selinux: remove the runtime disable functionality

After working with the larger SELinux-based distros for several
years, we're finally at a place where we can disable the SELinux
runtime disable functionality.  The existing kernel deprecation
notice explains the functionality and why we want to remove it:

  The selinuxfs "disable" node allows SELinux to be disabled at
  runtime prior to a policy being loaded into the kernel.  If
  disabled via this mechanism, SELinux will remain disabled until
  the system is rebooted.

  The preferred method of disabling SELinux is via the "selinux=0"
  boot parameter, but the selinuxfs "disable" node was created to
  make it easier for systems with primitive bootloaders that did not
  allow for easy modification of the kernel command line.
  Unfortunately, allowing for SELinux to be disabled at runtime makes
  it difficult to secure the kernel's LSM hooks using the
  "__ro_after_init" feature.

It is that last sentence, mentioning the '__ro_after_init' hardening,
which is the real motivation for this change, and if you look at the
diffstat you'll see that the impact of this patch reaches across all
the different LSMs, helping prevent tampering at the LSM hook level.

From a SELinux perspective, it is important to note that if you
continue to disable SELinux via "/etc/selinux/config" it may appear
that SELinux is disabled, but it is simply in an uninitialized state.
If you load a policy with `load_policy -i`, you will see SELinux
come alive just as if you had loaded the policy during early-boot.

It is also worth noting that the "/sys/fs/selinux/disable" file is
always writable now, regardless of the Kconfig settings, but writing
to the file has no effect on the system, other than to display an
error on the console if a non-zero/true value is written.

Finally, in the several years where we have been working on
deprecating this functionality, there has only been one instance of
someone mentioning any user visible breakage.  In this particular
case it was an individual's kernel test system, and the workaround
documented in the deprecation notice ("selinux=0" on the kernel
command line) resolved the issue without problem.

Acked-by: Casey Schaufler <casey@schaufler-ca.com>
Signed-off-by: Paul Moore <paul@paul-moore.com>

1 files changed, 3 insertions, 3 deletions

diff --git a/security/apparmor/lsm.c b/security/apparmor/lsm.c
index d6cc4812ca53..cebba4824e60 100644
--- a/security/apparmor/lsm.c
+++ b/security/apparmor/lsm.c
@@ -1209,13 +1209,13 @@ static int apparmor_inet_conn_request(const struct sock *sk, struct sk_buff *skb
 /*
  * The cred blob is a pointer to, not an instance of, an aa_label.
  */
-struct lsm_blob_sizes apparmor_blob_sizes __lsm_ro_after_init = {
+struct lsm_blob_sizes apparmor_blob_sizes __ro_after_init = {
 	.lbs_cred = sizeof(struct aa_label *),
 	.lbs_file = sizeof(struct aa_file_ctx),
 	.lbs_task = sizeof(struct aa_task_ctx),
 };
 
-static struct security_hook_list apparmor_hooks[] __lsm_ro_after_init = {
+static struct security_hook_list apparmor_hooks[] __ro_after_init = {
 	LSM_HOOK_INIT(ptrace_access_check, apparmor_ptrace_access_check),
 	LSM_HOOK_INIT(ptrace_traceme, apparmor_ptrace_traceme),
 	LSM_HOOK_INIT(capget, apparmor_capget),
@@ -1427,7 +1427,7 @@ static const struct kernel_param_ops param_ops_aaintbool = {
 	.get = param_get_aaintbool
 };
 /* Boot time disable flag */
-static int apparmor_enabled __lsm_ro_after_init = 1;
+static int apparmor_enabled __ro_after_init = 1;
 module_param_named(enabled, apparmor_enabled, aaintbool, 0444);
 
 static int __init apparmor_enabled_setup(char *str)