linux-kernel - Re: [RFC 0/3] WhiteEgret LSM module

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Message-ID: <20170530205002.GA9841@srcf.ucam.org>
Date:   Tue, 30 May 2017 21:50:02 +0100
From:   Matthew Garrett <mjg59@...gle.com>
To:     Masanobu Koike <masanobu2.koike@...hiba.co.jp>
Cc:     james.l.morris@...cle.com, serge@...lyn.com,
        linux-security-module@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC 0/3] WhiteEgret LSM module

On Tue, May 30, 2017 at 08:11:57PM +0900, Masanobu Koike wrote:
> An execution-whitelist, simply called whitelist, is a list
> of executable components (e.g., applications, libraries)
> that are approved to run on a host. The whitelist is used
> to decide whether executable components are permitted to
> execute or not. This mechanism can stop an execution of
> unknown software, so it helps to stop the execution of
> malicious code and other unauthorized software.
> The whitelisting-type execution control works best in the
> execution environments that are not changed for a long time,
> for example, servers and control devices in industrial
> control systems. This RFC provides a whitelisting-type
> execution control implementation WhiteEgret.

There's a few assumptions made here:

1) The system isn't subject to any form of offline attack. If it is, the 
attacker can simply replace either the whitelist agent or any of the 
executables.
2) The system contains no whitelisted executables that will execute 
arbitrary code. This means not shipping perl or python.
3) None of the whitelisted applications on the system will misbehave if 
fed invalid input.
4) It's impossible for a user to ptrace() any other process after it's 
been executed, and simply inject new code.

(3) is especially awkward. If you're implementing this as an LSM then 
you're giving up on being able to use any of the other LSMs to protect 
you against this - a vulnerability in a single application bypasses your 
entire security model.

The kernel already has support for application whitelisting in the form 
of IMA appraisal. All you need to do is sign the apps that you want 
whitelisted and then load a policy at runtime that enforces appraisal. 
Anything that's unsigned will then fail to execute. This deals with (1) 
(assuming that you load the policy from something that's validated 
earlier in the boot process), is no worse with respect to (2), and still 
allows you to use SELinux or Apparmor to mitigate (3) and (4). You also 
gain additional reliability by not having the system fail in the event 
of a bug in the whitelisting agent causing it to crash.

I think it would be helpful to have more details of exactly what 
circumstances this is intended to be used in and then figure out whether 
there's any way to use existing kernel functionality to provide the same 
benefits.
-- 
Matthew Garrett | mjg59@...f.ucam.org