[libvirt] [PATCH v3] docs: Provide documentation for SEV launch security
Singh, Brijesh
brijesh.singh at amd.com
Tue Jul 2 16:36:38 UTC 2019
On 7/2/19 10:59 AM, Erik Skultety wrote:
> Signed-off-by: Erik Skultety <eskultet at redhat.com>
> ---
>
> Since v2:
> - incorporated Brijesh's notes into the SEV description and fixed the info
> about SME
>
> docs/launch_security_sev.html.in | 521 +++++++++++++++++++++++++++++++
> 1 file changed, 521 insertions(+)
> create mode 100644 docs/launch_security_sev.html.in
>
Reviewed-by: Brijesh Singh <brijesh.singh at amd.com>
thanks
> diff --git a/docs/launch_security_sev.html.in b/docs/launch_security_sev.html.in
> new file mode 100644
> index 0000000000..923bb52b25
> --- /dev/null
> +++ b/docs/launch_security_sev.html.in
> @@ -0,0 +1,521 @@
> +<?xml version="1.0" encoding="UTF-8"?>
> +<!DOCTYPE html>
> +<html xmlns="http://www.w3.org/1999/xhtml">
> + <body>
> + <h1>Launch security with AMD SEV</h1>
> +
> + <ul id="toc"></ul>
> +
> + <p>
> + 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.
> + </p>
> +
> + <h2>AMD SEV</h2>
> + <p>
> + 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
> + <a href="https://developer.amd.com/sev/">AMD's developer portal</a>.
> + </p>
> +
> + <h2><a id="Host">Enabling SEV on the host</a></h2>
> + <p>
> + 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:
> + </p>
> +
> + <pre>
> +$ cat /proc/cpuinfo | grep sev
> +...
> +sme ssbd sev ibpb</pre>
> +
> + <p>
> + 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:
> + </p>
> +
> + <pre>
> +mem_encrypt=on kvm_amd.sev=1
> + </pre>
> +
> + <p>
> + To make the changes persistent, append the above to the variable holding
> + parameters of the kernel command line in
> + <code>/etc/default/grub</code> to preserve SEV settings across reboots
> + </p>
> +
> + <pre>
> +$ cat /etc/default/grub
> +...
> +GRUB_CMDLINE_LINUX="... mem_encrypt=on kvm_amd.sev=1"
> +$ grub2-mkconfig -o /boot/efi/EFI/<distro>/grub.cfg</pre>
> +
> + <p>
> + <code>mem_encrypt=on</code> turns on the SME memory encryption feature on
> + the host which protects against the physical attack on the hypervisor
> + memory. The <code>kvm_amd.sev</code> parameter actually enables SEV in
> + the kvm module. It can be set on the command line alongside
> + <code>mem_encrypt</code> like shown above, or it can be put into a
> + module config under <code>/etc/modprobe.d/</code>
> + </p>
> +
> + <pre>
> +$ cat /etc/modprobe.d/sev.conf
> +options kvm_amd sev=1
> + </pre>
> +
> + <p>
> + After rebooting the host, you should see SEV being enabled in the kernel:
> + </p>
> +
> + <pre>
> +$ cat /sys/module/kvm_amd/parameters/sev
> +1
> + </pre>
> +
> + <h2><a id="Virt">Checking SEV support in the virt stack</a></h2>
> + <p>
> + <b>Note: All of the commands bellow need to be run with root privileges.</b>
> + </p>
> +
> + <p>
> + First make sure you have the following packages in the specified versions:
> + </p>
> +
> + <ul>
> + <li>
> + libvirt >= 4.5.0 (>5.1.0 recommended due to additional SEV bugfixes)
> + </li>
> + <li>
> + QEMU >= 2.12.0
> + </li>
> + </ul>
> + <p>
> + To confirm that the virtualization stack supports SEV, run the following:
> + </p>
> +
> + <pre>
> +# virsh domcapabilities
> +<domainCapabilities>
> +...
> + <features>
> + ...
> + <sev supported='yes'>
> + <cbitpos>47</cbitpos>
> + <reducedPhysBits>1</reducedPhysBits>
> + </sev>
> + ...
