============================
Launch security with AMD SEV
============================

.. contents::

Storage encryption in modern public cloud computing is a common
practice. However, from the point of view of a user of these cloud
workloads, a significant amount of trust needs to be put in the cloud
platform security as well as integrity (was the hypervisor tampered?).
For this reason there's ever rising demand for securing data in use,
i.e. memory encryption. One of the solutions addressing this matter is
AMD SEV.

AMD Secure Encrypted Virtualization (SEV)
=========================================

SEV (Secure Encrypted Virtualization) is a feature extension of AMD's
SME (Secure Memory Encryption) intended for KVM virtual machines which
is supported primarily on AMD's EPYC CPU line. In contrast to SME, SEV
uses a unique memory encryption key for each VM. The whole encryption of
memory pages is completely transparent to the hypervisor and happens
inside dedicated hardware in the on-die memory controller. Each
controller includes a high-performance Advanced Encryption Standard
(AES) engine that encrypts data when it is written to DRAM and decrypts
it when read. For more details about the technology itself, you can
visit `AMD's developer portal <https://developer.amd.com/sev/>`__.

Enabling SEV on the host
========================

Before VMs can make use of the SEV feature you need to make sure your
AMD CPU does support SEV. You can check whether SEV is among the CPU
flags with:

::

   $ cat /proc/cpuinfo | grep sev
   ...
   sme ssbd sev ibpb

Next step is to enable SEV in the kernel, because it is disabled by
default. This is done by putting the following onto the kernel command
line:

::

   mem_encrypt=on kvm_amd.sev=1

To make the changes persistent, append the above to the variable holding
parameters of the kernel command line in ``/etc/default/grub`` to
preserve SEV settings across reboots

::

   $ cat /etc/default/grub
   ...
   GRUB_CMDLINE_LINUX="... mem_encrypt=on kvm_amd.sev=1"
   $ grub2-mkconfig -o /boot/efi/EFI/<distro>/grub.cfg

``mem_encrypt=on`` turns on the SME memory encryption feature on the
host which protects against the physical attack on the hypervisor
memory. The ``kvm_amd.sev`` parameter actually enables SEV in the kvm
module. It can be set on the command line alongside ``mem_encrypt`` like
shown above, or it can be put into a module config under
``/etc/modprobe.d/``

::

   $ cat /etc/modprobe.d/sev.conf
   options kvm_amd sev=1

After rebooting the host, you should see SEV being enabled in the
kernel:

::

   $ cat /sys/module/kvm_amd/parameters/sev
   1


Checking SEV support in the virt stack
======================================

**Note: All of the commands bellow need to be run with root
privileges.**

First make sure you have the following packages in the specified
versions:

-  libvirt >= 4.5.0 (>5.1.0 recommended due to additional SEV bugfixes)
-  QEMU >= 2.12.0

To confirm that the virtualization stack supports SEV, run the
following:

::

   # virsh domcapabilities
   <domainCapabilities>
   ...
     <features>
       ...
       <sev supported='yes'>
         <cbitpos>47</cbitpos>
         <reducedPhysBits>1</reducedPhysBits>
       </sev>
       ...
     </features>
   </domainCapabilities>

Note that if libvirt was already installed and libvirtd running before
enabling SEV in the kernel followed by the host reboot you need to force
libvirtd to re-probe both the host and QEMU capabilities. First stop
libvirtd:

::

   # systemctl stop libvirtd.service

Now you need to clean the capabilities cache:

::

   # rm -f /var/cache/libvirt/qemu/capabilities/*

If you now restart libvirtd, it will re-probe the capabilities and if
you now run:

::

   # virsh domcapabilities

SEV should be listed as supported. If you still see:

::

   <sev supported='no'/>

it means one of two things:

#. libvirt does support SEV, but either QEMU or the host does not
#. you have libvirt <=5.1.0 which suffered from getting a
   ``'Permission denied'`` on ``/dev/sev`` because of the default
   permissions on the character device which prevented QEMU from opening
   it during capabilities probing - you can either manually tweak the
   permissions so that QEMU has access to it or preferably install
   libvirt 5.1.0 or higher

VM Configuration
================

SEV is enabled in the XML by specifying the
`<launchSecurity> <https://libvirt.org/formatdomain.html#launchSecurity>`__
element. However, specifying ``launchSecurity`` isn't enough to boot an
SEV VM. Further configuration requirements are discussed below.

