[RFC PATCH v19 5/5] samples/should-exec: Add set-should-exec

["Mickaël Salaün" <mic@digikod.net>]

Add a simple tool to set SECBIT_SHOULD_EXEC_CHECK,
SECBIT_SHOULD_EXEC_RESTRICT, and their lock counterparts before
executing a command.  This should be useful to easily test against
script interpreters.

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>
Signed-off-by: Mickaël Salaün <mic@digikod.net>
Link: https://lore.kernel.org/r/20240704190137.696169-6-mic@digikod.net
---
 samples/Kconfig                       |  7 +++
 samples/Makefile                      |  1 +
 samples/should-exec/.gitignore        |  1 +
 samples/should-exec/Makefile          | 13 ++++
 samples/should-exec/set-should-exec.c | 88 +++++++++++++++++++++++++++
 5 files changed, 110 insertions(+)
 create mode 100644 samples/should-exec/.gitignore
 create mode 100644 samples/should-exec/Makefile
 create mode 100644 samples/should-exec/set-should-exec.c

diff --git a/samples/Kconfig b/samples/Kconfig
index b288d9991d27..d8f2639bc830 100644
--- a/samples/Kconfig
+++ b/samples/Kconfig
@@ -180,6 +180,13 @@ config SAMPLE_SECCOMP
 	  Build samples of seccomp filters using various methods of
 	  BPF filter construction.
 
+config SAMPLE_SHOULD_EXEC
+	bool "Should-exec secure bits examples"
+	depends on CC_CAN_LINK && HEADERS_INSTALL
+	help
+	  Build a tool to easily configure SECBIT_SHOULD_EXEC_CHECK,
+	  SECBIT_SHOULD_EXEC_RESTRICT and their lock counterparts.
+
 config SAMPLE_TIMER
 	bool "Timer sample"
 	depends on CC_CAN_LINK && HEADERS_INSTALL
diff --git a/samples/Makefile b/samples/Makefile
index b85fa64390c5..0e7a97fb222d 100644
--- a/samples/Makefile
+++ b/samples/Makefile
@@ -19,6 +19,7 @@ subdir-$(CONFIG_SAMPLE_PIDFD)		+= pidfd
 obj-$(CONFIG_SAMPLE_QMI_CLIENT)		+= qmi/
 obj-$(CONFIG_SAMPLE_RPMSG_CLIENT)	+= rpmsg/
 subdir-$(CONFIG_SAMPLE_SECCOMP)		+= seccomp
+subdir-$(CONFIG_SAMPLE_SHOULD_EXEC)	+= should-exec
 subdir-$(CONFIG_SAMPLE_TIMER)		+= timers
 obj-$(CONFIG_SAMPLE_TRACE_EVENTS)	+= trace_events/
 obj-$(CONFIG_SAMPLE_TRACE_CUSTOM_EVENTS) += trace_events/
diff --git a/samples/should-exec/.gitignore b/samples/should-exec/.gitignore
new file mode 100644
index 000000000000..ac46c614ec80
--- /dev/null
+++ b/samples/should-exec/.gitignore
@@ -0,0 +1 @@
+/set-should-exec
diff --git a/samples/should-exec/Makefile b/samples/should-exec/Makefile
new file mode 100644
index 000000000000..c4294278dd07
--- /dev/null
+++ b/samples/should-exec/Makefile
@@ -0,0 +1,13 @@
+# SPDX-License-Identifier: BSD-3-Clause
+
+userprogs-always-y := set-should-exec
+
+userccflags += -I usr/include
+
+.PHONY: all clean
+
+all:
+	$(MAKE) -C ../.. samples/should-exec/
+
+clean:
+	$(MAKE) -C ../.. M=samples/should-exec/ clean
diff --git a/samples/should-exec/set-should-exec.c b/samples/should-exec/set-should-exec.c
new file mode 100644
index 000000000000..b3c31106d916
--- /dev/null
+++ b/samples/should-exec/set-should-exec.c
@@ -0,0 +1,88 @@
+// SPDX-License-Identifier: BSD-3-Clause
+/*
+ * Simple tool to set SECBIT_SHOULD_EXEC_CHECK,  SECBIT_SHOULD_EXEC_RESTRICT,
+ * and their lock counterparts before executing a command.
+ *
+ * Copyright © 2024 Microsoft Corporation
+ */
+
+#define _GNU_SOURCE
+#define __SANE_USERSPACE_TYPES__
+#include <errno.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>
+
+static void print_usage(const char *argv0)
+{
+	fprintf(stderr, "usage: %s -c|-r [-l] -- <cmd> [args]...\n\n", argv0);
+	fprintf(stderr, "Execute a command with\n");
+	fprintf(stderr, "- SECBIT_SHOULD_EXEC_CHECK set: -c\n");
+	fprintf(stderr, "- SECBIT_SHOULD_EXEC_RESTRICT set: -r\n");
+	fprintf(stderr, "- SECBIT_SHOULD_EXEC_*_LOCKED set: -l\n");
+}
+
+int main(const int argc, char *const argv[], char *const *const envp)
+{
+	const char *cmd_path;
+	char *const *cmd_argv;
+	int opt, secbits, err;
+	bool has_policy = false;
+
+	secbits = prctl(PR_GET_SECUREBITS);
+
+	while ((opt = getopt(argc, argv, "crl")) != -1) {
+		switch (opt) {
+		case 'c':
+			secbits |= SECBIT_SHOULD_EXEC_CHECK;
+			has_policy = true;
+			break;
+		case 'r':
+			secbits |= SECBIT_SHOULD_EXEC_RESTRICT;
+			has_policy = true;
+			break;
+		case 'l':
+			secbits |= SECBIT_SHOULD_EXEC_CHECK_LOCKED;
+			secbits |= SECBIT_SHOULD_EXEC_RESTRICT_LOCKED;
+			break;
+		default:
+			print_usage(argv[0]);
+			return 1;
+		}
+	}
+
+	if (!argv[optind] || !has_policy) {
+		print_usage(argv[0]);
+		return 1;
+	}
+
+	err = prctl(PR_SET_SECUREBITS, secbits);
+	if (err) {
+		perror("Failed to set secure bit(s).");
+		fprintf(stderr,
+			"Hint: The running kernel may not support this feature.\n");
+		return 1;
+	}
+
+	fprintf(stderr, "SECBIT_SHOULD_EXEC_CHECK: %d\n",
+		!!(secbits & SECBIT_SHOULD_EXEC_CHECK));
+	fprintf(stderr, "SECBIT_SHOULD_EXEC_CHECK_LOCKED: %d\n",
+		!!(secbits & SECBIT_SHOULD_EXEC_CHECK_LOCKED));
+	fprintf(stderr, "SECBIT_SHOULD_EXEC_RESTRICT: %d\n",
+		!!(secbits & SECBIT_SHOULD_EXEC_RESTRICT));
+	fprintf(stderr, "SECBIT_SHOULD_EXEC_RESTRICT_LOCKED: %d\n",
+		!!(secbits & SECBIT_SHOULD_EXEC_RESTRICT_LOCKED));
+
+	cmd_path = argv[optind];
+	cmd_argv = argv + optind;
+	fprintf(stderr, "Executing command...\n");
+	execvpe(cmd_path, cmd_argv, envp);
+	fprintf(stderr, "Failed to execute \"%s\": %s\n", cmd_path,
+		strerror(errno));
+	return 1;
+}
-- 
2.45.2