Launch security with AMD SEV

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 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.

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:

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:

  1. libvirt does support SEV, but either QEMU or the host does not
  2. 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> 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>

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>