[PATCH v2] lockdown,selinux: avoid bogus SELinux lockdown permission checks

Paul Moore paul at paul-moore.com
Wed Jun 2 15:13:27 UTC 2021

On Wed, Jun 2, 2021 at 8:40 AM Daniel Borkmann <daniel at iogearbox.net> wrote:
> On 6/1/21 10:47 PM, Paul Moore wrote:
> > The thing I'm worried about would be the case where a LSM policy
> > change requires that an existing BPF program be removed or disabled.
> > I'm guessing based on the refcounting that there is not presently a
> > clean way to remove a BPF program from the system, but is this
> > something we could resolve?  If we can't safely remove a BPF program
> > from the system, can we replace/swap it with an empty/NULL BPF
> > program?
> Removing progs would somehow mean destroying those references from an
> async event and then /safely/ guaranteeing that nothing is accessing
> them anymore. But then if policy changes once more where they would
> be allowed again we would need to revert back to the original state,
> which brings us to your replace/swap question with an empty/null prog.
> It's not feasible either, because there are different BPF program types
> and they can have different return code semantics that lead to subsequent
> actions. If we were to replace them with an empty/NULL program, then
> essentially this will get us into an undefined system state given it's
> unclear what should be a default policy for each program type, etc.
> Just to pick one simple example, outside of tracing, that comes to mind:
> say, you attached a program with tc to a given device ingress hook. That
> program implements firewalling functionality, and potentially deep down
> in that program there is functionality to record/sample packets along
> with some meta data. Part of what is exported to the ring buffer to the
> user space reader may be a struct net_device field that is otherwise not
> available (or at least not yet), hence it's probe-read with mentioned
> helpers. If you were now to change the SELinux policy for that tc loader
> application, and therefore replace/swap the progs in the kernel that were
> loaded with it (given tc's lockdown policy was recorded in their sec blob)
> with an empty/NULL program, then either you say allow-all or drop-all,
> but either way, you break the firewalling functionality completely by
> locking yourself out of the machine or letting everything through. There
> is no sane way where we could reason about the context/internals of a
> given program where it would be safe to replace with a simple empty/NULL
> prog.

Help me out here, is your answer that the access check can only be
done at BPF program load time?  That isn't really a solution from a
SELinux perspective as far as I'm concerned.

I understand the ideas I've tossed out aren't practical from a BPF
perspective, but it would be nice if we could find something that does
work.  Surely you BPF folks can think of some way to provide a
runtime, not load time, check?

paul moore

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