On Sun, May 18, 2025 at 11:34 PM Paul Moore <p...@paul-moore.com> wrote: > > On Sun, May 18, 2025 at 11:52 AM Alexei Starovoitov > <alexei.starovoi...@gmail.com> wrote: > > On Sat, May 17, 2025 at 10:49 PM Paul Moore <p...@paul-moore.com> wrote: > > > On May 17, 2025 12:13:50 PM Alexei Starovoitov > > > <alexei.starovoi...@gmail.com> wrote: > > > > On Sat, May 17, 2025 at 8:03 AM Paul Moore <p...@paul-moore.com> wrote: > > > >> On Fri, May 16, 2025 at 7:49 PM Alexei Starovoitov > > > >> <alexei.starovoi...@gmail.com> wrote: > > > >>> On Fri, May 16, 2025 at 12:49 PM Paul Moore <p...@paul-moore.com> > > > >>> wrote: > > > >>>> > > > >>>> I think we need some clarification on a few of these details, it > > > >>>> would > > > >>>> be good if you could answer the questions below about the > > > >>>> authorization aspects of your design? > > > >>>> > > > >>>> * Is the signature validation code in the BPF verifier *always* going > > > >>>> to be enforced when a signature is passed in from userspace? In > > > >>>> other > > > >>>> words, in your design is there going to be either a kernel build time > > > >>>> or runtime configuration knob that could selectively enable (or > > > >>>> disable) signature verification in the BPF verifier? > > > >>> > > > >>> If there is a signature in union bpf_attr and it's incorrect > > > >>> the prog_load command will be rejected. > > > >>> No point in adding a knob to control that. > > > >> > > > >> I agree that when a signature is provided and that signature check > > > >> fails, the BPF load should be rejected. I'm simply trying to > > > >> understand how you envision your design handling all of the cases, not > > > >> just this one, as well as what build and runtime options you expect > > > >> for controlling various aspects of this behavior. > > > >> > > > >>>> * In the case where the signature validation code in the BPF verifier > > > >>>> is active, what happens when a signature is *not* passed in from > > > >>>> userspace? Will the BPF verifier allow the program load to take > > > >>>> place? Will the load operation be blocked? Will the load operation > > > >>>> be subject to a more granular policy, and if so, how do you plan to > > > >>>> incorporate that policy decision into the BPF program load path? > > > >>> > > > >>> If there is no signature the existing loading semantics will remain > > > >>> intact. > > > >>> We can discuss whether to add a sysctl or cgroup knob to disallow > > > >>> loading when signature is not present ... > > > >> > > > >> As mentioned earlier this week, if the BPF verifier is performing the > > > >> signature verification as KP described, we will need a LSM hook after > > > >> the verifier to serve as an access control point. Of course that > > > >> doesn't preclude the addition of some type of sysctl/cgroup/whatever > > > >> based access control, but the LSM hook would be needed regardless. > > > > > > > > No. New hook is not needed. > > > > > > It would be good for you to explain how the existing LSM hook is > > > sufficient > > > to authorize the loading of a BPF program using the signature validation > > > state determined in the BPF verifier. > > > > I already explained: > > .. a job of trivial LSM: > > if (prog_attr doesn't have signature && > > (task == .. || task is under certain cgroup || whatever)) > > disallow. > > I read that earlier reply as an example that covers a sample use case, > I didn't realize you were asserting that was the only approach you > were considering. Perhaps that was the source of confusion earlier, > we may disagree, but I don't intentionally "twist" words; not only is > that rude, it's just stupid in public, archived discussions. > > As I mentioned previously, we really need to see an explicit yes/no > flag from the BPF verifier to indicate that the signature on the BPF > program has been validated. It really should be as simple as adding a > bool to bpf_prog_aux which the BPF verifier sets to true upon > successful signature validation, and then an LSM can use this flag as > input to an access control decision in a hook placed after the > verifier. Are you objecting to the addition of a flag in the > bpf_prog_aux struct (or some other struct tightly coupled to the BPF > program), the LSM hook after the verifier, or both? It would also be > helpful if you can elaborate on the technical reasons behind these > objections.
Neither the aux field, nor the hook are required because: * If the signature is passed, it will be enforced, there are no "runtime aspects" that need to be configurable here. * What the LSM can specify a policy for is when a signature is not passed, for this, it does not need an aux field or a signature or the new hook, existing hooks are sufficient. - KP > > -- > paul-moore.com
