linux-kernel - Re: [PATCH 16/18] LSM: Allow arbitrary LSM ordering

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Date:   Sun, 16 Sep 2018 11:49:31 -0700
From:   Casey Schaufler <casey@...aufler-ca.com>
To:     Kees Cook <keescook@...omium.org>, James Morris <jmorris@...ei.org>
Cc:     John Johansen <john.johansen@...onical.com>,
        Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>,
        Paul Moore <paul@...l-moore.com>,
        Stephen Smalley <sds@...ho.nsa.gov>,
        "Schaufler, Casey" <casey.schaufler@...el.com>,
        LSM <linux-security-module@...r.kernel.org>,
        LKLM <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 16/18] LSM: Allow arbitrary LSM ordering

On 9/15/2018 5:30 PM, Kees Cook wrote:
> To prepare for having a third type of LSM ("shared blob"), this implements
> dynamic handling of LSM ordering. The visible change here is that the
> "security=" boot commandline is now a comma-separated ordered list of
> all LSMs, not just the single "exclusive" LSM. This means that the
> "minor" LSMs can now be disabled at boot time by omitting them from the
> commandline. Additionally LSM ordering becomes entirely mutable for LSMs
> with LSM_ORDER_MUTABLE ("capability" is not mutable and is always enabled
> first).

Today if I have Yama enabled and use security=apparmor I get a
module list of capability,yama,apparmor. With this change I would
get a different result, capability,apparmor. I am personally OK with
this, but I think others may see it as a violation of compatibility.

One solution is to leave security= as is, not affecting "minor"
modules and only allowing specification of one major module, and adding
another boot option security.stack= that overrides a security= option
and that takes the list as you've implemented here.

An icky alternative would be to say that any security= specification
with no commas in it retains the old behavior. So
	security=apparmor
	security=apparmor,
would get you
	capability,yama,apparmor
	capability,apparmor
respectively.

Another option would be to require negation on the minor modules,
such as
	security=apparmor,-loadpin

I can't honestly say which I like least or best.