samples/check-exec: Add an enlighten "inc" interpreter and 28 tests

Add a very simple script interpreter called "inc" that can evaluate two
different commands (one per line):
- "?" to initialize a counter from user's input;
- "+" to increment the counter (which is set to 0 by default).

It is enlighten to only interpret executable files according to
AT_EXECVE_CHECK and the related securebits:

  # Executing a script with RESTRICT_FILE is only allowed if the script
  # is executable:
  ./set-exec -f -- ./inc script-exec.inc # Allowed
  ./set-exec -f -- ./inc script-noexec.inc # Denied

  # Executing stdin with DENY_INTERACTIVE is only allowed if stdin is an
  # executable regular file:
  ./set-exec -i -- ./inc -i < script-exec.inc # Allowed
  ./set-exec -i -- ./inc -i < script-noexec.inc # Denied

  # However, a pipe is not executable and it is then denied:
  cat script-noexec.inc | ./set-exec -i -- ./inc -i # Denied

  # Executing raw data (e.g. command argument) with DENY_INTERACTIVE is
  # always denied.
  ./set-exec -i -- ./inc -c "+" # Denied
  ./inc -c "$(<script-ask.inc)" # Allowed

  # To directly execute a script, we can update $PATH (used by `env`):
  PATH="${PATH}:." ./script-exec.inc

  # To execute several commands passed as argument:

Add a complete test suite to check the script interpreter against all
possible execution cases:

  make TARGETS=exec kselftest-install
  ./tools/testing/selftests/kselftest_install/run_kselftest.sh

Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Christian Brauner <brauner@kernel.org>
Cc: Kees Cook <keescook@chromium.org>
Cc: Paul Moore <paul@paul-moore.com>
Cc: Serge Hallyn <serge@hallyn.com>
Signed-off-by: Mickaël Salaün <mic@digikod.net>
Link: https://lore.kernel.org/r/20241212174223.389435-8-mic@digikod.net
Signed-off-by: Kees Cook <kees@kernel.org>

