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Standard text is aligned to 80 colums in all .rst files. Signed-off-by: Peter Krempa <pkrempa@redhat.com>
558 lines
16 KiB
ReStructuredText
558 lines
16 KiB
ReStructuredText
============================
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Launch security with AMD SEV
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============================
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.. contents::
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Storage encryption in modern public cloud computing is a common
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practice. However, from the point of view of a user of these cloud
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workloads, a significant amount of trust needs to be put in the cloud
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platform security as well as integrity (was the hypervisor tampered?).
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For this reason there's ever rising demand for securing data in use,
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i.e. memory encryption. One of the solutions addressing this matter is
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AMD SEV.
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AMD Secure Encrypted Virtualization (SEV)
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=========================================
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SEV (Secure Encrypted Virtualization) is a feature extension of AMD's
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SME (Secure Memory Encryption) intended for KVM virtual machines which
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is supported primarily on AMD's EPYC CPU line. In contrast to SME, SEV
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uses a unique memory encryption key for each VM. The whole encryption of
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memory pages is completely transparent to the hypervisor and happens
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inside dedicated hardware in the on-die memory controller. Each
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controller includes a high-performance Advanced Encryption Standard
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(AES) engine that encrypts data when it is written to DRAM and decrypts
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it when read. For more details about the technology itself, you can
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visit `AMD's developer portal <https://developer.amd.com/sev/>`__.
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Enabling SEV on the host
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========================
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Before VMs can make use of the SEV feature you need to make sure your
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AMD CPU does support SEV. You can run ``virt-host-validate``
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(libvirt >= 6.5.0) to check if your host supports secure guests or you
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can follow the manual checks below.
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You can manually check whether SEV is among the CPU flags with:
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::
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$ grep -w sev /proc/cpuinfo
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...
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sme ssbd sev ibpb
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Next step is to enable SEV in the kernel, because it is disabled by
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default. This is done by putting the following onto the kernel command
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line:
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::
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mem_encrypt=on kvm_amd.sev=1
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To make the changes persistent, append the above to the variable holding
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parameters of the kernel command line in ``/etc/default/grub`` to
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preserve SEV settings across reboots
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::
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$ cat /etc/default/grub
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...
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GRUB_CMDLINE_LINUX="... mem_encrypt=on kvm_amd.sev=1"
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$ grub2-mkconfig -o /boot/efi/EFI/<distro>/grub.cfg
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``mem_encrypt=on`` turns on the SME memory encryption feature on the
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host which protects against the physical attack on the hypervisor
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memory. The ``kvm_amd.sev`` parameter actually enables SEV in the kvm
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module. It can be set on the command line alongside ``mem_encrypt`` like
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shown above, or it can be put into a module config under
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``/etc/modprobe.d/``
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::
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$ cat /etc/modprobe.d/sev.conf
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options kvm_amd sev=1
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After rebooting the host, you should see SEV being enabled in the
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kernel:
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::
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$ cat /sys/module/kvm_amd/parameters/sev
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1
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Checking SEV support in the virt stack
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======================================
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**Note: All of the commands below need to be run with root
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privileges.**
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First make sure you have the following packages in the specified
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versions:
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- libvirt >= 4.5.0 (>5.1.0 recommended due to additional SEV bugfixes)
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- QEMU >= 2.12.0
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To confirm that the virtualization stack supports SEV, run the
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following:
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::
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# virsh domcapabilities
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<domainCapabilities>
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...
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<features>
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...
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<sev supported='yes'>
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<cbitpos>47</cbitpos>
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<reducedPhysBits>1</reducedPhysBits>
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</sev>
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...
