linux-kernel - Re: [PATCH bpf-next v1 00/13] MAC and Audit policy using eBPF (KRSI)

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Message-ID: <8e035f4d-5120-de6a-7ac8-a35841a92b8a@tycho.nsa.gov>
Date:   Thu, 9 Jan 2020 14:47:53 -0500
From:   Stephen Smalley <sds@...ho.nsa.gov>
To:     KP Singh <kpsingh@...omium.org>, James Morris <jmorris@...ei.org>
Cc:     Kees Cook <keescook@...omium.org>,
        Casey Schaufler <casey@...aufler-ca.com>,
        open list <linux-kernel@...r.kernel.org>,
        bpf <bpf@...r.kernel.org>, linux-security-module@...r.kernel.org,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Thomas Garnier <thgarnie@...omium.org>,
        Michael Halcrow <mhalcrow@...gle.com>,
        Paul Turner <pjt@...gle.com>,
        Brendan Gregg <brendan.d.gregg@...il.com>,
        Jann Horn <jannh@...gle.com>,
        Matthew Garrett <mjg59@...gle.com>,
        Christian Brauner <christian@...uner.io>,
        Mickaël Salaün <mic@...ikod.net>,
        Florent Revest <revest@...omium.org>,
        Brendan Jackman <jackmanb@...omium.org>,
        Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        "Serge E. Hallyn" <serge@...lyn.com>,
        Mauro Carvalho Chehab <mchehab+samsung@...nel.org>,
        "David S. Miller" <davem@...emloft.net>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Nicolas Ferre <nicolas.ferre@...rochip.com>,
        Stanislav Fomichev <sdf@...gle.com>,
        Quentin Monnet <quentin.monnet@...ronome.com>,
        Andrey Ignatov <rdna@...com>, Joe Stringer <joe@...d.net.nz>,
        Paul Moore <paul@...l-moore.com>
Subject: Re: [PATCH bpf-next v1 00/13] MAC and Audit policy using eBPF (KRSI)

On 1/9/20 2:43 PM, KP Singh wrote:
> On 10-Jan 06:07, James Morris wrote:
>> On Thu, 9 Jan 2020, Stephen Smalley wrote:
>>
>>> On 1/9/20 1:11 PM, James Morris wrote:
>>>> On Wed, 8 Jan 2020, Stephen Smalley wrote:
>>>>
>>>>> The cover letter subject line and the Kconfig help text refer to it as a
>>>>> BPF-based "MAC and Audit policy".  It has an enforce config option that
>>>>> enables the bpf programs to deny access, providing access control. IIRC,
>>>>> in
>>>>> the earlier discussion threads, the BPF maintainers suggested that Smack
>>>>> and
>>>>> other LSMs could be entirely re-implemented via it in the future, and that
>>>>> such an implementation would be more optimal.
>>>>
>>>> In this case, the eBPF code is similar to a kernel module, rather than a
>>>> loadable policy file.  It's a loadable mechanism, rather than a policy, in
>>>> my view.
>>>
>>> I thought you frowned on dynamically loadable LSMs for both security and
>>> correctness reasons?
> 
> Based on the feedback from the lists we've updated the design for v2.
> 
> In v2, LSM hook callbacks are allocated dynamically using BPF
> trampolines, appended to a separate security_hook_heads and run
> only after the statically allocated hooks.
> 
> The security_hook_heads for all the other LSMs (SELinux, AppArmor etc)
> still remains __lsm_ro_after_init and cannot be modified. We are still
> working on v2 (not ready for review yet) but the general idea can be
> seen here:
> 
>    https://github.com/sinkap/linux-krsi/blob/patch/v1/trampoline_prototype/security/bpf/lsm.c
> 
>>
>> Evaluating the security impact of this is the next step. My understanding
>> is that eBPF via BTF is constrained to read only access to hook
>> parameters, and that its behavior would be entirely restrictive.
>>
>> I'd like to understand the security impact more fully, though.  Can the
>> eBPF code make arbitrary writes to the kernel, or read anything other than
>> the correctly bounded LSM hook parameters?
>>
> 
> As mentioned, the BPF verifier does not allow writes to BTF types.
> 
>>> And a traditional security module would necessarily fall
>>> under GPL; is the eBPF code required to be likewise?  If not, KRSI is a
>>> gateway for proprietary LSMs...
>>
>> Right, we do not want this to be a GPL bypass.
> 
> This is not intended to be a GPL bypass and the BPF verifier checks
> for license compatibility of the loaded program with GPL.

IIUC, it checks that the program is GPL compatible if it uses a function 
marked GPL-only.  But what specifically is marked GPL-only that is 
required for eBPF programs using KRSI?

> 
> - KP
> 
>>
>> If these issues can be resolved, this may be a "safe" way to support
>> loadable LSM applications.
>>
>> Again, I'd be interested in knowing how this is is handled in the
>> networking stack (keeping in mind that LSM is a much more invasive API,
>> and may not be directly comparable).
>>
>> -- 
>> James Morris
>> <jmorris@...ei.org>
>>