Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
From:   KP Singh <kpsingh@chromium.org>
Date:   Thu, 9 Jan 2020 20:43:02 +0100
To:     James Morris <jmorris@namei.org>
Cc:     Stephen Smalley <sds@tycho.nsa.gov>,
        Kees Cook <keescook@chromium.org>,
        KP Singh <kpsingh@chromium.org>,
        Casey Schaufler <casey@schaufler-ca.com>,
        open list <linux-kernel@vger.kernel.org>,
        bpf <bpf@vger.kernel.org>, linux-security-module@vger.kernel.org,
        Alexei Starovoitov <ast@kernel.org>,
        Daniel Borkmann <daniel@iogearbox.net>,
        Thomas Garnier <thgarnie@chromium.org>,
        Michael Halcrow <mhalcrow@google.com>,
        Paul Turner <pjt@google.com>,
        Brendan Gregg <brendan.d.gregg@gmail.com>,
        Jann Horn <jannh@google.com>,
        Matthew Garrett <mjg59@google.com>,
        Christian Brauner <christian@brauner.io>,
        =?iso-8859-1?Q?Micka=EBl_Sala=FCn?= <mic@digikod.net>,
        Florent Revest <revest@chromium.org>,
        Brendan Jackman <jackmanb@chromium.org>,
        Martin KaFai Lau <kafai@fb.com>,
        Song Liu <songliubraving@fb.com>, Yonghong Song <yhs@fb.com>,
        "Serge E. Hallyn" <serge@hallyn.com>,
        Mauro Carvalho Chehab <mchehab+samsung@kernel.org>,
        "David S. Miller" <davem@davemloft.net>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Nicolas Ferre <nicolas.ferre@microchip.com>,
        Stanislav Fomichev <sdf@google.com>,
        Quentin Monnet <quentin.monnet@netronome.com>,
        Andrey Ignatov <rdna@fb.com>, Joe Stringer <joe@wand.net.nz>,
        Paul Moore <paul@paul-moore.com>
Subject: Re: [PATCH bpf-next v1 00/13] MAC and Audit policy using eBPF (KRSI)
Message-ID: <20200109194302.GA85350@google.com>
References: <20191220154208.15895-1-kpsingh@chromium.org>
 <95036040-6b1c-116c-bd6b-684f00174b4f@schaufler-ca.com>
 <CACYkzJ5nYh7eGuru4vQ=2ZWumGPszBRbgqxmhd4WQRXktAUKkQ@mail.gmail.com>
 <201912301112.A1A63A4@keescook>
 <c4e6cdf2-1233-fc82-ca01-ba84d218f5aa@tycho.nsa.gov>
 <alpine.LRH.2.21.2001090551000.27794@namei.org>
 <e59607cc-1a84-cbdd-5117-7efec86b11ff@tycho.nsa.gov>
 <alpine.LRH.2.21.2001100437550.21515@namei.org>
 <e90e03e3-b92f-6e1a-132f-1b648d9d2139@tycho.nsa.gov>
 <alpine.LRH.2.21.2001100558550.31925@namei.org>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <alpine.LRH.2.21.2001100558550.31925@namei.org>
User-Agent: Mutt/1.10.1 (2018-07-13)
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

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.

- 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@namei.org>
>