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From:   KP Singh <kpsingh@chromium.org>
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Date:   Fri, 10 Jan 2020 16:27:58 +0100
To:     Stephen Smalley <sds@tycho.nsa.gov>
Cc:     KP Singh <kpsingh@chromium.org>, James Morris <jmorris@namei.org>,
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Subject: Re: [PATCH bpf-next v1 00/13] MAC and Audit policy using eBPF (KRSI)
Message-ID: <20200110152758.GA260168@google.com>
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On 09-Jan 14:47, Stephen Smalley wrote:
> 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?

Good point! If no-one objects, I can add it to the BPF_PROG_TYPE_LSM
specific verification for the v2 of the patch-set which would require
all BPF-LSM programs to be GPL.

- KP

> 
> > 
> > - 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>
> > > 
>