added ovh rescue blog article

content/blog/debian/ovh-rescue.md

@@ -0,0 +1,116 @@
+---
+title: 'Fixing an encrypted Debian system boot'
+description: 'From booting in UEFI mode to legacy BIOS mode'
+date: '2024-09-19'
+tags:
+- Debian
+---
+
+## Introduction
+
+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).
+
+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.
+
+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.
+
+## Mounting the partitions from the rescue image
+
+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:
+``` shell
+apt update -qq
+apt install -y cryptsetup lvm2
+```
+
+I refreshed my knowledge of the layout with
+``` shell
+blkid
+fdisk -l /dev/sdb
+```
+
+Opening the encrypted block device is done with:
+``` shell
+cryptsetup luksOpen /dev/sdb3 sda3_crypt
+```
+
+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.
+
+The LVM subsystem now needs to be activated with:
+``` shell
+vgchange -ay vg
+```
+
+Now to mount the partitions and chroot into our system:
+
+``` shell
+mount /dev/vg/root /mnt
+cd /mnt
+mount -R /dev dev
+mount -R /proc proc
+mount -R /sys sys
+chroot ./
+mount /boot
+```
+
+## Replacing the EFI partition with a BIOS boot partition
+
+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:
+``` shell
+fdisk /dev/sdb
+Command (m for help): d
+Partition number (1-3, default 3): 1
+Partition 1 has been deleted.
+
+Command (m for help): n
+Partition number (1,4-128, default 1): 1
+First sector (34-41943006, default 2048):
+Last sector, +/-sectors or +/-size{K,M,G,T,P} (2048-1050623, default 1050623):
+
+Created a new partition 1 of type 'Linux filesystem' and of size 512 MiB.
+
+Command (m for help): t
+Partition number (1-3, default 3): 1
+Partition type or alias (type L to list all): 4
+w
+```
+
+Reinstalling grub was a matter of:
+``` shell
+apt install grub-pc
+update-grub
+grub-install /dev/sdb
+```
+
+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:
+``` shell
+update-initramfs -u
+```
+
+## Cleanup
+
+Close the chroot session with either `C-d` or the `exit` command. Then umount all partitions with:
+``` shell
+cd /
+umount -R -l /mnt
+```
+
+Deactivate the LVM subsystem with:
+``` shell
+vgchange -an
+```
+
+Close the luks volume with:
+``` shell
+cryptsetup luksClose sda3_crypt
+```
+
+Sync all data to disks just in case:
+``` shell
+sync
+```
+
+Then reboot in normal mode from the OVH management webui.
+
+## Conclusion
+
+This was a fun repair operation!