Re: [PATCH bpf-next v7 4/8] bpf: lsm: Implement attach, detach and execution

[Kees Cook <keescook@chromium.org>]

On Fri, Mar 27, 2020 at 09:36:15AM -0700, Casey Schaufler wrote:
> On 3/27/2020 6:43 AM, Stephen Smalley wrote:
> > On 3/27/20 8:41 AM, KP Singh wrote:
> >> On 27-Mär 08:27, Stephen Smalley wrote:
> >>> On 3/26/20 8:24 PM, James Morris wrote:
> >>>> On Thu, 26 Mar 2020, KP Singh wrote:
> >>>>
> >>>>> +int bpf_lsm_verify_prog(struct bpf_verifier_log *vlog,
> >>>>> +            const struct bpf_prog *prog)
> >>>>> +{
> >>>>> +    /* Only CAP_MAC_ADMIN users are allowed to make changes to LSM hooks
> >>>>> +     */
> >>>>> +    if (!capable(CAP_MAC_ADMIN))
> >>>>> +        return -EPERM;
> >>>>> +
> >>>>
> >>>> Stephen, can you confirm that your concerns around this are resolved
> >>>> (IIRC, by SELinux implementing a bpf_prog callback) ?
> >>>
> >>> I guess the only residual concern I have is that CAP_MAC_ADMIN means
> >>> something different to SELinux (ability to get/set file security contexts
> >>> unknown to the currently loaded policy), so leaving the CAP_MAC_ADMIN check
> >>> here (versus calling a new security hook here and checking CAP_MAC_ADMIN in
> >>> the implementation of that hook for the modules that want that) conflates
> >>> two very different things.  Prior to this patch, there are no users of
> >>> CAP_MAC_ADMIN outside of individual security modules; it is only checked in
> >>> module-specific logic within apparmor, safesetid, selinux, and smack, so the
> >>> meaning was module-specific.
> >>
> >> As we had discussed, We do have a security hook as well:
> >>
> >> https://lore.kernel.org/bpf/20200324180652.GA11855@chromium.org/
> >>
> >> The bpf_prog hook which can check for BPF_PROG_TYPE_LSM and implement
> >> module specific logic for LSM programs. I thougt that was okay?
> >>
> >> Kees was in favor of keeping the CAP_MAC_ADMIN check here:
> >>
> >> https://lore.kernel.org/bpf/202003241133.16C02BE5B@keescook
> >>
> >> If you feel strongly and Kees agrees, we can remove the CAP_MAC_ADMIN
> >> check here, but given that we already have a security hook that meets
> >> the requirements, we probably don't need another one.
> >
> > I would favor removing the CAP_MAC_ADMIN check here, and implementing it in a bpf_prog hook for Smack and AppArmor if they want that.  SELinux would implement its own check in its existing bpf_prog hook.
> >
> The whole notion of one security module calling into another for permission
> to do something still gives me the heebee jeebees, but if more nimble minds
> than mine think this is a good idea I won't nack it.

Well, it's a hook into BPF prog creation, not the BPF LSM specifically,
so that's why I think it's general enough control without it being
directly weird. :)

As far as dropping CAP_MAC_ADMIN, yeah, that should be fine. Creating LSM
BPF programs already requires CAP_SYS_ADMIN, so for SELinux-less systems,
that's likely fine. If we need to change the BPF program creation access
control in the future we can revisit it then.

-- 
Kees Cook