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

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Message-ID: <20200109194302.GA85350@google.com>
Date:   Thu, 9 Jan 2020 20:43:02 +0100
From:   KP Singh <kpsingh@...omium.org>
To:     James Morris <jmorris@...ei.org>
Cc:     Stephen Smalley <sds@...ho.nsa.gov>,
        Kees Cook <keescook@...omium.org>,
        KP Singh <kpsingh@...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 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@...ei.org>
>