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

Mickaël Salaün
2024-07-04T19:02:15Z

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