Re: [RFC PATCH] selinux: add SELinux hooks for lockdown integrity and confidentiality

From: Paul Moore <paul@paul-moore.com>
To: Stephen Smalley <sds@tycho.nsa.gov>
Cc: selinux@vger.kernel.org
Subject: Re: [RFC PATCH] selinux: add SELinux hooks for lockdown integrity and confidentiality
Date: Fri, 8 Nov 2019 13:38:53 -0500	[thread overview]
Message-ID: <CAHC9VhQnoGLM7H-vR-kDBgdib5gPehVpNPr+d85VRFs5UYHnqQ@mail.gmail.com> (raw)
In-Reply-To: <14181dcc-aa6f-8fbb-6244-08ed39591f30@tycho.nsa.gov>

On Thu, Nov 7, 2019 at 1:07 PM Stephen Smalley <sds@tycho.nsa.gov> wrote:
> On 11/7/19 12:48 PM, Paul Moore wrote:
>  > On Thu, Oct 31, 2019 at 10:01 AM Stephen Smalley <sds@tycho.nsa.gov>
> wrote:
> > That is an interesting question: do we consider dmesg output to be
> > part of the stable kernel API?  My hunch would be "no", as I've seen
> > things change quite a bit there over the years, but IANL (I Am Not
> > Linus).  However, that said, logging a reason string via audit seems
> > like a good idea (especially since there is presently a many-to-one
> > mapping between reasons and the SELinux permission).  Further, while
> > the audit field name is part of the kernel API, the value is much more
> > open.
>
> Ok, any preferences on the audit field name or should we just create one
> and cc linux-audit on the next RFC?  lockdown_reason=?

Definitely CC linux-audit as I expect Steve will want to have his say.
FWIW, "lockdown_reason" seems reasonable to me.

> >> I also wasn't sure about the pr_warn() above.  If we reach it, it is
> >> effectively a kernel bug. We could mirror what the lockdown module does
> >> in lockdown_is_locked_down(), i.e. use WARN() instead.  Of course, the
> >> SELinux hook won't even be reached in this case if the lockdown module
> >> is enabled, but the lockdown module could be disabled so I guess we need
> >> to check it too.
> >
> > Since this seems security relevant, I wonder if we should be using SELINUX_ERR?
>
> The benefit of a WARN() is that it will give us a stack trace showing
> the offending caller in the kernel, which would be useful since it would
> be a buggy caller passing an invalid lockdown reason (LOCKDOWN_NONE or
>  >= LOCKDOWN_CONFIDENTIALITY_MAX).  pr_warn() or audit_log() won't give
> us that info.  We could do both of course.

It's a balance between development needs and freaking out
administrators (although perhaps rightly so).  I also worry a bit that
WARN can be disabled at build time so having something like
SELINUX_ERR could be a good fallback, assuming we did both.

-- 
paul moore
www.paul-moore.com