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

[KP Singh <kpsingh@chromium.org>]

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>
>