AMD 安全加密虚拟化 (AMD-SEV) 指南 #

对磁盘上储存的计算机数据进行加密这项功能已得到广泛部署，但 RAM 中储存的数据并未加密。这可能会导致这些数据暴露给通过软件或硬件嗅探入侵主机系统的攻击者。新的永久性内存技术让此问题变得更加严重：用户可从系统上取下 NVDIMM（非易失性内存模块）实体且其上的数据将保持不变，就像硬盘或 SSD 上的数据一样。如果未加密，储存的任何信息（如敏感数据、口令或机密密钥）都可能轻易泄露。

AMD 的 SEV（安全加密虚拟化）技术通过独有的密钥以透明方式加密每个 VM 的内存，可以保护 Linux KVM 虚拟机。SEV 还可计算出内存内容的签名，该签名可发送给 VM 的所有者，作为固件已正确加密内存的证明。SEV 特别适用于云计算环境，在此环境中，VM 托管在远程服务器上，并不在 VM 所有者的控制之下。这样可减少需要置于超级管理程序及其主机系统管理员控制之下的可信 VM 的数量。

在 SUSE Linux Enterprise 12 SP4 及以上版本和即将推出的 SUSE Linux Enterprise 15 维护版本中，内核、QEMU 和 libvirt 支持使用 SEV 创建和管理 VM。

VM 主机硬件必须支持 AMD 的 SEV 技术。要检测主机硬件是否支持 SEV，请检查 sev 属性是否包含在 libvirt 的功能中，并且其值是否设置恰当：

<domainCapabilities>
   ...
   <features>
   ...
   <sev supported='yes'/>
   ...
   </sev>
   </features>
</domainCapabilities>

此外，请确保 kvm_amd 内核模块已启用 sev 参数：

/sys/module/kvm_amd/parameters/sev = 1

VM 必须为新型 Q35 计算机类型且必须使用 UEFI 固件。

注意：Q35 中无 IDE 支持

Q35 计算机类型没有 IDE 控制器，不支持 IDE 磁盘。

所有 virtio 设备都需要配置 iommu='on' 属性（在其 <driver> 配置中）。此外，必须锁定对 VM 使用的所有内存区域的直接内存访问 (DMA)，以防发生交换。

<domain type='kvm'>
    <memory unit='KiB'>4194304</memory>
    <currentMemory unit='KiB'>4194304</currentMemory>
    <memoryBacking>
    <locked/>
    </memoryBacking>
    <os>
    <type arch='x86_64' machine='pc-q35-2.11'>hvm</type>
    <loader readonly='yes' type='pflash'>/usr/share/qemu/ovmf-x86_64-ms-4m-code.bin</loader>
    <nvram>/var/lib/libvirt/qemu/nvram/sles15-sev-guest_VARS.fd</nvram>
    <boot dev='hd'/>
    </os>
    <launchSecurity 1 type='sev'>
    <cbitpos>47</cbitpos> 2
    <reducedPhysBits>1</reducedPhysBits> 3
    <policy>0x0033</policy> 4
    </launchSecurity>
    <devices>
    <disk type='file' device='disk'>
    <driver iommu='on' name='qemu' type='raw'/>
    <target dev='vda' bus='virtio'/>
    <source file='/vmimages/sev-guest-disk.raw'/>
    </disk>
    <rng model='virtio'>
    <driver iommu='on'/>
    ...
    </rng>
    <memballoon model='virtio'>
    <driver iommu='on' /> 5
    ...
    </memballoon>
    <video>
    <model type='qxl' ram='65536' vram='65536' vgamem='16384' heads='1' primary='yes'/>
    </video>
    ...
    </devices>
    ...
    </domain>

Guest 策略是定义如下的 4 个无符号字节：

可选 dhCert 元素提供 Guest 所有者的 base64 编码 Diffie-Hellman (DH) 密钥。该密钥用于在 SEV 固件与 Guest 所有者之间协商主机密密钥。此主机密密钥之后会用于在 SEV 固件与 Guest 所有者之间建立可信通道。

可选 session 元素提供 Guest 所有者的 base64 编码会话 Blob，如 SEV API 规范中定义。请参见 SEV 规范中针对会话 Blob 格式的 LAUNCH_START 部分。

SEV 加密的 VM 还必须锁定其所有内存区域，以允许 DMA 并防止发生交换。必须使用 memoryBacking 的 locked子元素指定内存的显式锁定。可通过将虚拟机配置为使用大页来避免进行显式内存锁定。有关在 VM 中使用大页的详细信息，请参见虚拟化最佳实践Article “虚拟化最佳实践”, Section 4.1.1 “将 VM 主机服务器和 VM Guest 配置为使用大页”。

如果产生的开销与非 SEV VM 所需并无不同，那么固定内存时设置正确的硬限制就重要得多。如果限制过低，VM 将被终止。

未来，当硬件、固件和各层软件具有新的功能时，将会去除这些限制。

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