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</features>
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</domainCapabilities>
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Note that if libvirt (<6.5.0) was already installed and libvirtd running before
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enabling SEV in the kernel followed by the host reboot you need to force
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libvirtd to re-probe both the host and QEMU capabilities. First stop
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libvirtd:
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::
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# systemctl stop libvirtd.service
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Now you need to clean the capabilities cache:
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::
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# rm -f /var/cache/libvirt/qemu/capabilities/*
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If you now restart libvirtd, it will re-probe the capabilities and if
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you now run:
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::
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# virsh domcapabilities
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SEV should be listed as supported. If you still see:
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::
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<sev supported='no'/>
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it means one of two things:
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#. libvirt does support SEV, but either QEMU or the host does not
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#. you have libvirt <=5.1.0 which suffered from getting a
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``'Permission denied'`` on ``/dev/sev`` because of the default
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permissions on the character device which prevented QEMU from opening
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it during capabilities probing - you can either manually tweak the
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permissions so that QEMU has access to it or preferably install
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libvirt 5.1.0 or higher
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VM Configuration
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================
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SEV is enabled in the XML by specifying the
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`<launchSecurity> <https://libvirt.org/formatdomain.html#launch-security>`__
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element. However, specifying ``launchSecurity`` isn't enough to boot an
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SEV VM. Further configuration requirements are discussed below.
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Machine type
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------------
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Even though both Q35 and legacy PC machine types (for PC see also
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"virtio") can be used with SEV, usage of the legacy PC machine type is
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strongly discouraged, since depending on how your OVMF package was built
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(e.g. including features like SecureBoot or SMM) Q35 may even be
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required.
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Q35
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~~~
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::
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...
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<os>
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<type arch='x86_64' machine='pc-q35-3.0'>hvm</type>
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...
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</os>
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...
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i440fx (discouraged)
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~~~~~~~~~~~~~~~~~~~~
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::
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...
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<os>
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<type arch='x86_64' machine='pc-i440fx-3.0'>hvm</type>
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...
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</os>
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...
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Boot loader
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-----------
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SEV is only going to work with OVMF (UEFI), so you'll need to point
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libvirt to the correct OVMF binary.
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::
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...
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<os>
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<type arch='x86_64' machine='pc-q35-3.0'>hvm</type>
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<loader readonly='yes' type='pflash'>/usr/share/edk2/ovmf/OVMF_CODE.fd</loader>
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</os>
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...
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Memory
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------
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Internally, SEV expects that the encrypted memory pages won't be swapped
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out or move around so the VM memory needs to be pinned in physical RAM
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which will be handled by QEMU. Apart from that, certain memory regions
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allocated by QEMU itself (UEFI pflash, device ROMs, video RAM, etc.)
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have to be encrypted as well. This causes a conflict in how libvirt
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tries to protect the host. By default, libvirt enforces a memory hard
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limit on each VM's cgroup in order to protect the host from malicious
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QEMU to allocate and lock all the available memory. This limit
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corresponds to the total memory allocation for the VM given by
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``<currentMemory>`` element. However, trying to account for the
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additional memory regions QEMU allocates when calculating the limit in
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an automated manner is non-deterministic. One way to resolve this is to
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set the hard limit manually.
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Note: Figuring out the right number so that your guest boots and isn't
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killed is challenging, but 256MiB extra memory over the total guest RAM
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should suffice for most workloads and may serve as a good starting
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point. For example, a domain with 4GB memory with a 256MiB extra hard
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limit would look like this:
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::
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# virsh edit <domain>
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<domain>
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...
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<currentMemory unit='KiB'>4194304</currentMemory>
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<memtune>
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<hard_limit unit='KiB'>4456448</hard_limit>
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</memtune>
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...
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</domain>
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There's another, preferred method of taking care of the limits by using
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the\ ``<memoryBacking>`` element along with the ``<locked/>``
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subelement:
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::
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<domain>
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...
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<memoryBacking>
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<locked/>
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</memoryBacking>
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...
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</domain>
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What that does is that it tells libvirt not to force any hard limit
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(well, unlimited) upon the VM cgroup. The obvious advantage is that one
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doesn't need to determine the hard limit for every single SEV-enabled
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VM. However, there is a significant security-related drawback to this
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approach. Since no hard limit is applied, a malicious QEMU could perform
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a DoS attack by locking all of the host's available memory. The way to
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avoid this issue and to protect the host is to enforce a bigger hard
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limit on the master cgroup containing all of the VMs - on systemd this
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is ``machine.slice``.
