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

[Stephen Smalley]

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?

> - 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@xxxxxxxxx>