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, 0 insertions, 24 deletions

diff --git a/security/selinux/Kconfig b/security/selinux/Kconfig
index 4ea946123255..95a186ec0fcb 100644
--- a/security/selinux/Kconfig
+++ b/security/selinux/Kconfig
@@ -23,30 +23,6 @@ config SECURITY_SELINUX_BOOTPARAM
 
 	  If you are unsure how to answer this question, answer N.
 
-config SECURITY_SELINUX_DISABLE
-	bool "NSA SELinux runtime disable"
-	depends on SECURITY_SELINUX
-	select SECURITY_WRITABLE_HOOKS
-	default n
-	help
-	  This option enables writing to a selinuxfs node 'disable', which
-	  allows SELinux to be disabled at runtime prior to the policy load.
-	  SELinux will then remain disabled until the next boot.
-	  This option is similar to the selinux=0 boot parameter, but is to
-	  support runtime disabling of SELinux, e.g. from /sbin/init, for
-	  portability across platforms where boot parameters are difficult
-	  to employ.
-
-	  NOTE: selecting this option will disable the '__ro_after_init'
-	  kernel hardening feature for security hooks.   Please consider
-	  using the selinux=0 boot parameter instead of enabling this
-	  option.
-
-	  WARNING: this option is deprecated and will be removed in a future
-	  kernel release.
-
-	  If you are unsure how to answer this question, answer N.
-
 config SECURITY_SELINUX_DEVELOP
 	bool "NSA SELinux Development Support"
 	depends on SECURITY_SELINUX