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: Kees Cook <kees.cook@canonical.com>
Acked-by: Serge Hallyn <serue@us.ibm.com>
Signed-off-by: James Morris <jmorris@namei.org>

3 files changed, 28 insertions, 2 deletions

diff --git a/Documentation/sysctl/kernel.txt b/Documentation/sysctl/kernel.txt
index a4ccdd1981c..02b13495627 100644
--- a/Documentation/sysctl/kernel.txt
+++ b/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
diff --git a/kernel/module.c b/kernel/module.c
index f77ac320d0b..eeb3f7b1383 100644
--- a/kernel/module.c
+++ b/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 */
diff --git a/kernel/sysctl.c b/kernel/sysctl.c
index c5ef44ff850..2fb4246d27d 100644
--- a/kernel/sysctl.c
+++ b/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)
         {