Adam Williamson <adamw...@fedoraproject.org> writes:

> For background here, see:
> https://bugzilla.redhat.com/show_bug.cgi?id=2049849
>
> right now, when installing Fedora alongside a Windows install with
> BitLocker enabled, trying to boot Windows from the Fedora boot menu
> does not work.
>
> We waived the bug as a blocker for Fedora 36 on the basis upstream did
> not consider it fixable within the F36 timeframe. We agreed that if
> upstream still couldn't get this fixed for F37, we'd consider revising
> the criteria.
>
> Well, we're approaching F37 Final and the bug is still open, and
> there's no appreciable movement upstream, so I'm proposing the criteria
> change. I propose we change this:
>
> "The installer must be able to install into free space alongside an
> existing clean Windows installation and install a bootloader which can
> boot into both Windows and Fedora."
>
> to say:
>
> "The installer must be able to install into free space alongside an
> existing clean Windows installation. As long as the Windows
> installation does not have BitLocker enabled, the installer must also
> install a bootloader which can boot into both Windows and Fedora."

(Fedora grub2 maintainer hat on)

I'm fine with the proposed change.  I'm also fine with the original
text.

During boot, certain actions are taken that are recorded in the TPM.
These include, for instance, any loaders that are run - like grub2.  The
result is that if you load Windows from grub2 rather than the EFI
firmware, the TPM state will be different.  Bitlocker cares about this
TPM state.

So: if you install Windows and set up Bitlocker booting through grub, it
will continue to work through grub.  If you install Windows outside grub
(or it's pre-provisioned), it will continue to work outside grub.  If
you want to move from not using grub to using grub, then Bitlocker needs
to be reestablished with the new TPM values.

It is the opinion of the grub2 maintainers that this constitutes being
able to boot both Windows and Fedora today.  However, we also understand
that not everyone agrees with this, as evidenced by the existence of the
bug and this thread about changing RC.

The only way to get the TPM state to match not using a particular loader
is to not use a loader - i.e., have grub2 (or efibootmgr in Fedora
userspace) set EFI BootNext and reboot the machine.  But generally, if
users want to be booting Windows through grub, we recommend they
configure Bitlocker against those PCR values instead.

Be well,
--Robbie

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