menhera.sh/menhera.sh

#!/bin/bash
set -Eeuo pipefail

# config
WORKDIR="/tmp/menhera"
ROOTFS=""

# internal global variables
OLDROOT="/"
NEWROOT=""

# environment compatibility
__compat_restart_ssh() {
    if [ -x "$(command -v systemctl)" ]; then
        systemctl restart ssh
    elif [ -x "$(command -v service)" ]; then
        service ssh restart
    else
        echo "ERROR: Cannot restart SSH server" >&2
        exit 1
    fi
}

__compat_reload_init() {
    if [ -x "$(command -v systemctl)" ]; then
        systemctl daemon-reexec
    elif [ -x "$(command -v telinit)" ]; then
        telinit u
    else
        echo "ERROR: Cannot re-exec init" >&2
        exit 1
    fi
}

# helper functions
# https://stackoverflow.com/a/3232082/2646069
confirm() {
    # call with a prompt string or use a default
    read -r -p "${1:-Are you sure? [y/N]} " response
    case "$response" in
        [yY][eE][sS]|[yY]) 
            true
            ;;
        *)
            false
            ;;
    esac
}

# jobs
get_rootfs() {
    if [ -n ${ROOTFS} ]; then 
        echo "Getting rootfs URL..."

        # forgive me for parsing HTML with these shit
        # and hope it works
        ROOTFS_TIME=$(curl -fsSL "https://images.linuxcontainers.org/images/debian/stretch/amd64/default/?C=M;O=D" | grep -oP '(\d{8}_\d{2}:\d{2})' | head -n 1)
        
        ROOTFS="https://images.linuxcontainers.org/images/debian/stretch/amd64/default/${ROOTFS_TIME}/rootfs.squashfs"
    else 
        echo "\$ROOTFS is set to '$ROOTFS'"
    fi
}

sync_filesystem() {
    echo "Syncing..."
    sync
    sync
}

prepare_environment() {
    echo "Loading kernel modules..."
    modprobe overlay
    modprobe squashfs

    sysctl kernel.panic=10

    echo "Creating workspace in '${WORKDIR}'..."
    # workspace
    mkdir -p "${WORKDIR}"
    mount -t tmpfs tmpfs "${WORKDIR}"

    # new rootfs
    mkdir -p "${WORKDIR}/newroot"
    # readonly part of new rootfs
    mkdir -p "${WORKDIR}/newrootro"
    # writeable part of new rootfs
    mkdir -p "${WORKDIR}/newrootrw"
    # overlayfs workdir
    mkdir -p "${WORKDIR}/overlayfs_workdir"

    echo "Downloading temporary rootfs..."
    curl -L -C - -o "${WORKDIR}/rootfs.squashfs" "${ROOTFS}"
}

mount_new_rootfs() {
    echo "Mounting temporary rootfs..."
    mount -t squashfs "${WORKDIR}/rootfs.squashfs" "${WORKDIR}/newrootro"
    mount -t overlay overlay -o rw,lowerdir="${WORKDIR}/newrootro",upperdir="${WORKDIR}/newrootrw",workdir="${WORKDIR}/overlayfs_workdir" "${WORKDIR}/newroot"

    NEWROOT="${WORKDIR}/newroot"
}

install_software() {
    echo "Installing OpenSSH Server into new rootfs..."
    DEBIAN_FRONTEND=noninteractive chroot "${NEWROOT}" apt-get update -y
    DEBIAN_FRONTEND=noninteractive chroot "${NEWROOT}" apt-get -o Dpkg::Options::="--force-confdef" -o Dpkg::Options::="--force-confold" install -y ssh
}

copy_config() {
    echo "Copying important config into new rootfs..."
    ! cp -ax "${OLDROOT}/etc/resolv.conf" "${NEWROOT}/etc"
    ! cp -axr "${OLDROOT}/etc/ssh" "${NEWROOT}/etc"
    ! cp -ax "${OLDROOT}/etc/"{passwd,shadow} "${NEWROOT}/etc"
    ! cp -axr "${OLDROOT}/root/.ssh" "${NEWROOT}/root"

    chroot "${NEWROOT}" chsh -s /bin/bash root

    cat > "${NEWROOT}/etc/motd" <<EOF

Download menhera.sh at https://github.com/Jamesits/menhera.sh

!!!NOTICE!!!

This is a minimal RAM system created by menhera.sh. Feel free to format your disk, but don't blame anyone
except yourself if you lost important files or your system is broken.

If you think you've done something wrong, reboot immediately -- there is still hope.

Your original rootfs is at /mnt/oldroot. Be careful dealing with it.

Have a lot of fun...
EOF
}

swap_root() {
    echo "Swapping rootfs..."
    # prepare future mount point for our old rootfs
    mkdir -p "${WORKDIR}/newroot/mnt/oldroot"
    mount --make-rprivate /

    # swap root
    pivot_root "${WORKDIR}/newroot" "${WORKDIR}/newroot/mnt/oldroot"

    OLDROOT="/mnt/oldroot"
    NEWROOT="/"

    # move mounts
    for i in dev proc sys run; do mount --move "${OLDROOT}/$i" "${NEWROOT}/$i"; done
    mount -t tmpfs tmpfs "${NEWROOT}/tmp"

    mkdir -p "${WORKDIR}"
    mount --move "${OLDROOT}/${WORKDIR}" "${WORKDIR}"

    echo "Restarting SSH daemon..."
    __compat_restart_ssh
}

clear_processes() {
    echo "Disabling swap..."
    swapoff -a

    echo "Restarting systemd..."
    __compat_reload_init
    # hope 15s is enough
    sleep 15

    echo "Killing all programs still using the old root..."
    fuser -kvm "${OLDROOT}" -15
    # in most cases the parent process of this script will be killed, so goodbye
}

# main procedure

if [[ $EUID -ne 0 ]]; then
    echo "This script must be run as root" 
    exit 1
fi

echo -e "We will start a temporary RAM system as your recovery environment."
echo -e "Note that this script will kill programs and umount filesystems without prompting."
echo -e "Please confirm:"
echo -e "\tYou have closed all programs you can, and backed up all important data"
echo -e "\tYou can SSH into your system as root user"
confirm || exit -1

get_rootfs
sync_filesystem

prepare_environment
mount_new_rootfs
copy_config
install_software
swap_root

echo -e "If you are connecting from SSH, please create a second session to this host use root and"
echo -e "confirm you can get a shell."
echo -e "After your confirmation, we are going to kill the old SSH server."

if confirm; then 
    clear_processes
else
    echo -e "Please manually issue a reboot to recover your old OS. If you believe there is a bug in menhera.sh, "
    echo -e "raise a ticket at https://github.com/Jamesits/menhera.sh/issues ."
    exit 1
fi