added ovh rescue blog article
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content/blog/debian/ovh-rescue.md
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---
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title: 'Fixing an encrypted Debian system boot'
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description: 'From booting in UEFI mode to legacy BIOS mode'
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date: '2024-09-19'
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tags:
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- Debian
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---
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## Introduction
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Some time ago, I reinstalled one of my OVH vps instances. I used a virtual machine image of a Debian Linux that I initially prepared for a GCP host a few months ago. It was setup to boot with UEFI, and I discovered that OVH does not offer it (at least on its small VPS offering).
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It is a problem because this is a system with an encrypted root partition. In order to boot with an encrypted partition in BIOS mode, grub needs some extra space than it does not when in UEFI mode.
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I could rebuild an image from scratch, or I could hop onto an OVH rescue image and fix it. I took the later approach in order to refresh my rescue skills.
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## Mounting the partitions from the rescue image
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This system has an encrypted block device holding an LVM set of volumes. Since the rescue image does not have the necessary tools, I installed them with:
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``` shell
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apt update -qq
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apt install -y cryptsetup lvm2
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```
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I refreshed my knowledge of the layout with
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``` shell
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blkid
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fdisk -l /dev/sdb
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```
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Opening the encrypted block device is done with:
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``` shell
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cryptsetup luksOpen /dev/sdb3 sda3_crypt
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```
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Note that I am mounting a sdb device because we are in OVH rescue, but it was known as sda during the installation. I need to use the same name otherwise grub will mess up when I regenerate its configuration and the system will not reboot properly.
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The LVM subsystem now needs to be activated with:
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``` shell
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vgchange -ay vg
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```
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Now to mount the partitions and chroot into our system:
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``` shell
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mount /dev/vg/root /mnt
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cd /mnt
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mount -R /dev dev
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mount -R /proc proc
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mount -R /sys sys
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chroot ./
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mount /boot
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```
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## Replacing the EFI partition with a BIOS boot partition
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My system had an EFI partition in /dev/sdb1: this is not suitable for booting a grub2 system to an encrypted volume directly from BIOS. I replaced it with a BIOS boot partition with:
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``` shell
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fdisk /dev/sdb
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Command (m for help): d
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Partition number (1-3, default 3): 1
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Partition 1 has been deleted.
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Command (m for help): n
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Partition number (1,4-128, default 1): 1
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First sector (34-41943006, default 2048):
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Last sector, +/-sectors or +/-size{K,M,G,T,P} (2048-1050623, default 1050623):
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Created a new partition 1 of type 'Linux filesystem' and of size 512 MiB.
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Command (m for help): t
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Partition number (1-3, default 3): 1
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Partition type or alias (type L to list all): 4
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w
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```
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Reinstalling grub was a matter of:
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``` shell
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apt install grub-pc
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update-grub
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grub-install /dev/sdb
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```
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I am not sure whether it was necessary or not but I rebuilt the initramfs in case the set of modules needed by the kernel would be different:
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``` shell
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update-initramfs -u
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```
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## Cleanup
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Close the chroot session with either `C-d` or the `exit` command. Then umount all partitions with:
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``` shell
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cd /
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umount -R -l /mnt
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```
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Deactivate the LVM subsystem with:
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``` shell
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vgchange -an
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```
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Close the luks volume with:
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``` shell
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cryptsetup luksClose sda3_crypt
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```
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Sync all data to disks just in case:
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``` shell
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sync
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```
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Then reboot in normal mode from the OVH management webui.
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## Conclusion
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This was a fun repair operation!
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