Split dm-crypt for Qubes R4.0

Isolates device-mapper based secondary storage encryption (i.e. not the root filesystem) and LUKS1 header processing to DisposableVMs.

Instead of directly attaching an encrypted LUKS1 partition from a source VM such as sys-usb to a destination VM and decrypting it there, it works like this:

The encrypted partition is attached from the source VM to an offline device DisposableVM configured not to parse its content in any way: The kernel partition scanners, udev probes, and UDisks handling are disabled.
From there, the LUKS1 header is sent to a (short-lived) offline header DisposableVM prompting for the password, and the encryption key is sent back to the device DisposableVM, which validates that it received an AES-XTS key and creates the dm-crypt mapping.
Finally, the decrypted partition is attached from the device DisposableVM to the destination VM.

If the destination VM is compromised, it does not know the password or encryption key. It also cannot easily exfiltrate decrypted data to the disk in a form that would allow an attacker who seizes the disk contents later to read it. (But see below for caveats.)

Usage

qvm-block-split attach|at|a [--ro] [<k>] [<dst-vm>] <src-vm>:<device>
                detach|dt|d                         <src-vm>:<device>

                overwrite-everything-with-random    <src-vm>:<device>
                overwrite-header-with-random        <src-vm>:<device>
                overwrite-header-with-format [<k>]  <src-vm>:<device>
                overwrite-header-with-shell  [<k>]  <src-vm>:<device>
                modify-header-with-shell     [<k>]  <src-vm>:<device>

The <dst-vm> argument defaults to yet another DisposableVM.
<k> stands for an optional --key-file=[<key-vm>:]<file> argument.

As seen above, the qvm-block-split attach/detach commands accept a subset of the familiar qvm-block syntax, and some other commands are included:

Fully overwrite a device with random data
Overwrite just the LUKS1 header with random data
Format a new LUKS1 device with modern crypto parameters: AES-XTS with 256+256 (instead of 128+128) bit keys, SHA512 (instead of SHA1) PBKDF2 key derivation with 5 (instead of 0.1) seconds iteration time

Remaining attacks

After detaching, the password and/or key will linger in more RAM locations than without Split dm-crypt. Until there is a way to wipe the DisposableVMs' memory, and qvm-block-split is modified not to pass the key through dom0's memory, power off your computer when memory forensics is a concern.
If both the destination VM and the source VM/disk are compromised, they could establish a covert channel using e.g. read and write access patterns, slowly saving some amount of decrypted data to the disk.
If the source VM/disk is compromised and successfully exploits the header DisposableVM using a malicious LUKS1 header, a known AES-XTS key could be sent to the device DisposableVM and used to present malicious device content to the destination VM to potentially exploit it as well. Be suspicious if you do not see the expected filesystem data in the destination VM. Or simply use a DisposableVM as the destination VM.
Don't forget to overwrite your disk with random data before creating a LUKS1 volume on it. Otherwise, a compromised destination VM could trivially save decrypted data to the disk in its free space, by encoding each bit as an unmodified (still empty or in some other way nonrandom-looking) or modified (random-looking) 128 bit AES block.

Installation

Copy vm/ to a DisposableVM Template's TemplateVM (e.g. fedora-XX) - not to the DisposableVM Template itself (e.g. fedora-XX-dvm).

Inspect the code, and sudo make install; also install the pv (Pipe Viewer) package to be able to run the overwrite-everything-with-random command. Shut down the TemplateVM when finished.
Copy dom0/bin/qvm-block-split to dom0, e.g. into ~/bin/, inspect the code extra carefully, and chmod +x the script.
Either make your DisposableVM Template from step 1 the system-wide default:
```
 qubes-prefs default_dispvm fedora-XX-dvm
```
Or just let Split dm-crypt know what it is:
```
 echo TEMPLATE_FOR_DISPVMS=fedora-XX-dvm >/etc/split-dm-crypt.conf
```

Safety warning

The code's error handling is strict, and I haven't experienced any data loss during development. Nevertheless, this is an early release. Please ensure you have a backup of all drives that are connected to your computer.

Redistribution