Ubuntu Developers Figuring Out Dual-Boot Changes Ahead Of Ubuntu 22.04 LTS
The issue at hand is GRUB 2.06 has disabled os-prober by default as the feature for GRUB to detect other installed operating systems. OS-Prober is disabled by default upstream now due to security issues over it going through and mounting all partitions on the system when checking them for other operating systems and that could be taken advantage of if making use of file-system vulnerabilities.
Ubuntu relied on os-prober for finding other installed operating systems to be able to conveniently show them within the GRUB bootloader. This means with Ubuntu 22.04's move to GRUB 2.06, non-UEFI BIOS users can no longer boot any other OS while UEFI-based systems could still boot other operating systems if going through the UEFI bootloader. The main issue is obstructing boots for those still running the classical BIOS boot process.
Thus Ubuntu developers are left to decide whether to re-enable os-prober, which is something they aren't enthusiastic about doing given the valid security concerns. Or they are also considering a GRUB module that would fulfill some of the os-prober roles only when sought by the user, specialized handling just to look for Microsoft Windows on the disk to add a chainload boot option, or running os-prober just at install-time and then relying on that information for subsequent boots. There are also possibilities like enabling os-prober enabled if upgrading the Ubuntu installation from a prior version, etc.
Limiting os-prober to just running at install-time is something other Linux distributions have done and is their most likely path forward for continuing to support dual/multi-boot scenarios with Ubuntu. If you are a multi-boot Linux users, you can learn more or voice your opinions via ubuntu-devel.
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