前言
Android Verified Boot上有两个阶段，bootloader和init，每个阶段校验的内容都是不一样的。当然这个也不是绝对的，我觉得这个AVB的校验逻辑是可以用在很多的方面的，根据自己的启动方案可以设置很多的校验逻辑，这里就不展开了，说多了就不安全了。
这里暂时分为两个阶段：bootloader和init
第一阶段
在bootloader中，需要校验分区的函数变量：
// Library/avb/VerifiedBoot.c static CHAR8 *avb_verify_partition_name[] = { "boot", "dtbo", "vbmeta", "recovery", "vendor_boot" };
从上面的变量可以知道，在bootloader中，可能需要校验上面的5个分区。
为什么说是可能？因为最终的校验是需要根据vbmeta分区来决定的。
解析vbmeta镜像：
avbtool info_image --image vbmeta.img > vbmeta.img.info
vbmeta.img.info 内容
Minimum libavb version: 1.0 Header Block: 256 bytes Authentication Block: 576 bytes Auxiliary Block: 4032 bytes Public key (sha1): 2e22ae4a46cf9db9c24ab74cee91fa005ccb30e4 Algorithm: SHA256_RSA4096 Rollback Index: 0 Flags: 0 Release String: 'avbtool 1.1.0' Descriptors: Chain Partition descriptor: Partition Name: vbmeta_system Rollback Index Location: 2 Public key (sha1): fc92d9cba0628858d846fb9a18a7af72b05d7dc8 ... Hash descriptor: Image Size: 59932672 bytes Hash Algorithm: sha256 Partition Name: boot Salt: e691366c1c43ee5e23b342d65555ad8cfbadf77118dceb77e240c8e7d3e63ea6 Digest: 239648eb41f5a491c7c4d6b51b52a533bd9da98ba8800f58a0957f7341dd1686 Flags: 0 Hash descriptor: Image Size: 740733 bytes Hash Algorithm: sha256 Partition Name: dtbo Salt: d445a36d8154a774589dd51c49029ee388ecaac28212c8c6899f45dc5a51dbcf Digest: cac0bd59091464292bc83d6f1193afb1520c12a3849f2673ad3160bb951acf6d Flags: 0 Hash descriptor: Image Size: 1916928 bytes Hash Algorithm: sha256 Partition Name: vendor_boot Salt: 5f7b7c3592142d4f3645d7e675fb7865915e52e8b361ba330fccf00aeb1c4028 Digest: cf153ab9df9a5c34024417ea2b0b4dfd716d01ba9cd2d5bcf964fe7e25cdd802 Flags: 0 Hashtree descriptor: Version of dm-verity: 1 Image Size: 864256 bytes Tree Offset: 864256 Tree Size: 12288 bytes Data Block Size: 4096 bytes Hash Block Size: 4096 bytes FEC num roots: 2 FEC offset: 876544 FEC size: 8192 bytes Hash Algorithm: sha1 Partition Name: odm Salt: b6e1f57ae6939659355e83ad7fa57feb6b5eb15a3d16b96752f43cdc14918708 Root Digest: da99875b16661e72eec81d05d58dffdf09fe228d Flags: 0 Hashtree descriptor: Version of dm-verity: 1 Image Size: 1002233856 bytes Tree Offset: 1002233856 Tree Size: 7897088 bytes Data Block Size: 4096 bytes Hash Block Size: 4096 bytes FEC num roots: 2 FEC offset: 1010130944 FEC size: 7987200 bytes Hash Algorithm: sha1 Partition Name: vendor Salt: b6e1f57ae6939659355e83ad7fa57feb6b5eb15a3d16b96752f43cdc14918708 Root Digest: 336bd4885da274ae0de38b32b899f7b6169e676f Flags: 0
通过vbmeta.img.info可以知道，bootloader中需要校验 vbmeta、boot 、dtbo、vendor_boot四个分区。
第二阶段
init阶段的avb校验，可以通过dts和fstab确定需要校验的分区。
1、dts
android { compatible = "android,firmware"; vbmeta { compatible = "android,vbmeta"; parts = "vbmeta,boot,system,vendor,dtbo"; }; fstab { compatible = "android,fstab"; vendor { compatible = "android,vendor"; dev = "/dev/block/platform/soc/1d84000.ufshc/by-name/vendor"; type = "ext4"; mnt_flags = "ro,barrier=1,discard"; fsmgr_flags = "wait,slotselect,avb"; status = "ok"; }; }; };
