fix(libvirt): reject shell injection via guest init-script interface fields

[doublewhy]

Related issues

Closes #1116

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What breaks

A hostile value in a scenario definition could run arbitrary commands as root inside the guest VM. The guest's startup script was assembled as shell text, and three fields from the scenario — a network interface's MAC address, IP address, and CIDR prefix — were pasted into it without quoting or validation.

Concretely

Give a node an interface whose mac is:

aa:bb:cc:dd:ee:ff) ; rm -rf /outside #

The generated script closes the shell case arm early and runs whatever follows. ip and cidr_prefix were injectable the same way with $(...) or backticks. The generated line was:

    aa:bb:cc:dd:ee:ff) ; rm -rf /outside #)
      ip addr add 192.0.2.10/24 dev "$iface"

The same flaw was present in two files: techvault_appliance.py and guest_appliance.py.

Why it happens

Both _init_script functions built the interface arms with f-strings:

f"    {interface.get('mac')})",
f'      ip addr add {interface.get("ip")}/{interface.get("cidr_prefix")} dev "$iface"',

Only the hostname went through the module's existing _shell_quote. This is also inconsistent with the rest of the backend, which uses argv lists rather than shell text (cloudinit.py, drivers/oci.py).

The fix

Both files now share one helper that validates each field to its declared shape and refuses malformed input, then quotes it as a second layer:

• mac must match a MAC address pattern
• ip must parse via ipaddress.ip_address
• cidr_prefix must be an integer within range for that address family

A value that is not the shape it claims to be is a bug in the plan, not text to escape, so it raises ValueError — matching how techvault_matrix.py, which produces these values, already reports problems.

Two related items:

• _cpio_newc piped newline-joined file paths into cpio with check=False, so a filename containing a newline silently corrupted the initramfs archive. Such paths are now refused.
• str.isdigit accepts characters like ² that int then rejects, which leaked a raw interpreter message instead of the module's own. isdecimal matches what int actually accepts.

How I know

7 tests fail against the current techvault_appliance and pass after; 6 of the guest_appliance cases likewise.

guest_appliance.py had no coverage on this path on any machine without a static BusyBox and cpio installed, because its only test module needs them. Its script generator is now exercised directly, so the second injection point stays covered wherever the suite runs.

I also confirmed the quoting holds in a real shell. An IPv6 zone-ID payload that ipaddress happens to accept is printed literally rather than executed:

$ printf '%s\n' 'fe80::1%$(id)/64'
fe80::1%$(id)/64

Full nox -s verify green on Ubuntu 22.04 / Python 3.12, all six lanes, 91% total coverage.

🤖 Generated with Claude Code

[Brad-Edwards]

Please note in the comments or body which issue this PR closes. If no issue exists, please create one and link it.

Thank you!

[doublewhy]

Linked the focused guest-interface validation issue in the PR body: this PR closes #1116. Thank you.

[doublewhy]

Superseded by #1128, which is rebuilt on the current dev branch and closes #1094, #1095, #1105, and #1116. The successor includes the init-script validation fix without the stray .DS_Store, and adds the related reproducible-artifact and fresh-evidence corrections with current verification evidence.