[PATCH v23 5/8] samples/check-exec: Add set-exec

Mickaël Salaün
2024-12-12T17:42:52Z

Add a simple tool to set SECBIT_EXEC_RESTRICT_FILE or
SECBIT_EXEC_DENY_INTERACTIVE before executing a command. This is useful
to easily test against enlighten 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>
Cc: Serge Hallyn <serge@hallyn.com>
Signed-off-by: Mickaël Salaün <mic@digikod.net>
Link: https://lore.kernel.org/r/20241212174223.389435-6-mic@digikod.net
---

Changes since v19:
* Rename file and directory.
* Update securebits and related arguments.
* Remove useless call to prctl() when securebits are unchanged.
---
samples/Kconfig | 7 +++
samples/Makefile | 1 +
samples/check-exec/.gitignore | 1 +
samples/check-exec/Makefile | 14 ++++++
samples/check-exec/set-exec.c | 85 +++++++++++++++++++++++++++++++++++
5 files changed, 108 insertions(+)
create mode 100644 samples/check-exec/.gitignore
create mode 100644 samples/check-exec/Makefile
create mode 100644 samples/check-exec/set-exec.c

diff --git a/samples/Kconfig b/samples/Kconfig
index b288d9991d27..efa28ceadc42 100644
--- a/samples/Kconfig
+++ b/samples/Kconfig
@@ -291,6 +291,13 @@ config SAMPLE_CGROUP
help
Build samples that demonstrate the usage of the cgroup API.

+config SAMPLE_CHECK_EXEC
+ bool "Exec secure bits examples"
+ depends on CC_CAN_LINK && HEADERS_INSTALL
+ help
+ Build a tool to easily configure SECBIT_EXEC_RESTRICT_FILE and
+ SECBIT_EXEC_DENY_INTERACTIVE.
+
source "samples/rust/Kconfig"

endif # SAMPLES
diff --git a/samples/Makefile b/samples/Makefile
index b85fa64390c5..f988202f3a30 100644
--- a/samples/Makefile
+++ b/samples/Makefile
@@ -3,6 +3,7 @@

subdir-$(CONFIG_SAMPLE_AUXDISPLAY) += auxdisplay
subdir-$(CONFIG_SAMPLE_ANDROID_BINDERFS) += binderfs
+subdir-$(CONFIG_SAMPLE_CHECK_EXEC) += check-exec
subdir-$(CONFIG_SAMPLE_CGROUP) += cgroup
obj-$(CONFIG_SAMPLE_CONFIGFS) += configfs/
obj-$(CONFIG_SAMPLE_CONNECTOR) += connector/
diff --git a/samples/check-exec/.gitignore b/samples/check-exec/.gitignore
new file mode 100644
index 000000000000..3f8119112ccf
--- /dev/null
+++ b/samples/check-exec/.gitignore
@@ -0,0 +1 @@
+/set-exec
diff --git a/samples/check-exec/Makefile b/samples/check-exec/Makefile
new file mode 100644
index 000000000000..d9f976e3ff98
--- /dev/null
+++ b/samples/check-exec/Makefile
@@ -0,0 +1,14 @@
+# SPDX-License-Identifier: BSD-3-Clause
+
+userprogs-always-y := \
+ set-exec
+
+userccflags += -I usr/include
+
+.PHONY: all clean
+
+all:
+ $(MAKE) -C ../.. samples/check-exec/
+
+clean:
+ $(MAKE) -C ../.. M=samples/check-exec/ clean
diff --git a/samples/check-exec/set-exec.c b/samples/check-exec/set-exec.c
new file mode 100644
index 000000000000..ba86a60a20dd
--- /dev/null
+++ b/samples/check-exec/set-exec.c
@@ -0,0 +1,85 @@
+// SPDX-License-Identifier: BSD-3-Clause
+/*
+ * Simple tool to set SECBIT_EXEC_RESTRICT_FILE, SECBIT_EXEC_DENY_INTERACTIVE,
+ * 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 -f|-i -- <cmd> [args]...\n\n", argv0);
+ fprintf(stderr, "Execute a command with\n");
+ fprintf(stderr, "- SECBIT_EXEC_RESTRICT_FILE set: -f\n");
+ fprintf(stderr, "- SECBIT_EXEC_DENY_INTERACTIVE set: -i\n");
+}
+
+int main(const int argc, char *const argv[], char *const *const envp)
+{
+ const char *cmd_path;
+ char *const *cmd_argv;
+ int opt, secbits_cur, secbits_new;
+ bool has_policy = false;
+
+ secbits_cur = prctl(PR_GET_SECUREBITS);
+ if (secbits_cur == -1) {
+ /*
+ * This should never happen, except with a buggy seccomp
+ * filter.
+ */
+ perror("ERROR: Failed to get securebits");
+ return 1;
+ }
+
+ secbits_new = secbits_cur;
+ while ((opt = getopt(argc, argv, "fi")) != -1) {
+ switch (opt) {
+ case 'f':
+ secbits_new |= SECBIT_EXEC_RESTRICT_FILE |
+ SECBIT_EXEC_RESTRICT_FILE_LOCKED;
+ has_policy = true;
+ break;
+ case 'i':
+ secbits_new |= SECBIT_EXEC_DENY_INTERACTIVE |
+ SECBIT_EXEC_DENY_INTERACTIVE_LOCKED;
+ has_policy = true;
+ break;
+ default:
+ print_usage(argv[0]);
+ return 1;
+ }
+ }
+
+ if (!argv[optind] || !has_policy) {
+ print_usage(argv[0]);
+ return 1;
+ }
+
+ if (secbits_cur != secbits_new &&
+ prctl(PR_SET_SECUREBITS, secbits_new)) {
+ perror("Failed to set secure bit(s).");
+ fprintf(stderr,
+ "Hint: The running kernel may not support this feature.\n");
+ return 1;
+ }
+
+ 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;
+}
--
2.47.1