On Mon, Jan 6, 2020 at 4:25 PM Stephen Smalley [off-list ref] wrote:

On 12/19/19 2:22 PM, Paul Moore wrote:

quoted

Deprecate the CONFIG_SECURITY_SELINUX_DISABLE functionality.  The
code was originally developed to make it easier for Linux
distributions to support architectures where adding parameters to the
kernel command line was difficult.  Unfortunately, supporting runtime
disable meant we had to make some security trade-offs when it came to
the LSM hooks, as documented in the Kconfig help text:

   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.

Fortunately it looks as if that the original motivation for the
runtime disable functionality is gone, and Fedora/RHEL appears to be
the only major distribution enabling this capability at build time
so we are now taking steps to remove it entirely from the kernel.
The first step is to mark the functionality as deprecated and print
an error when it is used (what this patch is doing).  As Fedora/RHEL
makes progress in transitioning the distribution away from runtime
disable, we will introduce follow-up patches over several kernel
releases which will block for increasing periods of time when the
runtime disable is used.  Finally we will remove the option entirely
once we believe all users have moved to the kernel cmdline approach.

Signed-off-by: Paul Moore <paul@paul-moore.com>
---
  security/selinux/Kconfig     |    3 +++
  security/selinux/selinuxfs.c |    6 ++++++
  2 files changed, 9 insertions(+)

diff --git a/security/selinux/Kconfig b/security/selinux/Kconfig
index 996d35d950f7..580ac24c7aa1 100644
--- a/security/selinux/Kconfig
+++ b/security/selinux/Kconfig

@@ -42,6 +42,9 @@ config SECURITY_SELINUX_DISABLE
        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

diff --git a/security/selinux/selinuxfs.c b/security/selinux/selinuxfs.c
index 278417e67b4c..adbe2dd35202 100644
--- a/security/selinux/selinuxfs.c
+++ b/security/selinux/selinuxfs.c

@@ -281,6 +281,12 @@ static ssize_t sel_write_disable(struct file *file, const char __user *buf,
      int new_value;
      int enforcing;

+     /* NOTE: we are now officially considering runtime disable as
+      *       deprecated, and using it will become increasingly painful
+      *       (e.g. sleeping/blocking) as we progress through future
+      *       kernel releases until eventually it is removed */
+     pr_err("SELinux:  Runtime disable is deprecated, use selinux=0 on the kernel cmdline.\n");

Looking for examples of similar deprecations in the kernel, I notice
that they often use pr_warn_once() or WARN_ONCE() to avoid spamming the
kernel logs excessively.  They also often include the current process
name to identify the offending process.  In this case, it probably
matters little since this is only done (legitimately) by the init
process and only once, so up to you whether you bother amending it.

Yes, I saw that too and decided we were better off printing something
each time since it really should only ever happen once on a well
behaved system.

Also for some interfaces, they appear to document the intent to remove
it in a file under Documentation/ABI/obsolete/ and then later move that
to removed/ when fully removed.

Thanks, I wasn't aware of that, and couldn't find anything relevant
while grep'ing under Documentation/process.  There used to be a
Documentation/feature-removal.txt (or a file with a similar name)
which tracked these things, but I guess it migrated over to
Documentation/ABI during the last Documentation shuffle a couple of
years ago.

I'll send out an updated patch in just a moment.

-- 
paul moore
www.paul-moore.com