init: use host's /etc/passwd in rootful setups

This will ensure rootful containers (podman/docker) will not setup
passwordless sudo/su, and instead use host's passwords

It's still less secure than a full rootless setup, but it's better now
than an open gate to root.

Signed-off-by: Luca Di Maio <luca.dimaio1@gmail.com>

distrobox-init

@@ -1446,7 +1446,10 @@ fi
 
 # If we're running this script as root in a login shell (sudoless), we don't
 # have to bother setting up sudo.
-if [ "${container_user_uid}" -ne 0 ]; then
+#
+# Also if we're in a rootful container, we can just use host's /etc/shadow to
+# secure user passwords, so let's skip passwordless sudo too
+if [ "${container_user_uid}" -ne 0 ] && [ ! -r /run/host/etc/shadow ] && [ ! -e /etc/passwd.done ]; then
 	printf "distrobox: Setting up sudo...\n"
 	mkdir -p /etc/sudoers.d
 	# Do not check fqdn when doing sudo, it will not work anyways
@@ -1514,6 +1517,7 @@ if ! grep "^$(printf '%s' "${container_user_name}" | tr '\\' '.'):" /etc/passwd;
 			"${container_user_home}" "${SHELL:-"/bin/bash"}" >> /etc/passwd
 		printf "%s::1::::::" "${container_user_name}" >> /etc/shadow
 	fi
+	touch /etc/passwd.done
 # Ensure we're not using the specified SHELL. Run it only once, so that future
 # user's preferences are not overwritten at each start.
 elif [ ! -e /etc/passwd.done ]; then
@@ -1541,13 +1545,22 @@ elif [ ! -e /etc/passwd.done ]; then
 	touch /etc/passwd.done
 fi
 
-# We generate a random password to initialize the entry for the user and root.
-temporary_password="$(cat /proc/sys/kernel/random/uuid)"
-printf "%s\n%s\n" "${temporary_password}" "${temporary_password}" | passwd root
-printf "%s:%s" "${container_user_name}" "${temporary_password}" | chpasswd -e
-# Delete password for root and user
-printf "%s:" "root" | chpasswd -e
-printf "%s:" "${container_user_name}" | chpasswd -e
+# If we have read access to host's /etc/shadow, let's mount it read-only in the
+# container, so that sudo, and su has a password and that's the same as the host
+#
+# else we fallback to the usual setup with passwordless sudo/su user. This is
+# likely because we're in a rootless setup, so privilege escalation is not a concern.
+if [ -r /run/host/etc/shadow ]; then
+	mount_bind /run/host/etc/shadow /etc/shadow ro
+elif [ ! -e /etc/passwd.done ]; then
+	# We generate a random password to initialize the entry for the user and root.
+	temporary_password="$(cat /proc/sys/kernel/random/uuid)"
+	printf "%s\n%s\n" "${temporary_password}" "${temporary_password}" | passwd root
+	printf "%s:%s" "${container_user_name}" "${temporary_password}" | chpasswd -e
+	# Delete password for root and user
+	printf "%s:" "root" | chpasswd -e
+	printf "%s:" "${container_user_name}" | chpasswd -e
+fi
 
 # If we do not have profile files in the home, we should copy the
 # skeleton files, if present.