linux-kernel - [PATCH v3 0/1] Safe LSM (un)loading, and immutable hooks

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Date:   Thu, 29 Mar 2018 21:14:13 +0000
From:   Sargun Dhillon <sargun@...gun.me>
To:     linux-security-module@...r.kernel.org, linux-kernel@...r.kernel.org
Cc:     penguin-kernel@...ove.sakura.ne.jp, keescook@...omium.org,
        igor.stoppa@...wei.com, casey@...aufler-ca.com, jmorris@...ei.org,
        sds@...ho.nsa.gov, paul@...l-moore.com, plautrba@...hat.com
Subject: [PATCH v3 0/1] Safe LSM (un)loading, and immutable hooks

The biggest security benefit of this patchset is the introduction of
read-only hooks. Currently, if you have any LSMs with mutable hooks
it will render all heads, and list nodes mutable. This is a prime
place to attack, because being able to manipulate those hooks is a
way to bypass all LSMs easily.

By moving to hlist_head, it keeps a singly-linked, non-circular list.
This can always be marked as read only because we add a mutable "null"
hook. All immutable LSMs should be installed as immutable hooks and
sit before the null hook.

+------+   +-----------+   +-----------+   +-----------+   +------------------+
|      |   |           |   |           |   |           |   |                  |
| HEAD +---> Immutable +---> Immutable +---> Null hook +--->   Mutable Hook   |
|      |   |  Hook 1   |   |  Hook 2   |   |           |   |                  |
+------+   +-----------+   +-----------+   +-----------+   +------------------+
                 |               |                                  |
                 v               v                                  v
             Callback        Callback                           Callback

If LSMs have a model to be unloaded, or are compled as modules,
they should mark themselves mutable at runtime.

In order to provide safe code-unloading, there is a shared SRCU between
all security hooks. 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.

Because of all of this, we can now load LSMs at runtime, so those APIs
are exposed. It is up to the module author to check if
CONFIG_SECURITY_WRITABLE_HOOKS is enabled prior to trying to load.

Changes since:
v2:
	* 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
v1:
	* Add SRCU to allow for code-unloading
	* Add concurrency control around hook mutation

Sargun Dhillon (1):
  security: Add mechanism to safely (un)load LSMs after boot time

 include/linux/lsm_hooks.h  |  23 ++---
 security/Kconfig           |   2 +-
 security/apparmor/lsm.c    |   2 +-
 security/commoncap.c       |   2 +-
 security/security.c        | 210 ++++++++++++++++++++++++++++++++++++++-------
 security/selinux/hooks.c   |   5 +-
 security/smack/smack_lsm.c |   3 +-
 security/tomoyo/tomoyo.c   |   3 +-
 security/yama/yama_lsm.c   |   2 +-
 9 files changed, 196 insertions(+), 56 deletions(-)

-- 
2.14.1