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.)

-- 
Kees Cook