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>

samples/Kconfig

@@ -296,7 +296,9 @@ config SAMPLE_CHECK_EXEC
 	depends on CC_CAN_LINK && HEADERS_INSTALL
 	help
 	  Build a tool to easily configure SECBIT_EXEC_RESTRICT_FILE and
-	  SECBIT_EXEC_DENY_INTERACTIVE.
+	  SECBIT_EXEC_DENY_INTERACTIVE, and a simple script interpreter to
+	  demonstrate how they should be used with execveat(2) +
+	  AT_EXECVE_CHECK.
 
 source "samples/rust/Kconfig"
 

samples/check-exec/.gitignore

@@ -1 +1,2 @@
+/inc
 /set-exec

samples/check-exec/Makefile

@@ -1,6 +1,7 @@
 # SPDX-License-Identifier: BSD-3-Clause
 
 userprogs-always-y := \
+	inc \
 	set-exec
 
 userccflags += -I usr/include

samples/check-exec/inc.c

@@ -0,0 +1,205 @@
+// SPDX-License-Identifier: BSD-3-Clause
+/*
+ * Very simple script interpreter 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).
+ *
+ * See tools/testing/selftests/exec/check-exec-tests.sh and
+ * Documentation/userspace-api/check_exec.rst
+ *
+ * Copyright © 2024 Microsoft Corporation
+ */
+
+#define _GNU_SOURCE
+#include <errno.h>
+#include <linux/fcntl.h>
+#include <linux/prctl.h>
+#include <linux/securebits.h>
+#include <stdbool.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <sys/prctl.h>
+#include <unistd.h>
+
+/* Returns 1 on error, 0 otherwise. */
+static int interpret_buffer(char *buffer, size_t buffer_size)
+{
+	char *line, *saveptr = NULL;
+	long long number = 0;
+
+	/* Each command is the first character of a line. */
+	saveptr = NULL;
+	line = strtok_r(buffer, "\n", &saveptr);
+	while (line) {
+		if (*line != '#' && strlen(line) != 1) {
+			fprintf(stderr, "# ERROR: Unknown string\n");
+			return 1;
+		}
+		switch (*line) {
+		case '#':
+			/* Skips shebang and comments. */
+			break;
+		case '+':
+			/* Increments and prints the number. */
+			number++;
+			printf("%lld\n", number);
+			break;
+		case '?':
+			/* Reads integer from stdin. */
+			fprintf(stderr, "> Enter new number: \n");
+			if (scanf("%lld", &number) != 1) {
+				fprintf(stderr,
+					"# WARNING: Failed to read number from stdin\n");
+			}
+			break;
+		default:
+			fprintf(stderr, "# ERROR: Unknown character '%c'\n",
+				*line);
+			return 1;
+		}
+		line = strtok_r(NULL, "\n", &saveptr);
+	}
+	return 0;
+}
+
+/* Returns 1 on error, 0 otherwise. */
+static int interpret_stream(FILE *script, char *const script_name,
+			    char *const *const envp, const bool restrict_stream)
+{
+	int err;
+	char *const script_argv[] = { script_name, NULL };
+	char buf[128] = {};
+	size_t buf_size = sizeof(buf);
+
+	/*
+	 * We pass a valid argv and envp to the kernel to emulate a native
+	 * script execution.  We must use the script file descriptor instead of
+	 * the script path name to avoid race conditions.
+	 */
+	err = execveat(fileno(script), "", script_argv, envp,
+		       AT_EMPTY_PATH | AT_EXECVE_CHECK);
+	if (err && restrict_stream) {
+		perror("ERROR: Script execution check");
+		return 1;
+	}
+
+	/* Reads script. */
+	buf_size = fread(buf, 1, buf_size - 1, script);
+	return interpret_buffer(buf, buf_size);
+}
+
+static void print_usage(const char *argv0)
+{
+	fprintf(stderr, "usage: %s <script.inc> | -i | -c <command>\n\n",
+		argv0);
+	fprintf(stderr, "Example:\n");
+	fprintf(stderr, "  ./set-exec -fi -- ./inc -i < script-exec.inc\n");
+}
+
+int main(const int argc, char *const argv[], char *const *const envp)
+{
+	int opt;
+	char *cmd = NULL;
+	char *script_name = NULL;
+	bool interpret_stdin = false;
+	FILE *script_file = NULL;
+	int secbits;
+	bool deny_interactive, restrict_file;
+	size_t arg_nb;
+
+	secbits = prctl(PR_GET_SECUREBITS);
+	if (secbits == -1) {
+		/*
+		 * This should never happen, except with a buggy seccomp
+		 * filter.
+		 */
+		perror("ERROR: Failed to get securebits");
+		return 1;
+	}
+
+	deny_interactive = !!(secbits & SECBIT_EXEC_DENY_INTERACTIVE);
+	restrict_file = !!(secbits & SECBIT_EXEC_RESTRICT_FILE);
+
+	while ((opt = getopt(argc, argv, "c:i")) != -1) {
+		switch (opt) {
+		case 'c':
+			if (cmd) {
+				fprintf(stderr, "ERROR: Command already set");
+				return 1;
+			}
+			cmd = optarg;
+			break;
+		case 'i':
+			interpret_stdin = true;
+			break;
+		default:
+			print_usage(argv[0]);
+			return 1;
+		}
+	}
+
+	/* Checks that only one argument is used, or read stdin. */
+	arg_nb = !!cmd + !!interpret_stdin;
+	if (arg_nb == 0 && argc == 2) {
+		script_name = argv[1];
+	} else if (arg_nb != 1) {
+		print_usage(argv[0]);
+		return 1;
+	}
+
+	if (cmd) {
+		/*
+		 * Other kind of interactive interpretations should be denied
+		 * as well (e.g. CLI arguments passing script snippets,
+		 * environment variables interpreted as script).  However, any
+		 * way to pass script files should only be restricted according
+		 * to restrict_file.
+		 */
+		if (deny_interactive) {
+			fprintf(stderr,
+				"ERROR: Interactive interpretation denied.\n");
+			return 1;
+		}
+
+		return interpret_buffer(cmd, strlen(cmd));
+	}
+
+	if (interpret_stdin && !script_name) {
+		script_file = stdin;
+		/*
+		 * As for any execve(2) call, this path may be logged by the
+		 * kernel.
+		 */
+		script_name = "/proc/self/fd/0";
+		/*
+		 * When stdin is used, it can point to a regular file or a
+		 * pipe.  Restrict stdin execution according to
+		 * SECBIT_EXEC_DENY_INTERACTIVE but always allow executable
+		 * files (which are not considered as interactive inputs).
+		 */
+		return interpret_stream(script_file, script_name, envp,
+					deny_interactive);
+	} else if (script_name && !interpret_stdin) {
+		/*
+		 * In this sample, we don't pass any argument to scripts, but
+		 * otherwise we would have to forge an argv with such
+		 * arguments.
+		 */
+		script_file = fopen(script_name, "r");
+		if (!script_file) {
+			perror("ERROR: Failed to open script");
+			return 1;
+		}
+		/*
+		 * Restricts file execution according to
+		 * SECBIT_EXEC_RESTRICT_FILE.
+		 */
+		return interpret_stream(script_file, script_name, envp,
+					restrict_file);
+	}
+
+	print_usage(argv[0]);
+	return 1;
+}

