Linux as an IBM Secure Execution host or guest

6.6 LPAR mode KVM guest

With IBM® Secure Execution for Linux®, you can run encrypted Linux images on a public, private, or hybrid cloud with their in-use memory protected.

KVM guests in IBM Secure Execution mode are controlled by the ultravisor, which is a trusted firmware feature that uses memory-protection hardware to attain this protection.

IBM Secure Execution for Linux was introduced with IBM z15® and IBM LinuxONE III.

Kernel build options for KVM guests

The kernel of a KVM guest must be compiled with specific build options to enable IBM Secure Execution for Linux and its features.

Kernel builders: This information is intended for those who want to build their own kernel. Be aware that both compiling your own kernel or recompiling an existing distribution usually means that you have to maintain your kernel yourself.

The following figure summarizes the kernel configuration menu options that are relevant to IBM Secure Execution for Linux:

Figure 1. Kernel configuration menu

 Virtualization  --->
    Protected virtualization guest support              (CONFIG_PROTECTED_VIRTUALIZATION_GUEST)
    ...
  Device Drivers --->
    ...
    Character devices ---> 
       ...
       --- S/390 character device drivers (depends on S390) ---
       ...
       Ultravisor userspace API                          (CONFIG_S390_UV_UAPI)

The options have this meaning:

CONFIG_PROTECTED_VIRTUALIZATION_GUEST: provides base support for IBM Secure Execution for Linux.
CONFIG_S390_UV_UAPI: enables user-space interaction of KVM guests in IBM Secure Execution mode with the ultravisor. For example, attestation that a guest runs in IBM Secure Execution mode depends on this enablement.
Support for user-space interaction with the ultravisor can be compiled into the kernel or as a separate module, uvdevice.

Kernel parameters

Both KVM hosts and KVM guests must be set up to support IBM Secure Execution mode. This setup includes two kernel parameters, one for hosts and one for guests.

prot_virt=: By default, KVM hosts do not support guests in IBM Secure Execution mode. To support such guests, KVM hosts must boot in LPAR mode with the kernel parameter specification prot_virt=1.
KVM hosts that successfully start with support for IBM Secure Execution for Linux issue a kernel message like this: prot_virt: Reserving <amount>MB as ultravisor base storage.
swiotlb=: KVM guests in IBM Secure Execution mode require a bounce buffer for their virtio devices. Use the swiotlb= kernel parameter to assign 2 KB memory blocks for this bounce buffer. A suitable setting for most cases is swiotlb=262144, which corresponds to 512 MB.

Indicators for IBM Secure Execution mode

Two read-only sysfs attributes indicate whether a running Linux instance detects an environment of a KVM guest in IBM Secure Execution mode or of a KVM host that can run such guests.

/sys/firmware/uv/prot_virt_guest: The value of this attribute is 1 for Linux instances that detect their environment as consistent with that of a secure guest. For other instances, the value is 0 or the attribute does not exist.
/sys/firmware/uv/prot_virt_host: The value of this attribute is 1 for Linux instances that detect their environment as consistent with that of a secure host. For other instances, the value is 0. If the attribute does not exist, the Linux instance is not a KVM host in an environment that supports IBM Secure Execution for Linux.

Note: These values are indications, but do not prove that the Linux instance is a secure guest or host in the context of IBM Secure Execution for Linux. Use these indications for technical evaluations in trusted environments, but do not base security-related decisions on them.

The following example shows a Linux instance that runs as a KVM guest in IBM Secure Execution mode, but is not a KVM host that can run such guests.

# cat /sys/firmware/uv/prot_virt_guest
1
# cat /sys/firmware/uv/prot_virt_host
0

Attestation of IBM Secure Execution mode

Use attestation to prove that a guest runs in IBM Secure Execution mode on trusted IBM zSystems or IBM LinuxONE hardware, and optionally that the guest image runs on a specific hardware instance. Attestation requires hardware support and is available as of IBM z16™ and IBM LinuxONE 4.

Attestation depends on process steps on the ultravisor. Guests that support attestation have a character device /dev/uv for attestation-related communication with the ultravisor.

Attestation requires the uvdevice device driver. If this device driver has been compiled as a separate module, you might have to load it before the /dev/uv device becomes available. If applicable, issue the following command to load the module:

# modprobe uvdevice

More information

For more information about setting up KVM hosts and guests and about attestation, see Introducing IBM Secure Execution for Linux, SC34-7721.