GitHub Security Advisory: GHSA-wj2j-qwcf-cfcc
Package: github.com/lxc/incus-os/incus-osd (GO)
Severity: HIGH (CVSS 7.6)
CWE: CWE-522: Insufficiently Protected Credentials
Vulnerable versions: < 0.0.0[PII_REDACTED]2803-e3b35f230d23
Patched in: 0.0.0[PII_REDACTED]2803-e3b35f230d23

Summary: IncusOS has a LUKS encryption bypass due to insufficient TPM policy

Details:
The default configuration of systemd-cryptenroll as used by IncusOS through mkosi allows for an attacker with physical access to the machine to access the encrypted data without requiring any interaction by the system's owner or any tampering of Secure Boot state or kernel (UKI) boot image.

That's because in this configuration, the LUKS key is made available by the TPM so long as the system has the expected PCR7 value and the PCR11 policy matches. That default PCR11 policy importantly allows for the TPM to release the key to the booted system rather than just from the initrd part of the signed kernel image (UKI).

The attack relies on the attacker being able to substitute the original encrypted root partition for one that they control. By doing so, the system will prompt for a recovery key on boot, which the attacker has defined and can provide, before booting the system using the attacker's root partition rather than the system's original one.

The attacker only needs to put a systemd unit starting on system boot within their root partition to have the system run that logic on boot. That unit will then run in an environment where the TPM will allow for the retrieval of the encryption key of the real root disk, allowing the attacker to steal the LUKS volume key (immutable master key) and then use it against the real root disk, altering it or getting data out before putting the disk back the way it was and returning the system without a trace of this attack having happened.

This is all possible because the system will have still booted with Secure Boot enabled, will have measured and ran the expected bootloader and kernel image (UKI). The initrd selects the root disk based on GPT partition identifiers making it possible to easily substitute the real root disk for an attacker controlled one. This doesn't lead to any change in the TPM state and therefore allows for retrieval of the LUKS key by the attacker through a boot time systemd unit on their alternative root partition.

Reproducing steps are effectively:
 - Shutdown the system
 - Alter the GPT partition table to remove the GPT type UUID from the root partition
 - Resize the ESP partition to make space for the attacker's own root partition
 - Create a new LUKS encrypted ext4 partition in the space that was freed up and set the GPT type UUID to that of the original root partition
 - Populate that new root partition with a systemd unit and script which use `systemd-cryptenroll` to unlock and extract the key from the original root partition
 - Boot the system
 - When prompted, enter the passphrase of the new (attacker) root partition
 - Let the system boot IncusOS
 - Stop the system
 - Recover the encryption key that was extracted by the boot time systemd unit
 - Access the original root partition using it, steal or modify the data
 - Remove the new (attacker) root partition
 - Grow back the ESP
 - Restore the GPT type UUID on the root partition
 - Start the system back up, it will boot as expec