> On Jul 3, 2019, at 5:32 AM, Andy Lutomirski <luto@xxxxxxxxxx> wrote: > > On Tue, Jul 2, 2019 at 2:04 PM Kees Cook <keescook@xxxxxxxxxxxx> wrote: >> >> On Mon, Jul 01, 2019 at 06:59:13PM -0700, Andy Lutomirski wrote: >>> I think I'm understanding your motivation. You're not trying to make >>> bpf() generically usable without privilege -- you're trying to create >>> a way to allow certain users to access dangerous bpf functionality >>> within some limits. >>> >>> That's a perfectly fine goal, but I think you're reinventing the >>> wheel, and the wheel you're reinventing is quite complicated and >>> already exists. I think you should teach bpftool to be secure when >>> installed setuid root or with fscaps enabled and put your policy in >>> bpftool. If you want to harden this a little bit, it would seem >>> entirely reasonable to add a new CAP_BPF_ADMIN and change some, but >>> not all, of the capable() checks to check CAP_BPF_ADMIN instead of the >>> capabilities that they currently check. >> >> If finer grained controls are wanted, it does seem like the /dev/bpf >> path makes the most sense. open, request abilities, use fd. The open can >> be mediated by DAC and LSM. The request can be mediated by LSM. This >> provides a way to add policy at the LSM level and at the tool level. >> (i.e. For tool-level controls: leave LSM wide open, make /dev/bpf owned >> by "bpfadmin" and bpftool becomes setuid "bpfadmin". For fine-grained >> controls, leave /dev/bpf wide open and add policy to SELinux, etc.) >> >> With only a new CAP, you don't get the fine-grained controls. (The >> "request abilities" part is the key there.) > > Sure you do: the effective set. It has somewhat bizarre defaults, but > I don't think that's a real problem. Also, this wouldn't be like > CAP_DAC_READ_SEARCH -- you can't accidentally use your BPF caps. > > I think that a /dev capability-like object isn't totally nuts, but I > think we should do it well, and this patch doesn't really achieve > that. But I don't think bpf wants fine-grained controls like this at > all -- as I pointed upthread, a fine-grained solution really wants > different treatment for the different capable() checks, and a bunch of > them won't resemble capabilities or /dev/bpf at all. Thanks everyone again for great inputs. We will discuss this again and respin the set. Best, Song