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::
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# systemctl set-property machine.slice MemoryHigh=<value>
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To put even stricter measures in place which would involve the OOM
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killer, use
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::
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# systemctl set-property machine.slice MemoryMax=<value>
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instead. Alternatively, you can create a systemd config (don't forget to
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reload systemd configuration in this case):
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::
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# cat << EOF > /etc/systemd/system.control/machine.slice.d/90-MemoryMax.conf
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MemoryMax=<value>
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EOF
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The trade-off to keep in mind with the second approach is that the VMs
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can still perform DoS on each other.
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Virtio
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------
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In order to make virtio devices work, we need to use
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``<driver iommu='on'/>`` inside the given device XML element in order
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to enable DMA API in the virtio driver.
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Starting with QEMU 6.0.0 QEMU will set this for us by default. For earlier
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versions though, you will need to explicitly enable this in the device XML as
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follows::
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# virsh edit <domain>
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<domain>
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...
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<controller type='virtio-serial' index='0'>
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<driver iommu='on'/>
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</controller>
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<controller type='scsi' index='0' model='virtio-scsi'>
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<driver iommu='on'/>
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</controller>
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...
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<memballoon model='virtio'>
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<driver iommu='on'/>
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</memballoon>
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<rng model='virtio'>
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<backend model='random'>/dev/urandom</backend>
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<driver iommu='on'/>
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</rng>
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...
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<domain>
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If you for some reason want to use the legacy PC machine type, further
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changes to the virtio configuration is required, because SEV will not
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work with Virtio <1.0. In libvirt, this is handled by using the
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virtio-non-transitional device model (libvirt >= 5.2.0 required).
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Note: some devices like video devices don't support non-transitional
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model, which means that virtio GPU cannot be used.
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::
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<domain>
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...
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<devices>
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...
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<memballoon model='virtio-non-transitional'>
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<driver iommu='on'/>
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</memballoon>
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</devices>
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...
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</domain>
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Virtio-net
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~~~~~~~~~~
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With virtio-net it's also necessary to disable the iPXE option ROM as
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iPXE is not aware of SEV (at the time of this writing). This translates to the
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following XML:
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::
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<domain>
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...
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<interface type='network'>
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...
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<model type='virtio'/>
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<driver iommu='on'/>
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<rom enabled='no'/>
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</interface>
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...
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<domain>
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Checking SEV from within the guest
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==================================
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After making the necessary adjustments discussed in
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`VM Configuration`_, the VM should now boot successfully
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with SEV enabled. You can then verify that the guest has SEV enabled by
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running:
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::
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# dmesg | grep -i sev
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AMD Secure Encrypted Virtualization (SEV) active
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Limitations
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===========
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With older kernels (kernel <5.1) the boot disk cannot not be of type
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virtio-blk, instead, virtio-scsi needs to be used if virtio is desired.
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If you still cannot start an SEV VM, it could be because of wrong SELinux label
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on the ``/dev/sev`` device with selinux-policy <3.14.2.40 which prevents QEMU
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from touching the device. This can be resolved by upgrading the package, tuning
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the selinux policy rules manually to allow svirt_t to access the device (see
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``audit2allow`` on how to do that) or putting SELinux into permissive mode
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(discouraged).