Machine type
------------

Even though both Q35 and legacy PC machine types (for PC see also
"virtio") can be used with SEV, usage of the legacy PC machine type is
strongly discouraged, since depending on how your OVMF package was built
(e.g. including features like SecureBoot or SMM) Q35 may even be
required.

Q35
~~~

::

   ...
   <os>
     <type arch='x86_64' machine='pc-q35-3.0'>hvm</type>
     ...
   </os>
   ...

i440fx (discouraged)
~~~~~~~~~~~~~~~~~~~~

::

   ...
   <os>
     <type arch='x86_64' machine='pc-i440fx-3.0'>hvm</type>
     ...
   </os>
   ...

Boot loader
-----------

SEV is only going to work with OVMF (UEFI), so you'll need to point
libvirt to the correct OVMF binary.

::

   ...
   <os>
     <type arch='x86_64' machine='pc-q35-3.0'>hvm</type>
     <loader readonly='yes' type='pflash'>/usr/share/edk2/ovmf/OVMF_CODE.fd</loader>
   </os>
   ...

Memory
------

Internally, SEV expects that the encrypted memory pages won't be swapped
out or move around so the VM memory needs to be pinned in physical RAM
which will be handled by QEMU. Apart from that, certain memory regions
allocated by QEMU itself (UEFI pflash, device ROMs, video RAM, etc.)
have to be encrypted as well. This causes a conflict in how libvirt
tries to protect the host. By default, libvirt enforces a memory hard
limit on each VM's cgroup in order to protect the host from malicious
QEMU to allocate and lock all the available memory. This limit
corresponds to the total memory allocation for the VM given by
``<currentMemory>`` element. However, trying to account for the
additional memory regions QEMU allocates when calculating the limit in
an automated manner is non-deterministic. One way to resolve this is to
set the hard limit manually.

Note: Figuring out the right number so that your guest boots and isn't
killed is challenging, but 256MiB extra memory over the total guest RAM
should suffice for most workloads and may serve as a good starting
point. For example, a domain with 4GB memory with a 256MiB extra hard
limit would look like this:

::

   # virsh edit <domain>
   <domain>
     ...
     <currentMemory unit='KiB'>4194304</currentMemory>
     <memtune>
       <hard_limit unit='KiB'>4456448</hard_limit>
     </memtune>
     ...
   </domain>

There's another, preferred method of taking care of the limits by using
the\ ``<memoryBacking>`` element along with the ``<locked/>``
subelement:

::

   <domain>
     ...
     <memoryBacking>
       <locked/>
     </memoryBacking>
     ...
   </domain>

What that does is that it tells libvirt not to force any hard limit
(well, unlimited) upon the VM cgroup. The obvious advantage is that one
doesn't need to determine the hard limit for every single SEV-enabled
VM. However, there is a significant security-related drawback to this
approach. Since no hard limit is applied, a malicious QEMU could perform
a DoS attack by locking all of the host's available memory. The way to
avoid this issue and to protect the host is to enforce a bigger hard
limit on the master cgroup containing all of the VMs - on systemd this
is ``machine.slice``.

::

   # systemctl set-property machine.slice MemoryHigh=<value>

To put even stricter measures in place which would involve the OOM
killer, use

::

   # systemctl set-property machine.slice MemoryMax=<value>

instead. Alternatively, you can create a systemd config (don't forget to
reload systemd configuration in this case):

::

   # cat << EOF > /etc/systemd/system.control/machine.slice.d/90-MemoryMax.conf
   MemoryMax=<value>
   EOF

The trade-off to keep in mind with the second approach is that the VMs
can still perform DoS on each other.

