On Wed, Jun 2, 2021 at 8:40 AM Daniel Borkmann <daniel@xxxxxxxxxxxxx> 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 www.paul-moore.com
