Fedora CoreOS UKI PoC

How to reproduce

Start a secureboot VM

See under virtual_machine to generate a volume containing the secureboot keys. This can then be attached to the virtual machine as NVRAM.

The integrity demo requires the root disk to be formatted in ext.4. See uki.bu.

Once the coreOS VM is booted, follow these steps. NOTE: Everything should be ran as root.

1 Download and unpack our signed build of coreOS

Enable FS verity on the root filesystem and the ostree repo:

tune2fs -O verity /dev/disk/by-partlabel/root
ostree config set ex-fsverity.required true

Download and unencapsulate the conter, then deploy the ostree commit. See demo/1-ostree-commit.

2 Setup systemd-boot

Install systemd-boot with bootctl install. A package is missing in FCOS unfortunately. See demo/2-install-sd-boot.

3 Setup the UKI

You should be able to reboot into the new deployment and everything is sealed.

4 Put it to the test !!

In demo/ you will find two scripts named 4-demo-find-backing-file and 5-demo-tamper-file-block to show how fsverity will protect your system against tampering.

Firstly, find the backing file of your target :

# 4-demo-find-backing-file /usr/lib/os-release
The repo file for /usr/lib/os-release is:
/ostree/repo/objects/75/591953b65dc61722f77f54e416fbeb7e342ad6bd53b80a07862475b662852d.file

This script is just a helper to locate the backing file of a checked out file in the ostree repo.

Then, tamper the backing file (make sure to be root):

# 5-demo-tamper-file-block /ostree/repo/objects/75/591953b65dc61722f77f54e416fbeb7e342ad6bd53b80a07862475b662852d.file
/ostree/repo/objects/75/591953b65dc61722f77f54e416fbeb7e342ad6bd53b80a07862475b662852d.file is at offset on device /dev/disk/by-partlabel/root
Original initial block:
NAME="Fedora Linux"
VERSION="40.

Modifying initial block

Modified initial block:
NAME="FOOBAR Linux"
VERSION="40.

This script does a little bit more magic : it flushes the caches then locate the position of the file at the block level. This position is then overwrote with a few bytes to tamper the file content.

If you try to read the deployed file again you will get an error:

 cat /usr/lib/os-release 
cat: /usr/lib/os-release: Input/output error

And in the journal :

[  156.086877] fs-verity (vda3, inode 426106): FILE CORRUPTED! pos=0, level=-1, want_hash=sha256:672df088670aaa61c35d14aa148168dc0e312ae9294f903ef85c8c0b5a17037a, real_hash=sha256:9bf652778a15df188e9318e6105f7a6e9154abb22a40dee4220fca8422b3d813