Virtio
------

In order to make virtio devices work, we need to enable emulated IOMMU
on the devices so that virtual DMA can work.

::

   # virsh edit <domain>
   <domain>
     ...
     <controller type='virtio-serial' index='0'>
       <driver iommu='on'/>
     </controller>
     <controller type='scsi' index='0' model='virtio-scsi'>
       <driver iommu='on'/>
     </controller>
     ...
     <memballoon model='virtio'>
       <driver iommu='on'/>
     </memballoon>
     <rng model='virtio'>
       <backend model='random'>/dev/urandom</backend>
       <driver iommu='on'/>
     </rng>
     ...
   <domain>

If you for some reason want to use the legacy PC machine type, further
changes to the virtio configuration is required, because SEV will not
work with Virtio <1.0. In libvirt, this is handled by using the
virtio-non-transitional device model (libvirt >= 5.2.0 required).

Note: some devices like video devices don't support non-transitional
model, which means that virtio GPU cannot be used.

::

   <domain>
     ...
     <devices>
       ...
       <memballoon model='virtio-non-transitional'>
         <driver iommu='on'/>
       </memballoon>
     </devices>
     ...
   </domain>

Checking SEV from within the guest
==================================

After making the necessary adjustments discussed in
`Configuration <#Configuration>`__, the VM should now boot successfully
with SEV enabled. You can then verify that the guest has SEV enabled by
running:

::

   # dmesg | grep -i sev
   AMD Secure Encrypted Virtualization (SEV) active

Limitations
===========

Currently, the boot disk cannot be of type virtio-blk, instead,
virtio-scsi needs to be used if virtio is desired. This limitation is
expected to be lifted with future releases of kernel (the kernel used at
the time of writing the article is 5.0.14). If you still cannot start an
SEV VM, it could be because of wrong SELinux label on the ``/dev/sev``
device with selinux-policy <3.14.2.40 which prevents QEMU from touching
the device. This can be resolved by upgrading the package, tuning the
selinux policy rules manually to allow svirt_t to access the device (see
``audit2allow`` on how to do that) or putting SELinux into permissive
mode (discouraged).

Full domain XML examples
========================

Q35 machine
-----------

::

   <domain type='kvm'>
     <name>sev-dummy</name>
     <memory unit='KiB'>4194304</memory>
     <currentMemory unit='KiB'>4194304</currentMemory>
     <memoryBacking>
       <locked/>
     </memoryBacking>
     <vcpu placement='static'>4</vcpu>
     <os>
       <type arch='x86_64' machine='pc-q35-3.0'>hvm</type>
       <loader readonly='yes' type='pflash'>/usr/share/edk2/ovmf/OVMF_CODE.fd</loader>
       <nvram>/var/lib/libvirt/qemu/nvram/sev-dummy_VARS.fd</nvram>
     </os>
     <features>
       <acpi/>
       <apic/>
       <vmport state='off'/>
     </features>
     <cpu mode='host-model' check='partial'>
       <model fallback='allow'/>
     </cpu>
     <clock offset='utc'>
       <timer name='rtc' tickpolicy='catchup'/>
       <timer name='pit' tickpolicy='delay'/>
       <timer name='hpet' present='no'/>
     </clock>
     <on_poweroff>destroy</on_poweroff>
     <on_reboot>restart</on_reboot>
     <on_crash>destroy</on_crash>
     <pm>
       <suspend-to-mem enabled='no'/>
       <suspend-to-disk enabled='no'/>
     </pm>
     <devices>
       <emulator>/usr/bin/qemu-kvm</emulator>
       <disk type='file' device='disk'>
         <driver name='qemu' type='qcow2'/>
         <source file='/var/lib/libvirt/images/sev-dummy.qcow2'/>
         <target dev='sda' bus='scsi'/>
         <boot order='1'/>
       </disk>
       <controller type='virtio-serial' index='0'>
         <driver iommu='on'/>
       </controller>
       <controller type='scsi' index='0' model='virtio-scsi'>
         <driver iommu='on'/>
       </controller>
       <interface type='network'>
         <mac address='52:54:00:cc:56:90'/>
         <source network='default'/>
         <model type='virtio'/>
         <driver iommu='on'/>
       </interface>
       <graphics type='spice' autoport='yes'>
         <listen type='address'/>
         <gl enable='no'/>
       </graphics>
       <video>
         <model type='qxl'/>
       </video>
       <memballoon model='virtio'>
         <driver iommu='on'/>
       </memballoon>
       <rng model='virtio'>
         <driver iommu='on'/>
       </rng>
     </devices>
     <launchSecurity type='sev'>
       <cbitpos>47</cbitpos>
       <reducedPhysBits>1</reducedPhysBits>
       <policy>0x0003</policy>
     </launchSecurity>
   </domain>

