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

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Message-ID: <14ff822f-3ca5-7ebb-3df6-dd02249169d2@tycho.nsa.gov>
Date:   Fri, 27 Mar 2020 09:43:45 -0400
From:   Stephen Smalley <sds@...ho.nsa.gov>
To:     KP Singh <kpsingh@...omium.org>
Cc:     James Morris <jmorris@...ei.org>, linux-kernel@...r.kernel.org,
        bpf@...r.kernel.org, linux-security-module@...r.kernel.org,
        Brendan Jackman <jackmanb@...gle.com>,
        Florent Revest <revest@...gle.com>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Kees Cook <keescook@...omium.org>,
        Paul Turner <pjt@...gle.com>, Jann Horn <jannh@...gle.com>,
        Florent Revest <revest@...omium.org>,
        Brendan Jackman <jackmanb@...omium.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Paul Moore <paul@...l-moore.com>
Subject: Re: [PATCH bpf-next v7 4/8] bpf: lsm: Implement attach, detach and
 execution

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.