On Thu, 2 May 2019, Matthew Garrett wrote:

On Thu, May 2, 2019 at 2:07 PM James Morris [off-list ref] wrote:

quoted

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

quoted

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).

quoted

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
[off-list ref]