1 files changed, 205 insertions, 0 deletions

diff --git a/tools/testing/selftests/exec/check-exec-tests.sh b/tools/testing/selftests/exec/check-exec-tests.sh
new file mode 100755
index 000000000000..87102906ae3c
--- /dev/null
+++ b/tools/testing/selftests/exec/check-exec-tests.sh
@@ -0,0 +1,205 @@
+#!/usr/bin/env bash
+# SPDX-License-Identifier: GPL-2.0
+#
+# Test the "inc" interpreter.
+#
+# See include/uapi/linux/securebits.h, include/uapi/linux/fcntl.h and
+# samples/check-exec/inc.c
+#
+# Copyright © 2024 Microsoft Corporation
+
+set -u -e -o pipefail
+
+EXPECTED_OUTPUT="1"
+exec 2>/dev/null
+
+DIR="$(dirname $(readlink -f "$0"))"
+source "${DIR}"/../kselftest/ktap_helpers.sh
+
+exec_direct() {
+	local expect="$1"
+	local script="$2"
+	shift 2
+	local ret=0
+	local out
+
+	# Updates PATH for `env` to execute the `inc` interpreter.
+	out="$(PATH="." "$@" "${script}")" || ret=$?
+
+	if [[ ${ret} -ne ${expect} ]]; then
+		echo "ERROR: Wrong expectation for direct file execution: ${ret}"
+		return 1
+	fi
+	if [[ ${ret} -eq 0 && "${out}" != "${EXPECTED_OUTPUT}" ]]; then
+		echo "ERROR: Wrong output for direct file execution: ${out}"
+		return 1
+	fi
+}
+
+exec_indirect() {
+	local expect="$1"
+	local script="$2"
+	shift 2
+	local ret=0
+	local out
+
+	# Script passed as argument.
+	out="$("$@" ./inc "${script}")" || ret=$?
+
+	if [[ ${ret} -ne ${expect} ]]; then
+		echo "ERROR: Wrong expectation for indirect file execution: ${ret}"
+		return 1
+	fi
+	if [[ ${ret} -eq 0 && "${out}" != "${EXPECTED_OUTPUT}" ]]; then
+		echo "ERROR: Wrong output for indirect file execution: ${out}"
+		return 1
+	fi
+}
+
+exec_stdin_reg() {
+	local expect="$1"
+	local script="$2"
+	shift 2
+	local ret=0
+	local out
+
+	# Executing stdin must be allowed if the related file is executable.
+	out="$("$@" ./inc -i < "${script}")" || ret=$?
+
+	if [[ ${ret} -ne ${expect} ]]; then
+		echo "ERROR: Wrong expectation for stdin regular file execution: ${ret}"
+		return 1
+	fi
+	if [[ ${ret} -eq 0 && "${out}" != "${EXPECTED_OUTPUT}" ]]; then
+		echo "ERROR: Wrong output for stdin regular file execution: ${out}"
+		return 1
+	fi
+}
+
+exec_stdin_pipe() {
+	local expect="$1"
+	shift
+	local ret=0
+	local out
+
+	# A pipe is not executable.
+	out="$(cat script-exec.inc | "$@" ./inc -i)" || ret=$?
+
+	if [[ ${ret} -ne ${expect} ]]; then
+		echo "ERROR: Wrong expectation for stdin pipe execution: ${ret}"
+		return 1
+	fi
+}
+
+exec_argument() {
+	local expect="$1"
+	local ret=0
+	shift
+	local out
+
+	# Script not coming from a file must not be executed.
+	out="$("$@" ./inc -c "$(< script-exec.inc)")" || ret=$?
+
+	if [[ ${ret} -ne ${expect} ]]; then
+		echo "ERROR: Wrong expectation for arbitrary argument execution: ${ret}"
+		return 1
+	fi
+	if [[ ${ret} -eq 0 && "${out}" != "${EXPECTED_OUTPUT}" ]]; then
+		echo "ERROR: Wrong output for arbitrary argument execution: ${out}"
+		return 1
+	fi
+}
+
+exec_interactive() {
+	exec_stdin_pipe "$@"
+	exec_argument "$@"
+}
+
+ktap_test() {
+	ktap_test_result "$*" "$@"
+}
+
+ktap_print_header
+ktap_set_plan 28
+
+# Without secbit configuration, nothing is changed.
+
+ktap_print_msg "By default, executable scripts are allowed to be interpreted and executed."
+ktap_test exec_direct 0 script-exec.inc
+ktap_test exec_indirect 0 script-exec.inc
+
+ktap_print_msg "By default, executable stdin is allowed to be interpreted."
+ktap_test exec_stdin_reg 0 script-exec.inc
+
+ktap_print_msg "By default, non-executable scripts are allowed to be interpreted, but not directly executed."
+# We get 126 because of direct execution by Bash.
+ktap_test exec_direct 126 script-noexec.inc
+ktap_test exec_indirect 0 script-noexec.inc
+
+ktap_print_msg "By default, non-executable stdin is allowed to be interpreted."
+ktap_test exec_stdin_reg 0 script-noexec.inc
+
+ktap_print_msg "By default, interactive commands are allowed to be interpreted."
+ktap_test exec_interactive 0
+
+# With only file restriction: protect non-malicious users from inadvertent errors (e.g. python ~/Downloads/*.py).
+
+ktap_print_msg "With -f, executable scripts are allowed to be interpreted and executed."
+ktap_test exec_direct 0 script-exec.inc ./set-exec -f --
+ktap_test exec_indirect 0 script-exec.inc ./set-exec -f --
+
+ktap_print_msg "With -f, executable stdin is allowed to be interpreted."
+ktap_test exec_stdin_reg 0 script-exec.inc ./set-exec -f --
+
+ktap_print_msg "With -f, non-executable scripts are not allowed to be executed nor interpreted."
+# Direct execution of non-executable script is alwayse denied by the kernel.
+ktap_test exec_direct 1 script-noexec.inc ./set-exec -f --
+ktap_test exec_indirect 1 script-noexec.inc ./set-exec -f --
+
+ktap_print_msg "With -f, non-executable stdin is allowed to be interpreted."
+ktap_test exec_stdin_reg 0 script-noexec.inc ./set-exec -f --
+
+ktap_print_msg "With -f, interactive commands are allowed to be interpreted."
+ktap_test exec_interactive 0 ./set-exec -f --
+
+# With only denied interactive commands: check or monitor script content (e.g. with LSM).
+
+ktap_print_msg "With -i, executable scripts are allowed to be interpreted and executed."
+ktap_test exec_direct 0 script-exec.inc ./set-exec -i --
+ktap_test exec_indirect 0 script-exec.inc ./set-exec -i --
+
+ktap_print_msg "With -i, executable stdin is allowed to be interpreted."
+ktap_test exec_stdin_reg 0 script-exec.inc ./set-exec -i --
+
+ktap_print_msg "With -i, non-executable scripts are allowed to be interpreted, but not directly executed."
+# Direct execution of non-executable script is alwayse denied by the kernel.
+ktap_test exec_direct 1 script-noexec.inc ./set-exec -i --
+ktap_test exec_indirect 0 script-noexec.inc ./set-exec -i --
+
+ktap_print_msg "With -i, non-executable stdin is not allowed to be interpreted."
+ktap_test exec_stdin_reg 1 script-noexec.inc ./set-exec -i --
+
+ktap_print_msg "With -i, interactive commands are not allowed to be interpreted."
+ktap_test exec_interactive 1 ./set-exec -i --
+
+# With both file restriction and denied interactive commands: only allow executable scripts.
+
+ktap_print_msg "With -fi, executable scripts are allowed to be interpreted and executed."
+ktap_test exec_direct 0 script-exec.inc ./set-exec -fi --
+ktap_test exec_indirect 0 script-exec.inc ./set-exec -fi --
+
+ktap_print_msg "With -fi, executable stdin is allowed to be interpreted."
+ktap_test exec_stdin_reg 0 script-exec.inc ./set-exec -fi --
+
+ktap_print_msg "With -fi, non-executable scripts are not allowed to be interpreted nor executed."
+# Direct execution of non-executable script is alwayse denied by the kernel.
+ktap_test exec_direct 1 script-noexec.inc ./set-exec -fi --
+ktap_test exec_indirect 1 script-noexec.inc ./set-exec -fi --
+
+ktap_print_msg "With -fi, non-executable stdin is not allowed to be interpreted."
+ktap_test exec_stdin_reg 1 script-noexec.inc ./set-exec -fi --
+
+ktap_print_msg "With -fi, interactive commands are not allowed to be interpreted."
+ktap_test exec_interactive 1 ./set-exec -fi --
+
+ktap_finished