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，在有许多租户共享物理资源的云计算环境中更是如此。假设某个恶意租户的虚拟机利用超级管理程序 Bug 逃离了沙箱，并在内存中搜索敏感数据。

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。SEV 可以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。

当虚拟机正在处理敏感数据时，它可以存在于 CPU 寄存器以及内存中。如果处理出于某种原因（例如需要提供中断或与其他进程共享时间）而停止，虚拟机的 CPU 寄存器内容将会保存到超级管理程序内存中。超级管理程序可以读取此内存，即使启用了 SEV 也是如此。SEV-ES 可以防范这种情况，它会在虚拟机的处理停止时对所有 CPU 寄存器内容进行加密。SEV-ES 在 SEV 的基础上构建而成，能够为在多租户环境中运行的虚拟机缩小受攻击面。

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 libvirt 的功能中是否包含 sev 属性，并且该属性的值是否设置恰当：

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

此外，请确保已为 kvm_amd 内核模块启用 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。可以让 libvirt 自动选择启用了 SEV 或 SEV-ES 的适当 UEFI 固件，也可以手动指定。目前仅支持 /usr/share/qemu/ovmf-x86_64-code.bin 和 /usr/share/qemu/ovmf-x86_64-4m-code.bin 固件。有关如何使用 UEFI 固件以及自动选择功能的更多细节，请参见第 6.3 节 “安装 UEFI 支持”。

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-net 设备都需要配置为禁用 iPXE 选项 ROM。iPXE 目前与 SEV 和 SEV-ES 不兼容。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。其中包括 VM 的内存以及 QEMU 为支持 VM 运行而分配的任何内存区域，例如用于固件和变量储存的 UEFI pflash，视频 RAM 等。

<domain type='kvm'>
    <memory unit='KiB'>4194304</memory>
    <currentMemory unit='KiB'>4194304</currentMemory>
    <memoryBacking>
    <locked/> 1
    </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 2 type='sev'>
    <cbitpos>47</cbitpos> 3
    <reducedPhysBits>1</reducedPhysBits> 4
    <policy>0x0033</policy> 5
    <dhCert>AAAABBBB=CCCCCDDDDD</dhCert> 6
    <session>AAAABBBB=EEEEEFFFFF</session> 7
    </launchSecurity>
    <devices>
    <interface type='bridge'>
    <source bridge='br0'/>
    <model type='virtio'/>
    <rom enabled='no'/> 8
    </interface>
    ...
    </devices>
    ...
    </domain>

Guest 策略是定义如下的 4 个无符号字节：

virt-install 支持安装 SEV 和 SEV-ES 虚拟机。除了标准安装参数以外，还需要为 virt-install 提供用于满足 VM 要求的选项以及 --launchSecurity 选项。

下面的示例会启动通过 SEV-ES 提供保护的 SLES 15 SP4 虚拟机的网络安装。

   virt-install --name sles15sp4-sev-es --location http://192.168.0.1/install/sles15sp4/x86_64 --disk size=20 --network=bridge=br0,model=virtio,rom.bar=off 1 --vcpus 4 --memory 4096 --noautoconsole --events on_reboot=destroy --machine q35 --memtune hard_limit=4563402 --launchSecurity sev,policy=0x07 2 --boot firmware=efi 3 --vnc --serial pty

KubeVirt 从版本 0.49.0 开始支持运行 SEV Guest。该功能可通过启用 WorkloadEncryptionSEV 功能门控来激活：

> kubectl edit kubevirt kubevirt -n kubevirt
[...]
spec:
  configuration:
    developerConfiguration:
      featureGates:
      - WorkloadEncryptionSEV
[...]

要运行 SEV 加密的 Guest，必须在虚拟机规范中的 launchSecurity 域元素下包含 sev: {} 条目。此外，您还需要将 firmware/bootloader 参数配置为使用 efi 选项，并将 secureBoot 标志设置为 disabled。相应的 YAML 代码段将如下所示：

[...]
spec:
  domain:
    firmware:
      bootloader:
        efi:
          secureBoot: false
    launchSecurity:
      sev: {}
[...]

SUSE 不建议在第一代 AMD EPYC™ 7000 系列处理器（代号 Naples）上将 SEV 和 SEV-ES 功能与 SUSE Linux 产品搭配使用。建议至少使用第二代 7002 系列处理器（代号 Rome）。此外，SEV 和 SEV-ES VM 还存在以下限制。

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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