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Re: How to boot Windows when Bitlocker enabled with key sealed in TPM

From: Chris Murphy
Subject: Re: How to boot Windows when Bitlocker enabled with key sealed in TPM
Date: Fri, 25 Mar 2022 17:08:11 -0600

On Fri, Mar 25, 2022 at 5:00 PM Chris Murphy <> wrote:
> On Fri, Mar 25, 2022 at 2:32 PM Vladimir 'phcoder' Serbinenko
> <> wrote:
> >
> > On Fri, Mar 25, 2022 at 9:14 PM Chris Murphy <> 
> > wrote:
> > >
> > > For all practical purposes, this is functionally the end to dual boot
> > > in GRUB, if there is no work around, e.g. bootnext. Is that the
> > > direction GRUB maintainers want to go in?
> > Why don't you just update TPM with new values? Then it will get
> > unsealed when booted through GRUB
> How?
> The key is sealed in the TPM so first we need to get the key in order
> to (re)seal it with new PCR values. Correct? So we somehow need a way
> to boot only the Windows bootloader in order for measured boot to
> unseal the key, and then we'd need to somehow measure
> shim+grub+windows bootloaders together in order to seal the key with
> the new values for those three bootloaders used in that sequence. I
> have no idea if that's practical at all.
> The recovery key is not the one sealed in the TPM, they are separate
> keys in separate "keyslots".

The next problem is that when there's a Linux system update the
updates either shim or grub, the shim+grub+windows bootloader
measurements have changed and will again fail to unseal the key. It's
indistinguishable from the system having been compromised. And now you
get to do a clean install of Windows and Linux to get back to

Further, should the user need to reinstall Linux, or even boot Windows
directly from the firmware's boot manager - they wouldn't be able to.

This all sounds quite a lot more difficult than GRUB having the
ability to set a bootnext efi variable, and just reboot - let the
Windows bootloader handle all of this, and not involve Linux

Chris Murphy

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