AMD 安全加密虚拟化 (AMD-SEV) 指南 #

对磁盘上存储的计算机数据进行加密这项功能已得到广泛部署，但 RAM 中存储的数据并未加密。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，唯一受支持的固件为 /usr/share/qemu/ovmf-x86_64-code.bin 和 /usr/share/qemu/ovmf-x86_64-4m-code.bin。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。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存储的 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

VM 证明是校验可信软件组件是否在可信计算平台上正确实例化的过程。该过程通常涉及启动处于暂停状态的 VM，检索已实例化软件组件的启动度量，校验该度量，然后提供磁盘口令或其他密钥来解锁 VM 并恢复已暂停的引导过程。启动度量包括 VM 拥有者提供的加密项目，其中的加密信任根是 AMD SEV 平台。一旦启动度量得到校验，VM 拥有者就可以确认其软件组件未受到侵害并在可信平台上运行。

整个证明过程相当复杂，并且很可能会发生错误。必须小心确保该过程本身是安全的。例如，安全证明过程不能直接在运行 VM 的超级管理程序上执行，因为目标是证明超级管理程序不存在恶意行为，并且不会污染 VM。

SLES 15 SP4 中提供了用于证明的所有信息和 API，而 SLES 15 SP5 则引入了一个名为 virt-qemu-sev-validate 的简单实用程序，可用于满足多种证明用例的需要。例如，用于质量保证的自动化测试，以及不受大型商业管理堆栈管理的小规模 libvirt+KVM 部署。

virt-qemu-sev-vailidate 包含在 libvirt-client-qemu 软件包中，支持脱机和联机证明模式。virt-qemu-sev-validate 要求以命令行参数的形式为证明提供输入。如果在可信计算机上调用该实用程序，则可以信任 virt-qemu-sev-validate 的结果，因为不会从不可信的来源检索任何信息。联机模式不太安全，尤其是直接在运行 VM 的超级管理程序上执行时。

无论采用哪种模式，libvirt+KVM VM 的证明过程首先都会创建 VM 或来宾拥有者 (GO) 证书和会话 Blob，每次启动 VM 时，该证书和会话 Blob 都是唯一的。可以使用 sevctl 软件包中提供的 sevctl 实用程序创建该证书和 Blob。以下示例说明如何使用 sevctl session 命令创建所有与预启动 SEV 相关的项目。NAME 参数是可选的，允许为项目文件名指定前缀。使用 VM 名称作为前缀可便于稍后将项目与 VM 进行匹配。平台 Diffie-Hellman (DH) 证书的路径和所需的 SEV 策略是必需参数。

# sevctl session --name test-sev /data/sev/pdh.cert 7

sevctl session 命令生成四个文件：tick.bin、take.bin、godh.b64 和 session.b64。如果使用可选的 NAME 参数，则文件将以指定的值作为前缀。传输完整性密钥 (tik.bin) 和传输加密密钥 (tek.bin) 用于证明过程的校验阶段。在启动 VM 之前，来宾拥有者 Diffie-Hellman 密钥 (godh.b64) 和会话 Blob (session.b64) 将复制到 VM XML 配置中。有关更多细节，请参见 VM 配置部分中 launchSecurity 元素的 dhCert 和 session 子元素。

创建 VM 会话项目并更新 VM XML 配置后，可以在暂停状态下启动 VM，例如：

# virsh -c qemu+ssh://USER_NAME@HOST_NAME/system create --paused /path/to/vm.xml

在暂停状态下创建 VM 可以从超级管理程序检索启动度量，并将其与使用可信输入在可信主机上计算的度量进行比较。如果这些度量相同，则 VM 拥有者可以确信 VM 已在超级管理程序上正确实例化，并且可以安全地启动执行。以下命令演示如何使用 virsh domlaunchsecinfo 命令从宿主超级管理程序检索已暂停 VM 的启动度量和其他 SEV 相关信息。

# virsh -c qemu+ssh://username@hostname/system domlaunchsecinfo sevtest
sev-measurement: VZjxMSlu+UuYkWHN2mAxDVVYXRmL3wqTu84kwk+5QS+4OMii7hs6cMAmXNpmmyS/
sev-api-major  : 1
sev-api-minor  : 51
sev-build-id   : 3
sev-policy     : 7

然后，可以将检索到的启动度量提供给 virt-qemu-sev-validate，以校验 VM 是否已安全实例化。以下示例演示度量的完整脱机证明。

# virt-qemu-sev-validate --api-major 1 --api-minor 51 --build-id 3 --policy 7 \
 --firmware /usr/share/qemu/ovmf-x86_64-4m.bin --tik sevtest_tik.bin --tek sevtest_tek.bin --num-cpus 4 \
 --cpu-family 25 --cpu-model 1 --cpu-stepping 1 \
 --measurement QJ0oDpFmWj+bGZzFoMPbAxTuC6QD44W5w88x/hQM8toVsB75ci7V1YDfYoI9GTk

还可以在联机模式下使用 virt-qemu-sev-validate，在此模式下，将从宿主超级管理程序检索执行 VM 证明所需的信息。以下示例演示 VM 的联机证明，在此模式下，仅指定了宿主超级管理程序 URI、VM 名称以及关联的 TIK 和 TEK。virt-qemu-sev-validate 从宿主超级管理程序检索其余的信息，包括度量本身：

# virt-qemu-sev-validate --tik sevtest_tik.bin --tek sevtest_tek.bin \
 --connect qemu+ssh://USER_NAME@HOST_NAME/system --domain sevtest

校验 VM 启动度量后，VM 拥有者可以选择在 VM 中注入机密并恢复 VM 执行。注入机密的示例是提供一个密钥来解锁已加密的根磁盘。

# virsh -c qemu+ssh://USER_NAME@HOST_NAME/system domsetlaunchsecstate sevtest \
 --secrethdr hdr-str --secret secret-str
# virsh -c qemu+ssh://USER_NAME@HOST_NAME/system resume sevtest

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: {} 项。此外，您需要配置 firmware/bootloader 参数，以便在将 secureBoot 标志设置为 disabled 的情况下使用 efi 选项。相应的 YAML 代码段如下所示：

[...]
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、固件和特定软件层具有新的功能时，将会去除这些限制。

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If you have Invariant Sections, Front-Cover Texts and Back-Cover Texts, replace the “with...Texts.” line with this:

with the Invariant Sections being LIST THEIR TITLES, with the
Front-Cover Texts being LIST, and with the Back-Cover Texts being LIST.

If you have Invariant Sections without Cover Texts, or some other combination of the three, merge those two alternatives to suit the situation.

If your document contains nontrivial examples of program code, we recommend releasing these examples in parallel under your choice of free software license, such as the GNU General Public License, to permit their use in free software.