[PATCH V33 24/30] bpf: Restrict bpf when kernel lockdown is in confidentiality mode

[Matthew Garrett]

On Thu, Jun 27, 2019 at 1:16 PM Stephen Smalley <sds at tycho.nsa.gov> wrote:
> 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.