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Full domain XML examples
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========================
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Q35 machine
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-----------
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::
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<domain type='kvm'>
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<name>sev-dummy</name>
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<memory unit='KiB'>4194304</memory>
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<currentMemory unit='KiB'>4194304</currentMemory>
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<memoryBacking>
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<locked/>
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</memoryBacking>
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<vcpu placement='static'>4</vcpu>
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<os>
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<type arch='x86_64' machine='pc-q35-3.0'>hvm</type>
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<loader readonly='yes' type='pflash'>/usr/share/edk2/ovmf/OVMF_CODE.fd</loader>
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<nvram>/var/lib/libvirt/qemu/nvram/sev-dummy_VARS.fd</nvram>
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</os>
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<features>
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<acpi/>
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<apic/>
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<vmport state='off'/>
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</features>
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<cpu mode='host-model' check='partial'>
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<model fallback='allow'/>
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</cpu>
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<clock offset='utc'>
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<timer name='rtc' tickpolicy='catchup'/>
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<timer name='pit' tickpolicy='delay'/>
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<timer name='hpet' present='no'/>
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</clock>
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<on_poweroff>destroy</on_poweroff>
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<on_reboot>restart</on_reboot>
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<on_crash>destroy</on_crash>
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<pm>
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<suspend-to-mem enabled='no'/>
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<suspend-to-disk enabled='no'/>
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</pm>
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<devices>
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<emulator>/usr/bin/qemu-kvm</emulator>
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<disk type='file' device='disk'>
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<driver name='qemu' type='qcow2'/>
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<source file='/var/lib/libvirt/images/sev-dummy.qcow2'/>
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<target dev='sda' bus='scsi'/>
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<boot order='1'/>
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</disk>
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<controller type='virtio-serial' index='0'>
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<driver iommu='on'/>
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</controller>
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<controller type='scsi' index='0' model='virtio-scsi'>
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<driver iommu='on'/>
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</controller>
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<interface type='network'>
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<mac address='52:54:00:cc:56:90'/>
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<source network='default'/>
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<model type='virtio'/>
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<driver iommu='on'/>
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<rom enabled='no'/>
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</interface>
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<graphics type='spice' autoport='yes'>
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<listen type='address'/>
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<gl enable='no'/>
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</graphics>
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<video>
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<model type='qxl'/>
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</video>
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<memballoon model='virtio'>
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<driver iommu='on'/>
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</memballoon>
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<rng model='virtio'>
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<driver iommu='on'/>
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</rng>
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</devices>
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<launchSecurity type='sev'>
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<cbitpos>47</cbitpos>
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<reducedPhysBits>1</reducedPhysBits>
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<policy>0x0003</policy>
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</launchSecurity>
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</domain>
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PC-i440fx machine
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-----------------
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::
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<domain type='kvm'>
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<name>sev-dummy-legacy</name>
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<memory unit='KiB'>4194304</memory>
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<currentMemory unit='KiB'>4194304</currentMemory>
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<memtune>
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<hard_limit unit='KiB'>5242880</hard_limit>
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</memtune>
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<vcpu placement='static'>4</vcpu>
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<os>
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<type arch='x86_64' machine='pc-i440fx-3.0'>hvm</type>
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<loader readonly='yes' type='pflash'>/usr/share/edk2/ovmf/OVMF_CODE.fd</loader>
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<nvram>/var/lib/libvirt/qemu/nvram/sev-dummy_VARS.fd</nvram>
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<boot dev='hd'/>
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</os>
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<features>
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<acpi/>
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<apic/>
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<vmport state='off'/>
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</features>
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<cpu mode='host-model' check='partial'>
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<model fallback='allow'/>
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</cpu>
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<clock offset='utc'>
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<timer name='rtc' tickpolicy='catchup'/>
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<timer name='pit' tickpolicy='delay'/>
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<timer name='hpet' present='no'/>
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</clock>
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<on_poweroff>destroy</on_poweroff>
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<on_reboot>restart</on_reboot>
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<on_crash>destroy</on_crash>
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<pm>
|
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<suspend-to-mem enabled='no'/>
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<suspend-to-disk enabled='no'/>
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</pm>
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<devices>
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<emulator>/usr/bin/qemu-kvm</emulator>
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<disk type='file' device='disk'>
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<driver name='qemu' type='qcow2'/>
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<source file='/var/lib/libvirt/images/sev-dummy-seabios.qcow2'/>
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<target dev='sda' bus='sata'/>
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</disk>
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<interface type='network'>
|
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<mac address='52:54:00:d8:96:c8'/>
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<source network='default'/>
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<model type='virtio-non-transitional'/>
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<driver iommu='on'/>
|
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<rom enabled='no'/>
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</interface>
|
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<serial type='pty'>
|
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<target type='isa-serial' port='0'>
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<model name='isa-serial'/>
|
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</target>
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</serial>
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<console type='pty'>
|
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<target type='serial' port='0'/>
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</console>
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<input type='tablet' bus='usb'>
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<address type='usb' bus='0' port='1'/>
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</input>
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<input type='mouse' bus='ps2'/>
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|
<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>
|