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README
Apache-2.0

安全启动

简介

OpenHarmony Verified Boot (HVB)部件用于校验设备启动过程对系统镜像的完整性以及合法性;确保OpenHarmony设备启动的镜像包:

  • 来源合法
  • 不被篡改
  • 无法回滚到有漏洞的历史版本

HVB安全启动使用到的基础安全算法包括以下两类:

  • 完整性算法

编译时为系统镜像计算完整性摘要数据,启动时执行相同的完整性算法计算出摘要进行对比,从而确定镜像的完整性。

  • 签名算法

通过对摘要信息进行签名校验,确保镜像的来源可信。此过程依赖设备OEM厂商在初始的bootloader启动时通过安全运行环境提供可行根,从而建立完整的信任链。

系统架构

基本概念

HVB主要用于校验系统镜像,根据镜像的类型分为整包校验和分块校验两种方式:

  • 整包校验(hash)

整包校验适用于一次性加载的镜像,这种镜像只需要生成一个hash值即可,加载镜像时对整个镜像进行hash计算,与编译时的hash值进行对比确保完整性。

此校验方式的弊端是镜像过大时影响开机校验时间。

  • 按需校验(hashtree)

按需校验适用于大的文件系统镜像包,由内核在访问存储块时进行完整性校验。由于是按存储块进行hash值计算的,因此需要在编译时构造哈希树(hashtree)加快校验速度。此方案可以不用在开机时对整个进行进行完整性校验,而只对hashtree进行校验,加快系统启动速度。

此方案依赖内核开启dm-verity特性。

verity校验信息

为了实现安全启动,每个系统镜像都需要在末尾追加校验信息(verity),根据校验方式为两类,如下图所示:

verity校验信息主要包括

字段名称 说明
signature_info verity信息的签名内容。verity签名内容可包含多份pubkey,signature值信息,以兼容老的bootloader产品。
hash_info hash_info存放镜像的完整性信息,根据镜像校验类型可以是hash或hashtree信息。

Root Verity Table根校验信息列表

对于有多个镜像的系统,可以增加根校验信息列表(RVT, Root Verity Table)分区用于统一存放各个镜像verity信息的pubkey,如下图所示:

上游的bootloader只需要校验root_verity_table(RVT)的合法性,就可以通过该分区的pubkey列表逐个校验每个分区镜像的verity信息。

部件结构

如下图所示,HVB部件主要包括以下几个模块:

  • libhvb是安全启动的校验库,工作在设备侧。该模块可供bootloader集成,完成初始的安全镜像校验;也会被系统init调用,完成dm-verity使能。
  • hvbtool用于构造安全校验信息,由编译工具集成使用。

目录

仓目录结构如下:

base/startup/hvb/
├── libhvb         # libhvb模块代码
└── tools          # hvbtool工具代码

约束

目前仅支持标准系统设备。

说明

hvb特性的完整使能依赖以下几方面的适配:

Bootloader适配

Bootloader中安全启动校验流程示例如下:

如上图所示,Bootloader中可判断:

  • 设备是否解锁

Bootloader可通过可信的存储器来存放解锁标记。一般可通过可信信任根对解锁标记进行完整性校验,确保合法。

设备是否支持解锁以及解锁方式由厂商实现。

  • 镜像是否可信

Bootloader通过可信根信息对RVT内容进行可信认证。同时,需要对镜像中的rollback_index进行安全存放;防止回滚到有安全漏洞的版本。

OEM厂商在Bootloader中可使用libhvb中的接口对单个镜像的verity信息进行校验;也可根据rvt信息对多个镜像的verity信息进行校验。

且需要适配hvb_sysdeps.h中平台相关的函数。

Bootloader校验通过后,通过bootargs传递校验信息给内核,具体的参数定义如下:

bootargs名称 说明
ohos.boot.hvb.enable 方案一
hvb使能开关:
false:不需要使能hvb
true:需使能HVB
方案二
关联锁及验证状态时hvb使能开关:
orange: unlock状态下,不需要使能
green: lock状态下hvb校验成功,需要使能
yellow/red: lock状态下hvb校验失败,不需要使能
使能时,bootloader需传递以下hvb详细参数:
ohos.boot.hvb.version 当前只支持1.0版本
ohos.boot.hvb.hash_algo 当前只支持sha256
ohos.boot.hvb.digest HVB保护的所有用户态镜像以及RVT的verity信息摘要

init部件启动时会根据上述参数使能内核dm-verity,从而实现对system.img等镜像的安全校验。

Kernel适配

Kernel需要使能dm-verity功能,默认需要开启以下内核宏:

CONFIG_MD=y
CONFIG_BLK_DEV_DM_BUILTIN=y
CONFIG_BLK_DEV_DM=y
CONFIG_DM_BUFIO=y
CONFIG_DM_ZERO=y
CONFIG_DM_DELAY=m
CONFIG_DM_INIT=y
CONFIG_DM_UEVENT=y
CONFIG_DM_VERITY=y
CONFIG_DM_VERITY_VERIFY_ROOTHASH_SIG=y
CONFIG_DM_VERITY_FEC=y

fstab适配

init部件在挂载系统镜像时,会根据fstab配置的fs_mgr_flags选项决定哪些系统镜像需要使能hvb安全校验。如下所示,fs_mgr_flags添加上hvb表示该分区需要使能hvb安全校验:

#<src>                                                  <mnt_point> <type>   <mnt_flags and options>  <fs_mgr_flags>
/dev/block/platform/f8300000.ufs/by-name/system         /usr        ext4     ro,barrier=1             wait,required,hvb

相关仓

startup_init_lite

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OpenHarmony Verified Boot (HVB)部件用于校验设备启动过程中系统镜像的完整性以及合法性。 展开 收起
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