Re: [PATCH 23/27] bpf: Restrict kernel image access functions when the kernel is locked down

[Andy Lutomirski]

On Tue, Mar 26, 2019 at 11:57 AM James Morris <jmorris@xxxxxxxxx> wrote:
>
> On Mon, 25 Mar 2019, Andy Lutomirski wrote:
>
> > A while back, I suggested an approach to actually make this stuff
> > mergeable: submit a patch series that adds lockdown mode, enables it
> > by command line option (and maybe sysctl) *only* and has either no
> > effect or only a token effect.  Then we can add actual features to
> > lockdown mode one at a time and review them separately.
>
> This makes sense to me.
>
> >
> > And I'm going to complain loudly unless two things change about this
> > whole thing:
> >
> > 1. Lockdown mode becomes three states, not a boolean.  The states are:
> > no lockdown, best-effort-to-protect-kernel-integrity, and
> > best-effort-to-protect-kernel-secrecy-and-integrity.  And this BPF
> > mess illustrates why: most users will really strongly object to
> > turning off BPF when they actually just want to protect kernel
> > integrity.  And as far as I know, things like Secure Boot policy will
> > mostly care about integrity, not secrecy, and tracing and such should
> > work on a normal locked-down kernel.  So I think we need this knob.
>
> Another approach would be to make this entirely policy based:
>
> - Assign an ID to each lockdown point
> - Implement a policy mechanism where each ID is mapped to 0 or 1
> - Allow this policy to be specified statically or dynamically
>
> So,
>
>         kernel_is_locked_down("ioperm")
>
> becomes
>
>         kernel_is_locked_down(LOCKDOWN_IOPERM)
>
> and this function checks e.g.
>
>         if (lockdown_polcy[id]) {
>                 fail or warn;
>         }
>
> Thoughts?

I'm concerned that this gives too much useless flexibility to
administrators and user code in general.  If you can break kernel
integrity, you can break kernel integrity -- it shouldn't really
matter *how* you break it.

--Andy