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

[Stephen Smalley]

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? 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...

> This would be similar to the difference between iptables rules and
> loadable eBPF networking code.  I'd be interested to know how the
> eBPF networking scenarios are handled wrt kernel ABI.
>
>
> > Again, not arguing for or against, but wondering if people fully understand
> > the implications.  If it ends up being useful, people will build access
> > control systems with it, and it directly exposes a lot of kernel internals to
> > userspace.  There was a lot of concern originally about the LSM hook interface
> > becoming a stable ABI and/or about it being misused.  Exposing that interface
> > along with every kernel data structure exposed through it to userspace seems
> > like a major leap.
>
> Agreed this is a leap, although I'm not sure I'd characterize it as
> exposure to userspace -- it allows dynamic extension of the LSM API from
> userland, but the code is executed in the kernel.
>
> KP: One thing I'd like to understand better is the attack surface
> introduced by this.  IIUC, the BTF fields are read only, so the eBPF code
> should not be able to modify any LSM parameters, correct?
>
>
> >   Even if the mainline kernel doesn't worry about any kind
> > of stable interface guarantees for it, the distros might be forced to provide
> > some kABI guarantees for it to appease ISVs and users...
>
> How is this handled currently for other eBPF use-cases?