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

From: KP Singh <kpsingh@chromium.org>
To: Stephen Smalley <sds@tycho.nsa.gov>
Cc: James Morris <jmorris@namei.org>,
	linux-kernel@vger.kernel.org, bpf@vger.kernel.org,
	linux-security-module@vger.kernel.org,
	Brendan Jackman <jackmanb@google.com>,
	Florent Revest <revest@google.com>,
	Alexei Starovoitov <ast@kernel.org>,
	Daniel Borkmann <daniel@iogearbox.net>,
	Kees Cook <keescook@chromium.org>, Paul Turner <pjt@google.com>,
	Jann Horn <jannh@google.com>,
	Florent Revest <revest@chromium.org>,
	Brendan Jackman <jackmanb@chromium.org>,
	Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	Paul Moore <paul@paul-moore.com>
Subject: Re: [PATCH bpf-next v7 4/8] bpf: lsm: Implement attach, detach and execution
Date: Fri, 27 Mar 2020 15:29:43 +0100	[thread overview]
Message-ID: <20200327142943.GA23618@chromium.org> (raw)
In-Reply-To: <14ff822f-3ca5-7ebb-3df6-dd02249169d2@tycho.nsa.gov>

On 27-Mär 09:43, 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

Okay. For the scope of this series I will remove this check in the
next revision. If people feel strongly that we need it centrally
within the BPF infrastructure, we can do that as a separate patch and
discuss it there.

> a bpf_prog hook for Smack and AppArmor if they want that.  SELinux would
> implement its own check in its existing bpf_prog hook.

I think Smack and AppArmor can also use the same hook. Since we
already have a hook, I don't think anyone is blocked from
implementing policy logic for loading LSM BPF programs.

James/Kees does this sound okay?

- KP

> 
> 
> 