Bug#988876: grub-efi-amd64: outb command no longer works

[Colin Watson]

On Thu, May 20, 2021 at 06:13:16PM +0200, Marco Kühnel wrote:
> After a fresh install of debian buster a grub.cfg containing an outb command throws the error "outb command not found" (and, in my case, 
> cannot start lightdm thereafter). This problem does not appear when downgrading all installed grub packages to version 
> 2.02+dfsg1-20+deb10u3. 

Can you clarify whether you have UEFI Secure Boot enabled?  The outb
command had to be disabled in Secure Boot mode, because it could be used
to subvert boot security mechanisms.  Systems that use the outb command
will have to disable Secure Boot, or find some other way to achieve the
same effect.

>         outb 0x728 1
>         outb 0x710 2
>         outb 0x740 2
>         outb 0x750 0

I'm just noting explicitly here that these are local customizations to
grub.cfg (presumably by way of editing /etc/grub.d/10_linux or similar),
and not in the default setup.

-- 
Colin Watson (he/him)                              [cjwatson at debian.org]