From Fedora Project Wiki
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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.  
 
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.  
SELinux runtime disable is enabled via ''CONFIG_SECURITY_SELINUX_DISABLE'' kernel build option which 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.
 
Unfortunately, supporting runtime disable meant we had to make some security trade-offs when it comes to the kernel LSM hooks.
  
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<!-- REQUIRED FOR SYSTEM WIDE CHANGES -->
 
<!-- REQUIRED FOR SYSTEM WIDE CHANGES -->
N/A (not a System Wide Change)
 
  
 
Users should not be directly affected by this change.
 
Users should not be directly affected by this change.
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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 instead of '-'.
 
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 instead of '-'.
These users will be also able to load SELinux policy after boot.
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These users will also be able to load SELinux policy after boot.
  
 
== Dependencies ==
 
== Dependencies ==
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<!-- If you cannot complete your feature by the final development freeze, what is the backup plan?  This might be as simple as "Revert the shipped configuration".  Or it might not (e.g. rebuilding a number of dependent packages).  If you feature is not completed in time we want to assure others that other parts of Fedora will not be in jeopardy.  -->
 
<!-- If you cannot complete your feature by the final development freeze, what is the backup plan?  This might be as simple as "Revert the shipped configuration".  Or it might not (e.g. rebuilding a number of dependent packages).  If you feature is not completed in time we want to assure others that other parts of Fedora will not be in jeopardy.  -->
* Contingency mechanism: (What to do? Who will do it?) N/A (not a System Wide Change) <!-- REQUIRED FOR SYSTEM WIDE CHANGES -->
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* Contingency mechanism:  Revert the kernel build option change and build kernel with ''CONFIG_SECURITY_SELINUX_DISABLE=y''
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  <!-- REQUIRED FOR SYSTEM WIDE CHANGES -->
 
<!-- When is the last time the contingency mechanism can be put in place?  This will typically be the beta freeze. -->
 
<!-- When is the last time the contingency mechanism can be put in place?  This will typically be the beta freeze. -->
* Contingency deadline: N/A (not a System Wide Change) <!-- REQUIRED FOR SYSTEM WIDE CHANGES -->
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* Contingency deadline: Beta freeze <!-- REQUIRED FOR SYSTEM WIDE CHANGES -->
 
<!-- Does finishing this feature block the release, or can we ship with the feature in incomplete state? -->
 
<!-- Does finishing this feature block the release, or can we ship with the feature in incomplete state? -->
* Blocks release? N/A (not a System Wide Change), Yes/No <!-- REQUIRED FOR SYSTEM WIDE CHANGES -->
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* Blocks release? No <!-- REQUIRED FOR SYSTEM WIDE CHANGES -->
 
* Blocks product? product <!-- Applicable for Changes that blocks specific product release/Fedora.next -->
 
* Blocks product? product <!-- Applicable for Changes that blocks specific product release/Fedora.next -->
 
Revert the kernel build option change and build it with ''CONFIG SECURITY SELINUX DISABLE=y''
 
  
 
== Documentation ==
 
== Documentation ==

Revision as of 14:33, 26 August 2020


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 if they want to disable SELinux.

Owner


Current status

  • Targeted release: Fedora 34
  • Last updated: 2020-08-26
  • FESCo issue: <will be assigned by the Wrangler>
  • Tracker bug: <will be assigned by the Wrangler>
  • Release notes tracker: <will be assigned by the Wrangler>

Detailed Description

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 selinuxfs unmounted, userspace will detect SELinux as disabled. Internally SELinux will be enabled but not initialized so that there will be no SELinux checks applied.

NOTE: 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

  • Proposal owners:
    • Make sure kernel is built without CONFIG_SECURITY_SELINUX_DISABLE
    • Make sure relevant documentation is updated in a way that selinux=0 on kernel command line is preferred way to disable SELinux
    • Make sure the installer uses kernel command line instead of /etc/selinux/config to disable SELinux
    • Optional: selinux Ansible module should warn that SELinux needs to be disabled using selinux=0
    • Optional: linux-system-roles.selinux should disable SELinux using selinux=0
  • Other developers: N/A (not a System Wide Change)
  • Policies and guidelines: N/A (not a System Wide Change)
  • Trademark approval: N/A (not needed for this Change)

Upgrade/compatibility impact

Users should not be directly affected by this change.

How To Test

N/A (not a System Wide Change)

  1. Confirm that is disabled when selinux=0 is used on kernel command line
  2. Confirm that userspace consider SELinux disabled when SELINUX=disabled is used in /etc/selinux/config
  3. Confirm that userspace consider SELinux disabled when there's no /etc/selinux/config
  4. Confirm that 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 instead of '-'. These users will also be able to load SELinux policy after boot.

Dependencies

N/A (not a System Wide Change)

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
  • Blocks product? product

Documentation

TBD

Release Notes

TBD