fsmgr_flags = "wait,slotselect,avb"知道，根据fstab中分区的挂载参数，来确定需要进行校验的分区。
2. fstab.qcom
我们只需要关注fstab中的 wait,slotselect,avb= 挂载参数。
system /system ext4 ro,barrier=1,discard wait,slotselect,avb=vbmeta_system,logical,first_stage_mount,avb_keys=/avb/q-gsi.avbpubkey:/avb/r-gsi.avbpubkey:/avb/s-gsi.avbpubkey system_ext /system_ext ext4 ro,barrier=1,discard wait,slotselect,avb=vbmeta_system,logical,first_stage_mount product /product ext4 ro,barrier=1,discard wait,slotselect,avb=vbmeta_system,logical,first_stage_mount vendor /vendor ext4 ro,barrier=1,discard wait,slotselect,avb,logical,first_stage_mount odm /odm ext4 ro,barrier=1,discard wait,slotselect,avb,logical,first_stage_mount
其中 wait,slotselect,avb=vbmeta_system涉及的system、system_ext、product三个分区需要跟vbmeta_system分区进行校验。
而wait,slotselect,avb没有vbmeta_system参数的，则跟vbmeta分区进行校验，可以参考上面解析的vbmeta.img.info。
解析 vbmeta_system镜像以求证wait,slotselect,avb=vbmeta_system涉及的三个分区:
avbtool info_image --image vbmeta_system.img > vbmeta_system.img.info
vbmeta_system.img.info内容
Minimum libavb version: 1.0 Header Block: 256 bytes Authentication Block: 320 bytes Auxiliary Block: 2368 bytes Public key (sha1): fc92d9cba0628858d846fb9a18a7af72b05d7dc8 Algorithm: SHA256_RSA2048 Rollback Index: 1598918400 Flags: 0 Release String: 'avbtool 1.1.0' Descriptors: ... Hashtree descriptor: Version of dm-verity: 1 Image Size: 141250560 bytes Tree Offset: 141250560 Tree Size: 1122304 bytes Data Block Size: 4096 bytes Hash Block Size: 4096 bytes FEC num roots: 2 FEC offset: 142372864 FEC size: 1130496 bytes Hash Algorithm: sha1 Partition Name: product Salt: b6e1f57ae6939659355e83ad7fa57feb6b5eb15a3d16b96752f43cdc14918708 Root Digest: 8747dcab121c9cf740b106dc4b5545d685ee148f Flags: 0 Hashtree descriptor: Version of dm-verity: 1 Image Size: 2592661504 bytes Tree Offset: 2592661504 Tree Size: 20422656 bytes Data Block Size: 4096 bytes Hash Block Size: 4096 bytes FEC num roots: 2 FEC offset: 2613084160 FEC size: 20660224 bytes Hash Algorithm: sha1 Partition Name: system Salt: b6e1f57ae6939659355e83ad7fa57feb6b5eb15a3d16b96752f43cdc14918708 Root Digest: 489a406f49dd15c6c4ab3f86ae8c490837602f81 Flags: 0 Hashtree descriptor: Version of dm-verity: 1 Image Size: 418029568 bytes Tree Offset: 418029568 Tree Size: 3301376 bytes Data Block Size: 4096 bytes Hash Block Size: 4096 bytes FEC num roots: 2 FEC offset: 421330944 FEC size: 3334144 bytes Hash Algorithm: sha1 Partition Name: system_ext Salt: b6e1f57ae6939659355e83ad7fa57feb6b5eb15a3d16b96752f43cdc14918708 Root Digest: 3eabb06d98d7e4639fbca3fd21806ef36332f1d3 Flags: 0
可以看到vbmeta_system.img.info中包含product system system_ext三个分区校验信息。
总结
AVB校验 分成两个阶段
- bootloader 阶段 AVB 校验以下镜像
vbmeta.img、boot.img 、dtbo.img、vendor_boot.img
- init 阶段 AVB 校验以下镜像
vbmeta_system.img vbmeta.img vendor.img product.img odm.img system_ext.img
这里具体的校验方法前面也说了，以及哪些分区在那个步骤被校验也是可以自定义的，但是google这个框架推荐的区别，呗大多数使用。
最后感谢这前辈分享的blog
后面有时间再把这个两个阶段的代码层梳理一下。
