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Remove support for SELinux runtime disable

Summary

Remove support for SELinux runtime disable so that the LSM hooks can be hardened via read-only-after-initialization protections.

Migrate users to using selinux=0 on the kernel command line if they want to disable SELinux.

NOTE: By "disabling SELinux" here we mean that the kernel doesn't call into the SELinux subsystem at all. Switching SELinux between "permissive" and "enforcing" mode using setenforce(8) (which is often incorrectly called "disabling/enabling SELinux") is not affected and will remain fully functional. See the Fedora Documentation for more information about the SELinux modes.


Owner


Current status

  • Targeted release: Fedora 34
  • Last updated: 2020-09-16
  • FESCo issue: #2471
  • Tracker bug: <will be assigned by the Wrangler>
  • Release notes tracker: <will be assigned by the Wrangler>

Detailed Description

Currently, SELinux can be disabled using selinux=0 on the kernel command line, or in userspace via /etc/selinux/config. In the latter case, /etc/selinux/config is read by libselinux userspace library during boot and if it contains SELINUX=disabled, it writes 1 into /sys/fs/selinux/disable and unmounts /sys/fs/selinux.

Support for SELinux runtime disable via /etc/selinux/config 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 comes to the kernel LSM hooks.

Marking the kernel LSM hooks as read only provides some very nice security benefits, but it does mean that we can no longer disable SELinux at runtime. Toggling between enforcing and permissive mode while booted will remain unaffected and it will still be possible to disable SELinux by adding selinux=0 to the kernel command line via the boot loader (GRUB).

System with SELINUX=disabled in /etc/selinux/config will come up with /sys/fs/selinux unmounted, userspace will detect SELinux as disabled. Internally SELinux will be enabled but not initialized so that there will be no SELinux checks applied. This state is very similar to SELinux disabled - the hooks are active, but they mostly do almost nothing so there should be very little effect on the time spent in syscalls compared to SELinux fully disabled.

Runtime disable is considered deprecated by upstream, and using it will become increasingly painful (e.g. sleeping/blocking) through future kernel releases until eventually it is removed completely. Current kernel reports the following message during runtime disable: SELinux: Runtime disable is deprecated, use selinux=0 on the kernel cmdline

Additional info:

Feedback

Benefit to Fedora

Marking the LSM hooks as read-only provides extra security hardening against certain attacks, e.g. in case an attacker gains ability to write to random kernel memory locations, with support for disable SELinux runtime (CONFIG_SECURITY_SELINUX_DISABLE=y) they have a bigger chance to turn off (parts of) SELinux permission checking.

Scope

  • Other developers: N/A
  • Release engineering: #9742 (a check of an impact with Release Engineering is needed)
  • Policies and guidelines: N/A
  • Trademark approval: N/A (not needed for this Change)

Upgrade/compatibility impact

Users should not be directly affected by this change.

How To Test

  1. Install a kernel built with CONFIG_SECURITY_SELINUX_DISABLE disabled, e.g. from https://copr.fedorainfracloud.org/coprs/omos/drop-selinux-disable/.
  2. Confirm that SELinux is disabled when selinux=0 is used on kernel command line.
  3. Confirm that userspace considers SELinux disabled when SELINUX=disabled is used in /etc/selinux/config.
  4. Confirm that userspace considers SELinux disabled when there is no /etc/selinux/config.
  5. Confirm that the system works as expected in all previous cases.

User Experience

There's no visible change for users with SELinux enabled.

Users with SELINUX=disabled in /etc/selinux/config and without selinux=0 on kernel command line might notice that ps Z command uses kernel domain for processes, while with selinux=0 ps Z prints '-'. These users will also be able to load SELinux policy after boot.

Dependencies

Upstream kernel SELinux subsystem waits for this change in order to remove CONFIG_SECURITY_SELINUX_DISABLE functionality - https://lore.kernel.org/selinux/157836784986.560897.13893922675143903084.stgit@chester/#t

Contingency Plan

  • Contingency mechanism: Revert the kernel build option change and build kernel with CONFIG_SECURITY_SELINUX_DISABLE=y
  • Contingency deadline: Beta freeze
  • Blocks release? No

Documentation

TBD

Release Notes

TBD