[PATCH] module locking

[Rolf Fokkens]

This patch adds the option op locking module operations, which
means that no more insmod en rmmod operations are possible. This
hopefully satisfies some security requirements that state that no
modularized kernels should be used to be really safe in some
environents.

It can be used during boot (i.e. initrd) to load modules as needed
and afterwards locking module operations, making it a little harder
for hackers to tamper with kernel functionality... though changing
kmem still allows some changes...

It adds a /proc/sys/kernel/module-lock entry with default 0, which
means that module operations aren't locked. This value can be
changed to 1, which locks all module operations. Once the value is
changed to 1 it cannot be changed back to 0.

diff -ruN linux.orig/kernel/kmod.c linux/kernel/kmod.c
--- linux.orig/kernel/kmod.c	Wed Jul 18 03:23:50 2001
+++ linux/kernel/kmod.c	Sat Apr 27 22:23:07 2002
@@ -29,6 +29,7 @@
 #include <asm/uaccess.h>
 
 extern int max_threads;
+extern int module_lock;
 
 static inline void
 use_init_fs_context(void)
@@ -179,6 +180,8 @@
 	static atomic_t kmod_concurrent = ATOMIC_INIT(0);
 #define MAX_KMOD_CONCURRENT 50	/* Completely arbitrary value - KAO */
 	static int kmod_loop_msg;
+
+	if ( module_lock ) return -EPERM;
 
 	/* Don't allow request_module() before the root fs is mounted!  */
 	if ( ! current->fs->root ) {
diff -ruN linux.orig/kernel/module.c linux/kernel/module.c
--- linux.orig/kernel/module.c	Sat Nov 24 02:05:28 2001
+++ linux/kernel/module.c	Sat Apr 27 22:25:01 2002
@@ -68,6 +68,11 @@
 static int kmalloc_failed;
 
 /*
+ * This flag blocks any adding or removing of modules.
+ */
+int module_lock = 0;
+
+/*
  *	This lock prevents modifications that might race the kernel fault
  *	fixups. It does not prevent reader walks that the modules code
  *	does. The kernel lock does that.
@@ -296,7 +301,7 @@
 	struct module *mod;
 	unsigned long flags;
 
-	if (!capable(CAP_SYS_MODULE))
+	if (module_lock || !capable(CAP_SYS_MODULE))
 		return -EPERM;
 	lock_kernel();
 	if ((namelen = get_mod_name(name_user, &name)) < 0) {
@@ -351,7 +356,7 @@
 	unsigned long mod_user_size;
 	struct module_ref *dep;
 
-	if (!capable(CAP_SYS_MODULE))
+	if (module_lock || !capable(CAP_SYS_MODULE))
 		return -EPERM;
 	lock_kernel();
 	if ((namelen = get_mod_name(name_user, &name)) < 0) {
@@ -599,7 +604,7 @@
 	long error;
 	int something_changed;
 
-	if (!capable(CAP_SYS_MODULE))
+	if (module_lock || !capable(CAP_SYS_MODULE))
 		return -EPERM;
 
 	lock_kernel();
diff -ruN linux.orig/kernel/sysctl.c linux/kernel/sysctl.c
--- linux.orig/kernel/sysctl.c	Sat Apr 27 22:44:10 2002
+++ linux/kernel/sysctl.c	Sat Apr 27 22:44:16 2002
@@ -49,9 +49,13 @@
 extern atomic_t nr_queued_signals;
 extern int max_queued_signals;
 extern int sysrq_enabled;
+extern int module_lock;
 extern int core_uses_pid;
 extern int cad_pid;
 
+extern int proc_module_lock(ctl_table *table, int write, struct file *filp,
+                            void *buffer, size_t *lenp);
+
 /* this is needed for the proc_dointvec_minmax for [fs_]overflow UID and GID */
 static int maxolduid = 65535;
 static int minolduid;
@@ -209,6 +213,8 @@
 #ifdef CONFIG_KMOD
 	{KERN_MODPROBE, "modprobe", &modprobe_path, 256,
 	 0644, NULL, &proc_dostring, &sysctl_string },
+	{KERN_MODLOCK, "module-lock", &module_lock, sizeof (int),
+	 0644, NULL, &proc_module_lock},
 #endif
 #ifdef CONFIG_HOTPLUG
 	{KERN_HOTPLUG, "hotplug", &hotplug_path, 256,
@@ -998,6 +1004,23 @@
 {
     return do_proc_dointvec(table,write,filp,buffer,lenp,1,OP_SET);
 }
+
+/*
+ * Try to change the module-lock status. Once it's set it cannot
+ * be cleared however!
+ */
+int proc_module_lock(ctl_table *table, int write, struct file *filp,
+			void *buffer, size_t *lenp)
+{
+	int retval, sav;
+
+	sav = module_lock;
+	retval = do_proc_dointvec(table,write,filp,buffer,lenp,1,OP_SET);
+	if (!retval) module_lock = (sav || module_lock);
+
+	return retval;
+}
+
 
 /*
  *	init may raise the set.
diff -ruN linux.orig/include/linux/sysctl.h linux/include/linux/sysctl.h
--- linux.orig/include/linux/sysctl.h	Sat Apr 27 22:44:10 2002
+++ linux/include/linux/sysctl.h	Sat Apr 27 22:45:18 2002
@@ -124,6 +124,7 @@
 	KERN_CORE_USES_PID=52,		/* int: use core or core.%pid */
 	KERN_TAINTED=53,	/* int: various kernel tainted flags */
 	KERN_CADPID=54,		/* int: PID of the process to notify on CAD */
+	KERN_MODLOCK=65,	/* module_lock value */
 };