guest's might still be able to
>    enter PV mode, if the CPU has the right model.  This may be a
>    little surprising, but shouldn't actually be harmful.

This could be workable, I guess. Would like a second opinion, though.

> 
> To start a guest supporting Protected Virtualization using the new
> option use the command line arguments:
>     -object s390-pv-guest,id=pv0 -machine host-trust-limitation=pv0
> 
> Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
> ---
>  hw/s390x/pv.c | 61 +++++++++++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 61 insertions(+)
> 
> diff --git a/hw/s390x/pv.c b/hw/s390x/pv.c
> index ab3a2482aa..4bf3b345b6 100644
> --- a/hw/s390x/pv.c
> +++ b/hw/s390x/pv.c
> @@ -14,8 +14,11 @@
>  #include <linux/kvm.h>
>  
>  #include "cpu.h"
> +#include "qapi/error.h"
>  #include "qemu/error-report.h"
>  #include "sysemu/kvm.h"
> +#include "qom/object_interfaces.h"
> +#include "exec/host-trust-limitation.h"
>  #include "hw/s390x/ipl.h"
>  #include "hw/s390x/pv.h"
>  
> @@ -111,3 +114,61 @@ void s390_pv_inject_reset_error(CPUState *cs)
>      /* Report that we are unable to enter protected mode */
>      env->regs[r1 + 1] = DIAG_308_RC_INVAL_FOR_PV;
>  }
> +
> +#define TYPE_S390_PV_GUEST "s390-pv-guest"
> +#define S390_PV_GUEST(obj)                              \
> +    OBJECT_CHECK(S390PVGuestState, (obj), TYPE_S390_PV_GUEST)
> +
> +typedef struct S390PVGuestState S390PVGuestState;
> +
> +/**
> + * S390PVGuestState:
> + *
> + * The S390PVGuestState object is basically a dummy used to tell the
> + * host trust limitation system to use s390's PV mechanism.  guest.
> + *
> + * # $QEMU \
> + *         -object s390-pv-guest,id=pv0 \
> + *         -machine ...,host-trust-limitation=pv0
> + */
> +struct S390PVGuestState {
> +    Object parent_obj;
> +};
> +
> +static int s390_pv_kvm_init(HostTrustLimitation *gmpo, Error **errp)
> +{
> +    if (!s390_has_feat(S390_FEAT_UNPACK)) {
> +        error_setg(errp,
> +                   "CPU model does not support Protected Virtualization");
> +        return -1;
> +    }
> +
> +    return 0;
> +}

So here's where I'm confused: If I follow the code correctly, the
->kvm_init callback is invoked before kvm_arch_init() is called. The
kvm_arch_init() implementation for s390x checks whether
KVM_CAP_S390_PROTECTED is available, which is a pre-req for
S390_FEAT_UNPACK. Am I missing something? Can someone with access to PV
hardware check whether this works as intended?

> +
> +static void s390_pv_guest_class_init(ObjectClass *oc, void *data)
> +{
> +    HostTrustLimitationClass *gmpc = HOST_TRUST_LIMITATION_CLASS(oc);
> +
> +    gmpc->kvm_init = s390_pv_kvm_init;
> +}
> +
> +static const TypeInfo s390_pv_guest_info = {
> +    .parent = TYPE_OBJECT,
> +    .name = TYPE_S390_PV_GUEST,
> +    .instance_size = sizeof(S390PVGuestState),
> +    .class_init = s390_pv_guest_class_init,
> +    .interfaces = (InterfaceInfo[]) {
> +        { TYPE_HOST_TRUST_LIMITATION },
> +        { TYPE_USER_CREATABLE },
> +        { }
> +    }
> +};
> +
> +static void
> +s390_pv_register_types(void)
> +{
> +    type_register_static(&s390_pv_guest_info);
> +}
> +
> +type_init(s390_pv_register_types);