Re: [PATCH v2 bpf-next 1/4] bpf: unprivileged BPF access via /dev/bpf

[Song Liu <songliubraving@fb.com>]

Hi Andy, Lorenz, and all, 

> On Jul 2, 2019, at 2:32 PM, Andy Lutomirski <luto@kernel.org> wrote:
> 
> On Tue, Jul 2, 2019 at 2:04 PM Kees Cook <keescook@chromium.org> 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.

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