KVM: x86: Add capability to grant VM access to privileged SGX attribute

KVM: x86: Add capability to grant VM access to privileged SGX attribute

Add a capability, KVM_CAP_SGX_ATTRIBUTE, that can be used by userspace
to grant a VM access to a priveleged attribute, with args[0] holding a
file handle to a valid SGX attribute file.

The SGX subsystem restricts access to a subset of enclave attributes to
provide additional security for an uncompromised kernel, e.g. to prevent
malware from using the PROVISIONKEY to ensure its nodes are running
inside a geniune SGX enclave and/or to obtain a stable fingerprint.

To prevent userspace from circumventing such restrictions by running an
enclave in a VM, KVM restricts guest access to privileged attributes by
default.

Cc: Andy Lutomirski <[email protected]>
Signed-off-by: Sean Christopherson <[email protected]>
Signed-off-by: Kai Huang <[email protected]>
Message-Id: <0b099d65e933e068e3ea934b0523bab070cb8cea.1618196135[email protected]>
Signed-off-by: Paolo Bonzini <[email protected]>