From f1591a1579c44ea8127678e5cd0f89b22ecdc495 Mon Sep 17 00:00:00 2001 From: Paul Gortmaker Date: Wed, 21 Jun 2023 10:13:35 -0700 Subject: dm-verity: add sample systemd separate hash example and doc Create a wks.in that allows an out-of-the-box build of a bootable USB image using systemd and the hash data as a separate device or partition. A focus here was to ensure we used proper GPT names and GPT types, and the GPT UUIDs that are based on splitting the root hash. Signed-off-by: Paul Gortmaker Signed-off-by: Armin Kuster --- docs/dm-verity-systemd-hash-x86-64.txt | 43 +++++++++++++++++++++++++++++++ wic/systemd-bootdisk-dmverity-hash.wks.in | 18 +++++++++++++ 2 files changed, 61 insertions(+) create mode 100644 docs/dm-verity-systemd-hash-x86-64.txt create mode 100644 wic/systemd-bootdisk-dmverity-hash.wks.in diff --git a/docs/dm-verity-systemd-hash-x86-64.txt b/docs/dm-verity-systemd-hash-x86-64.txt new file mode 100644 index 0000000..673b810 --- /dev/null +++ b/docs/dm-verity-systemd-hash-x86-64.txt @@ -0,0 +1,43 @@ +dm-verity and x86-64 and systemd - separate hash device +------------------------------------------------------- + +Everything said in "dm-verity-systemd-x86-64.txt" applies here. +However booting under QEMU is not tested - only on real hardware. +So for your MACHINE you need to choose "genericx86-64". + +Also, you'll need to point at the hash specific WKS file: + +WKS_FILES += " systemd-bootdisk-dmverity-hash.wks.in" + +The fundamental difference is to use a separate device/partition for +storage of the hash data -- instead of "hiding" it beyond the filesystem +in what is essentially a 5-10% oversized partition. This takes any manual +math calculations of size/offset out of the picture, and uses the kernel's +natural behaviour of compartmentalizing devices to ensure they are separate. + +The example hash.wks file added here essentially adds a hash-only partition +directly after the filesystem partition. So the filesystem partition is +no longer "oversized" and no offsets are needed/used. + +Since we are now using multiple partitions, we make a better effort to use +accepted GPT partition types and UUIDs based on the roothash. This means +easier sysadmin level use/debugging based on cfdisk output etc. + +Generating the separate root hash image is driven off enabling this: + DM_VERITY_SEPARATE_HASH = "1" + +Two other variables control the GPT UUIDs - set to x86-64 defaults: + + DM_VERITY_ROOT_GUID ?= "4f68bce3-e8cd-4db1-96e7-fbcaf984b709" + DM_VERITY_RHASH_GUID ?= "2c7357ed-ebd2-46d9-aec1-23d437ec2bf5" + +See: https://uapi-group.org/specifications/specs/discoverable_partitions_specification/ + +Finally, the UUIDs (not the "partition types" above) are based off of +the root node hash value as per the systemd "autodetect" proposed standard. +These will obviously change with every update/rebuild of the root image. + +While not strictly coupled to any functionality at this point in time, it +does aid in easier debugging, and puts us in alignment with using systemd +inside the initramfs to replace manual veritysetup like configuration we +currently do in the initramfs today, should we decide to do so later on. diff --git a/wic/systemd-bootdisk-dmverity-hash.wks.in b/wic/systemd-bootdisk-dmverity-hash.wks.in new file mode 100644 index 0000000..e400593 --- /dev/null +++ b/wic/systemd-bootdisk-dmverity-hash.wks.in @@ -0,0 +1,18 @@ +# short-description: Create an EFI disk image with systemd-boot and separate hash dm-verity +# A dm-verity variant of the regular wks for IA machines. We need to fetch +# the partition images from the IMGDEPLOYDIR as the rootfs source plugin will +# not recreate the exact block device corresponding with the hash tree. We must +# not alter the label or any other setting on the image. +# Based on OE-core's systemd-bootdisk.wks and meta-security's beaglebone-yocto-verity.wks.in file +# +# This .wks only works with the dm-verity-img class and separate hash data. (DM_VERITY_SEPARATE_HASH) + +part /boot --source bootimg-efi --sourceparams="loader=systemd-boot,initrd=microcode.cpio" --ondisk sda --label msdos --active --align 1024 --use-uuid + +# include the root+hash part with the dynamic hash/UUIDs from the build. +include ${STAGING_VERITY_DIR}/${IMAGE_BASENAME}.${DM_VERITY_IMAGE_TYPE}.wks.in + +# add "console=ttyS0,115200" or whatever you need to the --append="..." +bootloader --ptable gpt --timeout=5 --append="root=/dev/mapper/rootfs" + +part swap --ondisk sda --size 44 --label swap1 --fstype=swap --use-uuid -- cgit v1.2.3-54-g00ecf