> + </features>
> +</domainCapabilities></pre>
> + <p>
> + 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:
> + </p>
> +
> + <pre>
> +# systemctl stop libvirtd.service
> + </pre>
> +
> + <p>
> + Now you need to clean the capabilities cache:
> + </p>
> +
> + <pre>
> +# rm -f /var/cache/libvirt/qemu/capabilities/*
> + </pre>
> +
> + <p>
> + If you now restart libvirtd, it will re-probe the capabilities and if
> + you now run:
> + </p>
> +
> + <pre>
> +# virsh domcapabilities
> + </pre>
> +
> + <p>
> + SEV should be listed as supported. If you still see:
> + </p>
> +
> + <pre>
> +<sev supported='no'/>
> + </pre>
> +
> + <p>
> + it means one of two things:
> + <ol>
> + <li>
> + libvirt does support SEV, but either QEMU or the host does not
> + </li>
> + <li>
> + you have libvirt <=5.1.0 which suffered from getting a
> + <code>'Permission denied'</code> on <code>/dev/sev</code> 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
> + </li>
> + </ol>
> + </p>
> +
> + <h2><a id="Configuration">VM Configuration</a></h2>
> + <p>
> + SEV is enabled in the XML by specifying the
> + <a href="https://libvirt.org/formatdomain.html#launchSecurity"><launchSecurity> </a> element. However, specifying <code>launchSecurity</code> isn't
> + enough to boot an SEV VM. Further configuration requirements are discussed
> + below.
> + </p>
> +
> + <h3><a id="Machine">Machine type</a></h3>
> + <p>
> + 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.
> + </p>
> +
> + <h5>Q35</h5>
> +<pre>
> +...
> +<os>
> + <type arch='x86_64' machine='pc-q35-3.0'>hvm</type>
> + ...
> +</os>
> +...</pre>
> +
> + <h5>i440fx (discouraged)</h5>
> + <pre>
> +...
> +<os>
> + <type arch='x86_64' machine='pc-i440fx-3.0'>hvm</type>
> + ...
> +</os>
> +...
> + </pre>
> +
> + <h3><a id="Boot">Boot loader</a></h3>
> + <p>
> + SEV is only going to work with OVMF (UEFI), so you'll need to point libvirt to
> + the correct OVMF binary.
> + </p>
> + <pre>
> +...
> +<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>
> +...</pre>
> +
> + <h3><a id="Memory">Memory</a></h3>
> + <p>
> + 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 <code><currentMemory></code> 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.
> +
> + <p>
> + 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:
> + </p>
> + </p>
> +
> + <pre>
> +# virsh edit <domain>
> +<domain>
> + ...
> + <currentMemory unit='KiB'>4194304</currentMemory>
> + <memtune>
> + <hard_limit unit='KiB'>4456448</hard_limit>
> + </memtune>
> + ...
> +</domain></pre>
> + <p>
> + There's another, preferred method of taking care of the limits by
> + using the<code><memoryBacking></code> element along with the
> + <code><locked/></code> subelement:
> + </p>
> +
> + <pre>
> +<domain>
> + ...
> + <memoryBacking>
> + <locked/>
> + </memoryBacking>
> + ...
> +</domain></pre>
> +
> + <p>
> + 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 <code>machine.slice</code>.
> + </p>
> +
> + <pre>
> +# systemctl set-property machine.slice MemoryHigh=<value></pre>
> +
> + <p>
> + To put even stricter measures in place which would involve the OOM killer, use
> + <pre>
> +# systemctl set-property machine.slice MemoryMax=<value></pre>
> + instead. Alternatively, you can create a systemd config (don't forget
> + to reload systemd configuration in this case):
> + <pre>
> +# cat << EOF > /etc/systemd/system.control/machine.slice.d/90-MemoryMax.conf
> +MemoryMax=<value>
> +EOF</pre>
> + The trade-off to keep in mind with the second approach is that the VMs
> + can still perform DoS on each other.
> + </p>
> +
> + <h3><a id="Virtio">Virtio</a></h3>
> + <p>
> + In order to make virtio devices work, we need to enable emulated IOMMU
> + on the devices so that virtual DMA can work.
> + </p>
> +
> + <pre>
> +# 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></pre>
> +
> + <p>
> + 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).
> +
> + <p>
> + Note: some devices like video devices don't
> + support non-transitional model, which means that virtio GPU cannot be used.
> + </p>
> + </p>
> +
> + <pre>
> +<domain>
> + ...
> + <devices>
> + ...
> + <memballoon model='virtio-non-transitional'>
> + <driver iommu='on'/>
> + </memballoon>
> + </devices>
> + ...
> +</domain></pre>
> +
> + <h2><a id="Limitations">Limitations</a></h2>
> + <p>
> + 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 <code>/dev/sev</code> 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 <code>audit2allow</code> on how to do that) or putting SELinux into permissive mode (discouraged).
> + </p>
> +
> + <h2><a id="Examples">Full domain XML examples</a></h2>
> +
> + <h5>Q35 machine</h5>
> + <pre>
> +<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></pre>
> +
> + <h5>PC-i440fx machine:</h5>
> + <pre>
> +<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></pre>
> + </body>
> +</html>
> --
> 2.21.0
>
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