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

[Stephen Smalley]

On 3/27/20 8:41 AM, KP Singh wrote:
> On 27-MÃr 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@xxxxxxxxxxxx/
>
> 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:
>
> 202003241133.16C02BE5B@keescook">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.