cvmutils 0.1.5

Tools for confidential virtual machines

Prepare OS image to run with encrypted root volume

Purpose

This tool prepares RHEL/Fedora image to run with encrypted root volume. The key to the volume is sealed to the target TPM.

Image pre-requisites

• GUID Partition Table (GPT)
• Image should contain ESP partition, GUID C12A7328-F81F-11D2-BA4B-00A0C93EC93B
• Image should contain 'Linux root (x86-64)' partition, GUID 4F68BCE3-E8CD-4DB1-96E7-FBCAF984B709
• Root partition should have ext4 filesystem
• The expected post boot PCR7 (sha256) value must be known, it must passed to the 'deploy' stage as '--pcr7' option or via an override file on the ESP ('--pcrs-json' option).
• Alternatively, PCR7 value can be predicted ('--pcr7 auto' option), this requires providing either UEFI profile ('--uefi-profile') or Azure Disk Profile ('--az-disk-profile'), examples are present in 'test-data'.
• PCR4 can also be used in the set of PCRs for root volume key sealing. Its expected value can be specified directly ('--pcr4 sha256value') or can be predicted by passing '--pcr4 auto' option.
• ESP may contain a JSON file containing the expected PCR values in JSON format:

{
    "bank": "sha256",
    "pcrs": [4, 7],
    "4": "0x7A94FFE8A7729A566D3D3C577FCB4B6B1E671F31540375F80EAE6382AB785E35",
    "7": "0xB5710BF57D25623E4019027DA116821FA99F5C81E9E38B87671CC574F9281439"
}

The name of the file can be passed as '--pcrs-json' option to 'deploy' stage, in that case values from the file override '--pcr4'/'--pcr7' values from the command line. The file can specify additional PCRs.

• ESP may contain "efivars.json" file in Azure format:

{
    "type": "Microsoft.Compute/disks",
    "properties": {
        "uefiSettings": {
            "Boot0004": {
                "guid": "Yd/ki8qT0hGqDQDgmAMrjA==",
                "attributes": "Bw==",
                "value": "AQAAAGIAUwBoAGkAbQAgAGIAbwBvAHQAIAB0AG8AIAA1AC4AMQA0AC4AMAAtADIAMwA4AF8AdQBrAGkAXwB0AGUAcwB0ADEAOQAuAGUAbAA5AC4AeAA4ADYAXwA2ADQAAAAEASoAAgAAAAAoAAAAAAAAAOAHAAAAAABibF3EAi9J4o1TPhDbQRiuAgIEBDQAXABFAEYASQBcAHIAZQBkAGgAYQB0AFwAcwBoAGkAbQB4ADYANAAuAGUAZgBpAAAAf/8EAFwARQBGAEkAXABMAGkAbgB1AHgAXAB2AG0AbABpAG4AdQB6AC0ANQAuADEANAAuADAALQAyADMAOABfAHUAawBpAF8AdABlAHMAdAAxADkALgBlAGwAOQAuAHgAOAA2AF8ANgA0AC0AdgBpAHIAdAAuAGUAZgBpAAAA"
            }
        }
    }
}

in case it does, its size and content will be written to a special 'Linux reserved' GUID 8DA63339-0007-60C0-C436-083AC8230908) partition starting at offset 2048 * 512 = 1048576.

TPM2

SRK public key is required for 'deploy' phase. It can be obtained with

$ systemd-analyze srk > public.srk  

or

$ tpm2_readpublic -c 0x81000001 -o public.srk -t primary.handle

Basic usage

$ ./encrypt-rhel-image.py encrypt /path/to/image.vhd  
$ ./encrypt-rhel-image.py deploy -s /path/to/public.srk --pcr7 auto /path/to/image.vhd  

Running in a container

The tool can run in a container and 'Containerfile.fedora'/'Containerfile.c9s' are provided as examples to build it. Optionally, 'quay.io/vkuznets/encrypt-rhel-image' (c9s based) can be used directly. Here is an example for Ubuntu 20.04. Assuming all data is stored in '/data' on the host,

# apt update && apt install docker.io  
# docker pull quay.io/vkuznets/encrypt-rhel-image:latest  
# modprobe nbd  
# docker run -it --privileged --mount type=bind,source=/data,target=/data --mount type=bind,source=/run/udev,target=/run/udev --mount type=bind,source=/dev,target=/dev quay.io/vkuznets/encrypt-rhel-image:latest /usr/bin/encrypt-rhel-image.py encrypt -v /data/image.qcow2  
# docker run -it --privileged --mount type=bind,source=/data,target=/data --mount type=bind,source=/run/udev,target=/run/udev --mount type=bind,source=/dev,target=/dev quay.io/vkuznets/encrypt-rhel-image:latest /usr/bin/encrypt-rhel-image.py deploy -s /data/public.srk -r /data/recovery_key --pcr7 auto --uefi-profile /data/uefi-profile-ovmf.json -v /data/image.qcow2  

Red Hat UBI can be used for the 'deploy' phase. As qemu-nbd is currently missing in the UBI, the image must be passed to the container as a device. RHEL UBI container can also be used sealing:

# podman build -f Containerfile.ubi9 -t encrypt-rhel-image-ubi9  
# qemu-nbd -c /dev/nbd2 /var/lib/libvirt/images/rhel9-azure-cvm-20240820-2.qcow2  
# podman run -it --privileged --mount type=bind,source=/var/lib/libvirt/images,target=/mnt --mount type=bind,source=/root,target=/root --mount type=bind,source=/run/udev,target=/run/udev --mount type=bind,source=/dev,target=/dev localhost/encrypt-rhel-image-ubi9:latest /usr/bin/encrypt-rhel-image.py deploy -s /root/public.srk -r /root/recovery_key --pcr7 auto --uefi-profile /root/encrypt-rhel-image/test-data/uefi-profiles/uefi-profile-ovmf.json -v /dev/nbd2  
# qemu-nbd --disconnect /dev/nbd2  

Testing

The tool comes with unit tests which can be executed with 'pytest'. Note that 'test-data' submodule must be checked out.

$ git submodule update
$ pytest

Dependencies

Basic:

• python3.x

'Encrypt' phase:

• qemu-nbd
• e2fsprogs
• cryptsetup
• growpart (optional)

'Deploy' phase additionally requires:

• openssl
• systemd-cryptenroll >= 255

TODO:

• Add support for UKI extensions ($ESP/EFI/Linux/<uki-name.efi>.extra.d/) to PCR4/7 predictors.