linux-kernel - Re: [PATCH V32 01/27] Add the ability to lock down access to the running kernel image

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Date:   Fri, 3 May 2019 09:19:49 +1000 (AEST)
From:   James Morris <jmorris@...ei.org>
To:     Matthew Garrett <mjg59@...gle.com>
cc:     LSM List <linux-security-module@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        David Howells <dhowells@...hat.com>,
        Linux API <linux-api@...r.kernel.org>,
        Andy Lutomirski <luto@...nel.org>
Subject: Re: [PATCH V32 01/27] Add the ability to lock down access to the
 running kernel image

On Thu, 2 May 2019, Matthew Garrett wrote:

> On Thu, May 2, 2019 at 2:07 PM James Morris <jmorris@...ei.org> wrote:
> > One possible direction is to (as previously mentioned) assign IDs to each
> > callsite and be able to check this ID against a simple policy array
> > (allow/deny).  The default policy choices could be reduced to 'all' or
> > 'none' during kconfig, and allow a custom policy to be loaded later if
> > desired.
> 
> Ok. My primary concern around this is that it's very difficult to use
> correctly in anything other than the "all" or "none" modes. If a new
> kernel feature is added with integrated lockdown support, if an admin
> is simply setting the flags of things they wish to block then this
> will be left enabled - and may violate the admin's expectations around
> integrity. On the other hand, if an admin is simply setting the flags
> of things they wish to permit, then adding lockdown support to an
> existing kernel feature may result in that feature suddenly being
> disabled, which may also violate the admin's expectations around the
> flags providing a stable set of behaviour.

Understood. Most uses will likely be either a distro or an embedded 
system, who I'm assuming would provide a useful policy by default, and 
perhaps a high-level abstraction for modification.

> Given that, would you prefer such a policy expression to look like?

Perhaps a write-once policy, injected from userspace during early boot?

The policy could be simply a list of:

lockdown_feature true|false


> 
> > Within the policy check hook, we could add a new LSM hook, which would
> > allow an LSM to restrictively override the lockdown policy with its own
> 
> Ok, that makes sense. If we take this approach, does there need to be
> a separate policy mechanism at all? Users who want fine-grained
> control would be able to set the behaviour to "None" and then use
> their choice of LSM to express more fine-grained control.

Right, and there could be a stackable LSM which just does fine-grained 
policy (per above).


> 
> > This doesn't really address the completeness / maintenance issue (i.e. "do
> > we have everything covered and how do we ensure this on an ongoing
> > basis?", and "what will this new lockdown feature break?"), although it
> > should make it easier to add new lockdown callsites as they don't have to
> > be enabled by the user.
> 
> I can start on this.

Cool!

-- 
James Morris
<jmorris@...ei.org>