Hi Andy, Lorenz, and all, 

On Jul 2, 2019, at 2:32 PM, Andy Lutomirski [off-list ref] wrote:

On Tue, Jul 2, 2019 at 2:04 PM Kees Cook [off-list ref] wrote:

quoted

On Mon, Jul 01, 2019 at 06:59:13PM -0700, Andy Lutomirski wrote:

quoted

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.

With 5.3-rc1 out, I am back on this. :)

How about we modify the set as:
  1. Introduce sys_bpf_with_cap() that takes fd of /dev/bpf. 
  2. Better handling of capable() calls through bpf code. I guess the
     biggest problem here is is_priv in verifier.c:bpf_check(). 

With this approach, we will be able to pass the fd around, so it should 
also solve problem for Go. 

Please let me know your comments/suggestions on this direction. 

Thanks,
Song