[systemd-devel] Measured systemd-sysext

[Dimitris Karakasilis]

Hi,

we (at kairos.io) are trying to understand how systemd-sysext extensions 
can also be made tamper-proof by being measured in a system that boots 
in UKI mode.

Specifically, when Kairos boots in "UKI mode", the whole operating 
system is signed and measurements in PCRs 7 and 11 are gating the 
decryption of the disk partition.
This renders the OS unusable in case of various changes (e.g. changes in 
UEFI databases).

systemd-sysext extensions though, are not part of this process so they 
are a possible attack target.
Reading through [the systemd-stub 
docs](https://www.freedesktop.org/software/systemd/man/latest/systemd-stub.html#) 
it seems that systemd-sysext raw images can be measured automatically in 
PCR 13 but it's not clear to us how this measurement can be used to 
prevent them from being copied in the extensions directory and made 
available.

- What prevents a malicious user from directly copying those raw files 
into the extensions dir manually? (sudo permissions only)?
- If we somehow check for the measurements on TPM13 (e.g. by binding 
disk decryption to PCR 13 as well),
how can new extensions be added? If you add or remove a new one, 
measurements won't match.
- What about extensions upgrades? Don't they change the measurement too?

- The docs read:

```
On access they should be further validated: in case of the credentials 
case by encrypting/authenticating them via TPM, as exposed by 
systemd-creds encrypt -T (see systemd-creds(1) for details); in case of 
the system extension images by using signed Verity images.
```
what does the `using signed Verity images` part mean?:

- these: 
https://uapi-group.org/specifications/specs/discoverable_partitions_specification/#verity 
?
- these: https://docs.kernel.org/admin-guide/device-mapper/verity.html ?


Looking forward for some pointers to the right direction, thanks!
Dimitris Karakasilis
(on behalf of the Kairos team)
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <https://lists.freedesktop.org/archives/systemd-devel/attachments/20240524/328f4f0a/attachment.htm>