On Thu, Jun 27, 2019 at 4:16 PM Matthew Garrett [off-list ref] wrote:

On Thu, Jun 27, 2019 at 1:16 PM Stephen Smalley [off-list ref] wrote:

quoted

That would only allow the LSM to further lock down the system above the
lockdown level set at boot, not grant exemptions for specific
functionality/interfaces required by the user or by a specific
process/program. You'd have to boot with lockdown=none (or your
lockdown=custom suggestion) in order for the LSM to allow anything
covered by the integrity or confidentiality levels.  And then the kernel
would be unprotected prior to full initialization of the LSM, including
policy load.

It seems like one would want to be able to boot with lockdown=integrity
to protect the kernel initially, then switch over to allowing the LSM to
selectively override it.

One option would be to allow modules to be "unstacked" at runtime, but
there's still something of a problem here - how do you ensure that
your userland can be trusted to load a new policy before it does so?
If you're able to assert that your early userland is trustworthy
(perhaps because it's in an initramfs that's part of your signed boot
payload), there's maybe an argument that most of the lockdown
integrity guarantees are unnecessary before handoff - just using the
lockdown LSM to protect against attacks via kernel parameters would be
sufficient.

I think that, if you don't trust your system enough to avoid
compromising itself before policy load, then your MAC policy is more
or less dead in the water.  It seems to be that it ought to be good
enough to boot with lockdown=none and then have a real policy loaded
along with the rest of the MAC policy.  Or, for applications that need
to be stricter, you accept that MAC policy can't override lockdown.