Bug#1057873: systemd-boot: allow user postinstall script to be able to sign the bootloader

[Matteo Settenvini]

Package: systemd-boot
Version: 255-1
Severity: important

Dear Maintainer,

as per https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=1033725 and
https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=996202, there seems to be no
willingness to sign esp/EFI/systemd/systemd-bootx64.efi and
esp/EFI/BOOT/BOOTX64.EFI with the Debian CA.

Sidenote: (Maybe this decision should be revisited? We are a couple of years
later and systemd-boot is the only proper Linux bootloader able to do
measured boot).

Instead, the solution pointed out is that the user should have their own
keys. I do just that, and I use sbctl accordingly for both UKI images and
systemd-boot. This works well, also with sbsign instead of
sbctl (the latter being unavailable as a package in Debian).

Unfortunately, one has to manually remember to sign the bootloader
in the EFI partition after each re-install of the systemd-boot package.

Would it be possible to provide a configuration / script file so that
one can sign the bootloader before installing it?

I can obviously create a dpkg-diversion and wrap bootctl myself by
invoking sbsign manually, but I think it would be better if this was a
more generic solution.

Best of all, systemd-boot.efi.signed would be provided by a
systemd-boot-signed package by Debian itself. :-)

From the bootctl man page:

SIGNED .EFI FILES
bootctl install and update will look for a systemd-boot file ending
with the ".efi.signed" suffix first, and copy that instead of the
normal ".efi" file. This allows distributions or end-users to provide
signed images for UEFI SecureBoot.

Thanks,
Matteo Settenvini

-- System Information:
Debian Release: trixie/sid
APT prefers unstable-debug
APT policy: (500, 'unstable-debug'), (500, 'unstable'), (1, 'experimental-debug'), (1, 'experimental')
Architecture: amd64 (x86_64)
Foreign Architectures: i386

Kernel: Linux 6.5.0-5-amd64 (SMP w/24 CPU threads; PREEMPT)
Locale: LANG=it_IT.utf8, LC_CTYPE=it_IT.utf8 (charmap=UTF-8), LANGUAGE not set
Shell: /bin/sh linked to /usr/bin/dash
Init: systemd (via /run/systemd/system)
LSM: AppArmor: enabled

Versions of packages systemd-boot depends on:
ii libc6 2.37-13
ii libsystemd-shared 255-1
ii systemd-boot-efi 255-1

Versions of packages systemd-boot recommends:
ii efibootmgr 18-1

systemd-boot suggests no packages.

-- no debconf information

[Matteo Settenvini]

For completeness, this is my current diversion for /usr/bin/bootctl:

------------------------------------------------

#!/bin/sh

set -e

# dpkg-divert --divert /usr/bin/bootctl --rename bootctl.real
# mv /usr/bin/bootctl{,.real}

keydir="/usr/share/secureboot/keys/db/"
bl="/usr/lib/systemd/boot/efi/systemd-bootx64.efi"

# if [ "${bl}" -nt "${bl}.signed" ]; then
if [ ! -f "${bl}.signed" -o "${bl}" -nt "${bl}.signed" ]; then
echo "Signing ${bl} with sbsign"
sbsign --key "${keydir}/db.key" --cert "${keydir}/db.pem"
"${bl}"
fi

exec /usr/bin/bootctl.real "$@"

------------------------------------------------

As can be deduced, I have the following structure thanks to sbctl, that
is reusable by sbsign:

# find /usr/share/secureboot/
/usr/share/secureboot/
/usr/share/secureboot/keys
/usr/share/secureboot/keys/PK
/usr/share/secureboot/keys/PK/PK.key
/usr/share/secureboot/keys/PK/PK.pem
/usr/share/secureboot/keys/KEK
/usr/share/secureboot/keys/KEK/KEK.key
/usr/share/secureboot/keys/KEK/KEK.pem
/usr/share/secureboot/keys/db
/usr/share/secureboot/keys/db/db.key
/usr/share/secureboot/keys/db/db.pem
/usr/share/secureboot/keys/dbx
/usr/share/secureboot/keys/dbx/dbx.key
/usr/share/secureboot/keys/dbx/dbx.pem
/usr/share/secureboot/GUID
/usr/share/secureboot/files.db
/usr/share/secureboot/bundles.db



I then have /etc/kernel/install.conf:

------------------------------------------------

layout=uki
uki_generator=ukify
initrd_generator=dracut

------------------------------------------------

And in /etc/kernel/uki.conf:

-----------------------------------------------

[UKI]
SecureBootSigningTool=sbsign
SecureBootPrivateKey=/usr/share/secureboot/keys/db/db.key
SecureBootCertificate=/usr/share/secureboot/keys/db/db.pem

-----------------------------------------------

This is enough for me to achieve full secure boot with a ukified image
via dracut. Yay! Next step for me is getting measured boot to work with
the TPM.

While this above is certainly a workable solution, it requires quite a
bit of effort for the non-initiated.

It would be good if achieving good security in Debian became easier and
easier over time ^_^.

Cheers,
Matteo Settenvini