samples/check-exec/run-script-ask.inc

@@ -0,0 +1,9 @@
+#!/usr/bin/env sh
+# SPDX-License-Identifier: BSD-3-Clause
+
+DIR="$(dirname -- "$0")"
+
+PATH="${PATH}:${DIR}"
+
+set -x
+"${DIR}/script-ask.inc"

samples/check-exec/script-ask.inc

@@ -0,0 +1,5 @@
+#!/usr/bin/env inc
+# SPDX-License-Identifier: BSD-3-Clause
+
+?
++

samples/check-exec/script-exec.inc

@@ -0,0 +1,4 @@
+#!/usr/bin/env inc
+# SPDX-License-Identifier: BSD-3-Clause
+
++

samples/check-exec/script-noexec.inc

@@ -0,0 +1,4 @@
+#!/usr/bin/env inc
+# SPDX-License-Identifier: BSD-3-Clause
+
++

tools/testing/selftests/exec/.gitignore

@@ -11,9 +11,11 @@ non-regular
 null-argv
 /check-exec
 /false
+/inc
 /load_address.*
 !load_address.c
 /recursion-depth
+/set-exec
 xxxxxxxx*
 pipe
 S_I*.test

tools/testing/selftests/exec/Makefile

@@ -10,9 +10,9 @@ ALIGN_PIES        := $(patsubst %,load_address.%,$(ALIGNS))
 ALIGN_STATIC_PIES := $(patsubst %,load_address.static.%,$(ALIGNS))
 ALIGNMENT_TESTS   := $(ALIGN_PIES) $(ALIGN_STATIC_PIES)
 
-TEST_PROGS := binfmt_script.py
+TEST_PROGS := binfmt_script.py check-exec-tests.sh
 TEST_GEN_PROGS := execveat non-regular $(ALIGNMENT_TESTS)
-TEST_GEN_PROGS_EXTENDED := false
+TEST_GEN_PROGS_EXTENDED := false inc set-exec script-exec.inc script-noexec.inc
 TEST_GEN_FILES := execveat.symlink execveat.denatured script subdir
 # Makefile is a run-time dependency, since it's accessed by the execveat test
 TEST_FILES := Makefile
@@ -26,6 +26,8 @@ EXTRA_CLEAN := $(OUTPUT)/subdir.moved $(OUTPUT)/execveat.moved $(OUTPUT)/xxxxx*
 
 include ../lib.mk
 
+CHECK_EXEC_SAMPLES := $(top_srcdir)/samples/check-exec
+
 $(OUTPUT)/subdir:
 	mkdir -p $@
 $(OUTPUT)/script: Makefile
@@ -45,3 +47,11 @@ $(OUTPUT)/load_address.static.0x%: load_address.c
 		-fPIE -static-pie $< -o $@
 $(OUTPUT)/false: false.c
 	$(CC) $(CFLAGS) $(LDFLAGS) -static $< -o $@
+$(OUTPUT)/inc: $(CHECK_EXEC_SAMPLES)/inc.c
+	$(CC) $(CFLAGS) $(LDFLAGS) $< -o $@
+$(OUTPUT)/set-exec: $(CHECK_EXEC_SAMPLES)/set-exec.c
+	$(CC) $(CFLAGS) $(LDFLAGS) $< -o $@
+$(OUTPUT)/script-exec.inc: $(CHECK_EXEC_SAMPLES)/script-exec.inc
+	cp $< $@
+$(OUTPUT)/script-noexec.inc: $(CHECK_EXEC_SAMPLES)/script-noexec.inc
+	cp $< $@