[PATCH v7 0/6] Safe LSM (un)loading, and immutable hooks

Sargun Dhillon sargun at sargun.me
Wed Apr 25 08:58:46 UTC 2018

The primary security benefit of this patchset is the introduction of
read-only hooks, even if some security modules have mutable hooks.
Currently, if you have any LSMs with mutable hooks it will render all
heads, and list nodes mutable. These are a prime place to attack because
being able to manipulate those hooks is a way to bypass all LSMs easily
and to create a persistent, covert channel to intercept nearly all

There is a shares SRCU between all security hooks to facilitate safe
LSM- unloading. This SRCU is very cheap for runtime overhead on reads,
but there is synchronization around it for unloads. There is only a cost
to pay at unload time, which is based on the execution time of longest
chain of callbacks after synchronization begins.

If CONFIG_SECURITY_WRITABLE_HOOKS is enabled, then hooks can be loaded
at runtime. The module can check the return code of security_add_hooks
to determine whether or not they can install their hooks, independently
of checking for the Kconfig value. In the future, we may make it so that
runtime hook loading is also disabled, after boot. We can do that in a
follow-up patch.

If CONFIG_SECURITY_UNREGISTRABLE_HOOKS is enabled, then hooks can be
unloaded at runtime. This behaviour can be disabled by setting
security.allow_unregister_hooks to 0. Once set, it requires a reboot
to be reset.

rule, and can unregister at any time. SELinux exposes a mechanism
to disable itself, prior to policy loading. This can be disabled
at compilation time, and it depends on CONFIG_SECURITY_WRITABLE_HOOKS.
Changing this behaviour would require breaking the uapi.

This patch can allow for a new style of LSMs. There are many cases
where LSM "policies" would be better defined in some formal
programming language, like C. This is either due to flexibility,
or performance, for functions in the hot path.

It also unlocks development usecases, but those come as a secondary
benefit to the earlier feature. There has been an appetite for
out-of-tree LSMs:
Casey's work has been great enabling minor LSM stacking, and a variety
of LSMs have been proposed on list that haven't made it into the kernel
but fit into the minor LSM stacking model.

One of the other big changes is the introduction of the lsm_info
structure. This encapsulates various information about the LSM
itself, and can be used at a later time.

This patch was tested with CONFIG_PROVE_LOCKING to prove the correctness
of the locking in lock_existing_hooks. It grabs the module lock
to prevent new hooks from being loaded, or current hooks from being
unloaded. It then grabs the lsm_info_lock, which is only grabbed
on hook modification, or inspection of the loaded LSMs.

The other thing that this introduces is monitoring of LKM
unloading. If an LSM's LKM is unloaded when it is not
supposed to be, it will crash the system.

Thanks to Tetsuo for their review, and feedback.

Changes since:
  * introduce LSM info
  * Crash by default instead of leaving it up to module authors
    to deal with things going wrong
  * Monitor module unloading, to ensure that attempts to
    unload the LKM for an LSM crashes the system.
    "CONFIG_SECURITY_UNREGISTRABLE_HOOKS" which enables arbitrary hook
    deregistration. If disabled, hooks are not allowed to deregister.
    SELinux is exempt.
  * Rename security.allow_unload_hooks to
  * Make it so that both security_add_hooks, and security_delete_hooks
    return errors upon being unsuccessful, allowing module authors to
    make the best decision for their use case.
  * Introduce the configuration flag "CONFIG_SECURITY_UNLOADABLE_MODULES"
    to disable module unloading
  * Introduce the kernel parameter security.allow_unload_hooks
  * Instead of taking the approach of a "null hook", using the approach of
    a second set of hooks -- this was mostly done through the
    FOR_EACH_SECURITY_HOOK_MUTABLE macro, which gets compiled out if
  * Split out hlist_head patch
  * Apply Tetsuo's changes to clean up functions which are not
      covered by call_int_hook / call_void_hook
  * Disable NULL hook checking when uneeded
  * Add SRCU to allow for code-unloading
  * Add concurrency control around hook mutation

Sargun Dhillon (6):
  security: Move LSM registration arguments to struct lsm_info
  security: Make security_hook_heads private
  security: Introduce mutable (RW) hooks
  security: Expose security_add_hooks externally and add error handling
  security: Panic on forced unloading of security module
  security: Add SECURITY_UNREGISTRABLE_HOOKS to allow for hook removal

 include/linux/lsm_hooks.h                     |  67 ++--
 scripts/gcc-plugins/randomize_layout_plugin.c |   2 -
 security/Kconfig                              |  12 +
 security/apparmor/lsm.c                       |   8 +-
 security/commoncap.c                          |   8 +-
 security/inode.c                              |  56 +++-
 security/loadpin/loadpin.c                    |   6 +-
 security/security.c                           | 421 +++++++++++++++++++++-----
 security/security.h                           |  11 +
 security/selinux/hooks.c                      |  16 +-
 security/smack/smack_lsm.c                    |   5 +-
 security/tomoyo/tomoyo.c                      |   6 +-
 security/yama/yama_lsm.c                      |   6 +-
 13 files changed, 491 insertions(+), 133 deletions(-)
 create mode 100644 security/security.h


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