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Let's grab another so far unused PCR, and measure all sysext images into
it that we load from the ESP. Note that this is possibly partly redundant,
since sysext images should have dm-verity enabled, and that is hooked up
to IMA. However, measuring this explicitly has the benefit that we can
measure filenames too, easily, and that all without need for IMA or
anything like that.
This means: when booting a unified sd-stub kernel through sd-boot we'll
now have:
1. PCR 11: unified kernel image payload (i.e. kernel, initrd, boot
splash, dtb, osrelease)
2. PCR 12: kernel command line (i.e. the one embedded in the image, plus
optionally an overriden one) + any credential files picked up by
sd-stub
3. PCR 13: sysext images picked up by sd-stub
And each of these three PCRs should carry just the above, and start from
zero, thus be pre-calculatable.
Thus, all components and parameters of the OS boot process (i.e.
everything after the boot loader) is now nicely pre-calculable.
NOTE: this actually replaces previous measuring of the syext images into
PCR 4. I added this back in 845707aae2,
following the train of thought, that sysext images for the initrd should
be measured like the initrd itself they are for, and according to my
thinking that would be a unified kernel which is measured by firmware
into PCR 4 like any other UEFI executables.
However, I think we should depart from that idea. First and foremost
that makes it harder to pre-calculate PCR 4 (since we actually measured
quite incompatible records to the TPM event log), but also I think
there's great value in being able to write policies that bind to the
used sysexts independently of the earlier boot chain (i.e. shim, boot
loader, unified kernel), hence a separate PCR makes more sense.
Strictly speaking, this is a compatibility break, but I think one we can
get away with, simply because the initrd sysext images are currently not
picked up by systemd-sysext yet in the initrd, and because of that we
can be reasonably sure noone uses this yet, and hence relies on the PCR
register used. Hence, let's clean this up before people actually do
start relying on this.
Here we grab a new – on Linux so far unused (by my Googling skills, that
is) – and measure all static components of the PE kernel image into.
This is useful since for the first time we'll have a PCR that contains
only a PCR of the booted kernel, nothing else. That allows putting
together TPM policies that bind to a specific kernel (+ builtin initrd),
without having to have booted that kernel first. PCRs can be
pre-calculated. Yay!
You might wonder, why we measure just the discovered PE sections we are
about to use, instead of the whole PE image. That's because of the next
step I have in mind: PE images should also be able to carry an
additional section that contains a signature for its own expected,
pre-calculated PCR values. This signature data should then be passed
into the booted kernel and can be used there in TPM policies. Benefit:
TPM policies can now be bound to *signatures* of PCRs, instead of the
raw hash values themselves. This makes update management a *lot* easier,
as policies don't need to be updated whenever a kernel is updated, as
long as the signature is available. Now, if the PCR signature is
embedded in the kernel PE image it cannot be of a PCR hash of the kernel
PE image itself, because that would be a chicken-and-egg problem. Hence,
by only measuring the relavent payload sections (and that means
excluding the future section that will contain the PCR hash signature)
we avoid this problem, naturally.
So, typically systemd-boot is referenced as sd-boot, due to te usual
shorter naming in ESP resources. systemd-stub didnt do that so far,
since it never appears as separate files in the ESP. However it's super
annoying that you can find "man sd-boot", but not the very closely
related "man sd-stub". Let's fix that, and also add an "sd-stub" alias
to the "systemd-stub" man page.
Some types of credentials that a user would want to pass
into the initrd do not depend on the specific kernel/initrd
version. For instance, this can include SSH keys, rootfs
encryption keys, dm-integrity keys, and so on. This
introduces a directory where such credentials can be placed
so that any kernel image will load them
