linux-kernel - [RFC 3/3] LSM: Add an example sample dynamic LSM

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Date:   Sun, 26 Nov 2017 22:16:31 +0000
From:   Sargun Dhillon <sargun@...gun.me>
To:     linux-security-module@...r.kernel.org
Cc:     keescook@...omium.org, igor.stoppa@...wei.com,
        casey@...aufler-ca.com, linux-kernel@...r.kernel.org
Subject: [RFC 3/3] LSM: Add an example sample dynamic LSM

This adds an example LSM that utilizes the features added by the
dynamically loadable LSMs patch. Once the module is unloaded, the
command is once again allowed. It prevents the user from running:
date --set="October 21 2015 16:29:00 PDT

The behaviour can be verified by looking at:
/sys/kernel/security/dynamic_hooks/settime

Signed-off-by: Sargun Dhillon <sargun@...gun.me>
---
 samples/Kconfig           |  6 ++++++
 samples/Makefile          |  2 +-
 samples/lsm/Makefile      |  4 ++++
 samples/lsm/lsm_example.c | 46 ++++++++++++++++++++++++++++++++++++++++++++++
 4 files changed, 57 insertions(+), 1 deletion(-)
 create mode 100644 samples/lsm/Makefile
 create mode 100644 samples/lsm/lsm_example.c

diff --git a/samples/Kconfig b/samples/Kconfig
index c332a3b..283f442 100644
--- a/samples/Kconfig
+++ b/samples/Kconfig
@@ -117,4 +117,10 @@ config SAMPLE_STATX
 	help
 	  Build example userspace program to use the new extended-stat syscall.
 
+config SAMPLE_DYNAMIC_LSM
+	tristate "Build LSM examples -- loadable modules only"
+	depends on SECURITY_DYNAMIC_HOOKS_FS && m
+	help
+	  This builds an example dynamic LSM
+
 endif # SAMPLES
diff --git a/samples/Makefile b/samples/Makefile
index db54e76..9d23835 100644
--- a/samples/Makefile
+++ b/samples/Makefile
@@ -3,4 +3,4 @@
 obj-$(CONFIG_SAMPLES)	+= kobject/ kprobes/ trace_events/ livepatch/ \
 			   hw_breakpoint/ kfifo/ kdb/ hidraw/ rpmsg/ seccomp/ \
 			   configfs/ connector/ v4l/ trace_printk/ blackfin/ \
-			   vfio-mdev/ statx/
+			   vfio-mdev/ statx/ lsm/
diff --git a/samples/lsm/Makefile b/samples/lsm/Makefile
new file mode 100644
index 0000000..d4ccb94
--- /dev/null
+++ b/samples/lsm/Makefile
@@ -0,0 +1,4 @@
+# builds the loadable LSM example kernel modules;
+# then to use one (as root):  insmod <module_name.ko>
+# and to unload: rmmod module_name
+obj-$(CONFIG_SAMPLE_DYNAMIC_LSM) += lsm_example.o
diff --git a/samples/lsm/lsm_example.c b/samples/lsm/lsm_example.c
new file mode 100644
index 0000000..509ae43
--- /dev/null
+++ b/samples/lsm/lsm_example.c
@@ -0,0 +1,46 @@
+/*
+ * This sample hooks into the "path_chroot"
+ *
+ * Once you run it, the following will not be allowed:
+ * date --set="October 21 2015 16:29:00 PDT"
+ */
+
+#include <linux/kernel.h>
+#include <linux/module.h>
+#include <linux/lsm_hooks.h>
+
+static const char lsm_name[] = "example";
+
+static int settime_cb(const struct timespec *ts, const struct timezone *tz)
+{
+	/* We aren't allowed to travel to October 21 2015 16:29 PDT */
+	if (ts->tv_sec >= 1445470140 && ts->tv_sec < 1445470200)
+		return -EPERM;
+
+	return 0;
+}
+
+DYNAMIC_SECURITY_HOOK(my_hook, lsm_name, settime, settime_cb);
+
+static int __init lsm_init(void)
+{
+	int ret;
+
+	ret = security_add_dynamic_hook(&my_hook);
+	if (!ret)
+		pr_info("Successfully installed example dynamic LSM\n");
+	else
+		pr_err("Unable to install dynamic LSM - %d\n", ret);
+
+	return ret;
+}
+
+static void __exit lsm_exit(void)
+{
+	security_remove_dynamic_hook(&my_hook);
+	pr_info("Removed example dynamic LSM\n");
+}
+
+module_init(lsm_init)
+module_exit(lsm_exit)
+MODULE_LICENSE("GPL");
-- 
2.9.3