PC-i440fx machine
-----------------

::

   <domain type='kvm'>
     <name>sev-dummy-legacy</name>
     <memory unit='KiB'>4194304</memory>
     <currentMemory unit='KiB'>4194304</currentMemory>
     <memtune>
       <hard_limit unit='KiB'>5242880</hard_limit>
     </memtune>
     <vcpu placement='static'>4</vcpu>
     <os>
       <type arch='x86_64' machine='pc-i440fx-3.0'>hvm</type>
       <loader readonly='yes' type='pflash'>/usr/share/edk2/ovmf/OVMF_CODE.fd</loader>
       <nvram>/var/lib/libvirt/qemu/nvram/sev-dummy_VARS.fd</nvram>
       <boot dev='hd'/>
     </os>
     <features>
     <acpi/>
     <apic/>
     <vmport state='off'/>
     </features>
     <cpu mode='host-model' check='partial'>
       <model fallback='allow'/>
     </cpu>
     <clock offset='utc'>
       <timer name='rtc' tickpolicy='catchup'/>
       <timer name='pit' tickpolicy='delay'/>
       <timer name='hpet' present='no'/>
     </clock>
     <on_poweroff>destroy</on_poweroff>
     <on_reboot>restart</on_reboot>
     <on_crash>destroy</on_crash>
     <pm>
       <suspend-to-mem enabled='no'/>
       <suspend-to-disk enabled='no'/>
     </pm>
     <devices>
       <emulator>/usr/bin/qemu-kvm</emulator>
       <disk type='file' device='disk'>
         <driver name='qemu' type='qcow2'/>
         <source file='/var/lib/libvirt/images/sev-dummy-seabios.qcow2'/>
         <target dev='sda' bus='sata'/>
       </disk>
       <interface type='network'>
         <mac address='52:54:00:d8:96:c8'/>
         <source network='default'/>
         <model type='virtio-non-transitional'/>
       </interface>
       <serial type='pty'>
         <target type='isa-serial' port='0'>
           <model name='isa-serial'/>
         </target>
       </serial>
       <console type='pty'>
         <target type='serial' port='0'/>
       </console>
       <input type='tablet' bus='usb'>
         <address type='usb' bus='0' port='1'/>
       </input>
       <input type='mouse' bus='ps2'/>
       <input type='keyboard' bus='ps2'/>
       <graphics type='spice' autoport='yes'>
         <listen type='address'/>
         <gl enable='no'/>
       </graphics>
       <video>
         <model type='qxl' ram='65536' vram='65536' vgamem='16384' heads='1' primary='yes'/>
       </video>
       <memballoon model='virtio-non-transitional'>
         <driver iommu='on'/>
       </memballoon>
         <rng model='virtio-non-transitional'>
       <driver iommu='on'/>
       </rng>
     </devices>
     <launchSecurity type='sev'>
       <cbitpos>47</cbitpos>
       <reducedPhysBits>1</reducedPhysBits>
       <policy>0x0003</policy>
     </launchSecurity>
   </domain>
