modules: sysctl to block module loading

Implement a sysctl file that disables module-loading system-wide since
there is no longer a viable way to remove CAP_SYS_MODULE after the system
bounding capability set was removed in 2.6.25.

Value can only be set to "1", and is tested only if standard capability
checks allow CAP_SYS_MODULE.  Given existing /dev/mem protections, this
should allow administrators a one-way method to block module loading
after initial boot-time module loading has finished.
Signed-off-by: NKees Cook <kees.cook@canonical.com>
Acked-by: NSerge Hallyn <serue@us.ibm.com>
Signed-off-by: NJames Morris <jmorris@namei.org>

Documentation/sysctl/kernel.txt

@@ -30,6 +30,7 @@ show up in /proc/sys/kernel:
 - kstack_depth_to_print       [ X86 only ]
 - l2cr                        [ PPC only ]
 - modprobe                    ==> Documentation/debugging-modules.txt
+- modules_disabled
 - msgmax
 - msgmnb
 - msgmni
@@ -179,6 +180,16 @@ kernel stack.
 
 ==============================================================
 
+modules_disabled:
+
+A toggle value indicating if modules are allowed to be loaded
+in an otherwise modular kernel.  This toggle defaults to off
+(0), but can be set true (1).  Once true, modules can be
+neither loaded nor unloaded, and the toggle cannot be set back
+to false.
+
+==============================================================
+
 osrelease, ostype & version:
 
 # cat osrelease

kernel/module.c

@@ -778,6 +778,9 @@ static void wait_for_zero_refcount(struct module *mod)
 	mutex_lock(&module_mutex);
 }
 
+/* Block module loading/unloading? */
+int modules_disabled = 0;
+
 SYSCALL_DEFINE2(delete_module, const char __user *, name_user,
 		unsigned int, flags)
 {
@@ -785,7 +788,7 @@ SYSCALL_DEFINE2(delete_module, const char __user *, name_user,
 	char name[MODULE_NAME_LEN];
 	int ret, forced = 0;
 
-	if (!capable(CAP_SYS_MODULE))
+	if (!capable(CAP_SYS_MODULE) || modules_disabled)
 		return -EPERM;
 
 	if (strncpy_from_user(name, name_user, MODULE_NAME_LEN-1) < 0)
@@ -2349,7 +2352,7 @@ SYSCALL_DEFINE3(init_module, void __user *, umod,
 	int ret = 0;
 
 	/* Must have permission */
-	if (!capable(CAP_SYS_MODULE))
+	if (!capable(CAP_SYS_MODULE) || modules_disabled)
 		return -EPERM;
 
 	/* Only one module load at a time, please */

kernel/sysctl.c

@@ -113,6 +113,7 @@ static int ngroups_max = NGROUPS_MAX;
 
 #ifdef CONFIG_MODULES
 extern char modprobe_path[];
+extern int modules_disabled;
 #endif
 #ifdef CONFIG_CHR_DEV_SG
 extern int sg_big_buff;
@@ -533,6 +534,17 @@ static struct ctl_table kern_table[] = {
 		.proc_handler	= &proc_dostring,
 		.strategy	= &sysctl_string,
 	},
+	{
+		.ctl_name	= CTL_UNNUMBERED,
+		.procname	= "modules_disabled",
+		.data		= &modules_disabled,
+		.maxlen		= sizeof(int),
+		.mode		= 0644,
+		/* only handle a transition from default "0" to "1" */
+		.proc_handler	= &proc_dointvec_minmax,
+		.extra1		= &one,
+		.extra2		= &one,
+	},
 #endif
 #if defined(CONFIG_HOTPLUG) && defined(CONFIG_NET)
 	{