Not just any UEFI, UEFI with SecureBoot enabled specifically. On SecureBoot-enabled systems, you're booting a Microsoft-signed executable that chainloads Grub, which then loads Linux. This has given Microsoft a power they should not have: the power to refuse to sign future versions of that first executable.
I think the whole "MS-signed Grub" is a way by Ubuntu (and possibly others) to facilitate Linux installation by "non pros". And it's not that bad, I think.
I, personally, use Arch Linux and the installer isn't signed with anything. They have a "do your own signing" policy. I had to disable SecureBoot to install it, because I couldn't be bothered to create a new ISO, but then I registered my own keys to the UEFI and boot directly my own signed kernel [0], without using GRUB at all.
I did all this on multiple generation HP "enterprise" laptops and desktops and it worked without any issue. And those systems don't come with any sort of Linux support from HP.
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[0] For the curious, this is EFISTUB combined with sbsign, that bundles the kernel image, initrd and CPU microcode in a single EFI binary and signs it. This is then booted directly by the UEFI, and you can even register multiple binaries and have them show up in the UEFI boot selection menu.
Pointing out the power that MS have is not FUD. If I suggested that there is a chance they would abuse it, that might be, but I didn't do that.
> or limited to specific systems with broken UEFI implementations.
A very quick Google search suggests there are quite a few of those, from mainstream vendors such as Dell and Lenovo.
> I, personally, use Arch Linux and the installer isn't signed with anything.
True, I oversimplified. The major distros use MS-signed shims, the rest mostly don't.
Also Ventoy needs to install user-defined keys into UEFI to make it work with Secure Boot.
So my assumption is that - at least on Thinkpads - you can add your own user-defined keys; the only Caudine Fork where a Microsoft blessed loader might be involved is the one used by the distro installer.
Or am I missing something
There is no reason to believe that this wasn't implemented thus far because of the high ethical standards Microsoft folks have historically adhered to and while we are to believe Microsoft has turned over a new leaf much of the current leadership was involved at the executive level in old bad Microsoft.