Re: [PATCH bpf-next 0/8] New BPF map and BTF security LSM hooks

[Kees Cook <keescook@chromium.org>]

On Wed, Apr 12, 2023 at 02:06:23PM -0400, Paul Moore wrote:
> On Wed, Apr 12, 2023 at 1:47 PM Kees Cook <keescook@chromium.org> wrote:
> > On Wed, Apr 12, 2023 at 12:49:06PM -0400, Paul Moore wrote:
> > > On Wed, Apr 12, 2023 at 12:33 AM Andrii Nakryiko <andrii@kernel.org> wrote:
> > > >
> > > > Add new LSM hooks, bpf_map_create_security and bpf_btf_load_security, which
> > > > are meant to allow highly-granular LSM-based control over the usage of BPF
> > > > subsytem. Specifically, to control the creation of BPF maps and BTF data
> > > > objects, which are fundamental building blocks of any modern BPF application.
> > > >
> > > > These new hooks are able to override default kernel-side CAP_BPF-based (and
> > > > sometimes CAP_NET_ADMIN-based) permission checks. It is now possible to
> > > > implement LSM policies that could granularly enforce more restrictions on
> > > > a per-BPF map basis (beyond checking coarse CAP_BPF/CAP_NET_ADMIN
> > > > capabilities), but also, importantly, allow to *bypass kernel-side
> > > > enforcement* of CAP_BPF/CAP_NET_ADMIN checks for trusted applications and use
> > > > cases.
> > >
> > > One of the hallmarks of the LSM has always been that it is
> > > non-authoritative: it cannot unilaterally grant access, it can only
> > > restrict what would have been otherwise permitted on a traditional
> > > Linux system.  Put another way, a LSM should not undermine the Linux
> > > discretionary access controls, e.g. capabilities.
> > >
> > > If there is a problem with the eBPF capability-based access controls,
> > > that problem needs to be addressed in how the core eBPF code
> > > implements its capability checks, not by modifying the LSM mechanism
> > > to bypass these checks.
> >
> > I think semantics matter here. I wouldn't view this as _bypassing_
> > capability enforcement: it's just more fine-grained access control.
> >
> > For example, in many places we have things like:
> >
> >         if (!some_check(...) && !capable(...))
> >                 return -EPERM;
> >
> > I would expect this is a similar logic. An operation can succeed if the
> > access control requirement is met. The mismatch we have through-out the
> > kernel is that capability checks aren't strictly done by LSM hooks. And
> > this series conceptually, I think, doesn't violate that -- it's changing
> > the logic of the capability checks, not the LSM (i.e. there no LSM hooks
> > yet here).
> 
> Patch 04/08 creates a new LSM hook, security_bpf_map_create(), which
> when it returns a positive value "bypasses kernel checks".  The patch
> isn't based on either Linus' tree or the LSM tree, I'm guessing it is
> based on a eBPF tree, so I can't say with 100% certainty that it is
> bypassing a capability check, but the description claims that to be
> the case.
> 
> Regardless of how you want to spin this, I'm not supportive of a LSM
> hook which allows a LSM to bypass a capability check.  A LSM hook can
> be used to provide additional access control restrictions beyond a
> capability check, but a LSM hook should never be allowed to overrule
> an access denial due to a capability check.
> 
> > The reason CAP_BPF was created was because there was nothing else that
> > would be fine-grained enough at the time.
> 
> The LSM layer predates CAP_BPF, and one could make a very solid
> argument that one of the reasons LSMs exist is to provide
> supplementary controls due to capability-based access controls being a
> poor fit for many modern use cases.

I generally agree with what you say, but we DO have this code pattern:

         if (!some_check(...) && !capable(...))
                 return -EPERM;

It looks to me like this series can be refactored to do the same. I
wouldn't consider that to be a "bypass", but I would agree the current
series looks too much like "bypass", and makes reasoning about the
effect of the LSM hooks too "special". :)

-- 
Kees Cook