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

[Alexei Starovoitov]

On Fri, Jan 10, 2020 at 04:27:58PM +0100, KP Singh wrote:
> 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.

I don't think it's a good idea to enforce license on the program.
The kernel doesn't do it for modules.
For years all of BPF tracing progs were GPL because they have to use
GPL-ed helpers to do anything meaningful.
So for KRSI just make sure that all helpers are GPL-ed as well.