linux-kernel - Re: [PATCH v3 1/1] security: Add CONFIG_LSM_AUTO to handle default LSM stack ordering

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Message-ID: <202210172153.C65BF23D5E@keescook>
Date:   Mon, 17 Oct 2022 22:55:30 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Paul Moore <paul@...l-moore.com>
Cc:     Casey Schaufler <casey@...aufler-ca.com>,
        Nicolas Iooss <nicolas.iooss@....org>,
        Mickaël Salaün <mic@...ikod.net>,
        James Morris <jmorris@...ei.org>,
        "Serge E . Hallyn" <serge@...lyn.com>,
        linux-kernel@...r.kernel.org,
        linux-security-module@...r.kernel.org,
        Mickaël Salaün <mic@...ux.microsoft.com>
Subject: Re: [PATCH v3 1/1] security: Add CONFIG_LSM_AUTO to handle default
 LSM stack ordering

On Mon, Oct 17, 2022 at 09:45:21PM -0400, Paul Moore wrote:
> The code sorta cares about ordering, at least to the extent that the
> LSMs will behave differently depending on the ordering, e.g. a LSM

Right -- this is why I've been so uncomfortable with allowing
arbitrarily reordering of the LSM list from lsm=. There are orderings we
know work, and others may have undesirable side-effects. I'd much rather
the kernel be specific about the order.

> I personally would like to preserve the existing concept where "built"
> does *not* equate to "enabled" by default.

Yup, understood. I didn't think I was going to win over anyone on that
one, but figured I'd just point it out again. ;)

> > I *still* think there should be a way to leave ordering alone and have
> > separate enable/disable control.
> 
> My current opinion is that enabling a LSM and specifying its place in
> an ordered list are one in the same.  The way LSM stacking as
> currently done almost requires the ability to specify an order if an
> admin is trying to meet an security relevant operation visibility
> goal.

As in an admin wants to see selinux rejections instead of loadpin
rejections for a blocked module loading?

Hmmm. Is this a realistic need?

> We can have defaults, like we do know, but I'm in no hurry to remove
> the ability to allow admins to change the ordering at boot time.

My concern is with new LSMs vs the build system. A system builder will
be prompted for a new CONFIG_SECURITY_SHINY, but won't be prompted
about making changes to CONFIG_LSM to include it.

Even booting with "lsm.debug" isn't entirely helpful to helping someone
construct the "lsm=" option they actually want... I guess I can fix that
part, at least. :)

-- 
Kees Cook