[PATCH bpf-next v4 3/7] bpf: Introduce BPF_MODIFY_RETURN

[Stephen Smalley]

On Thu, Mar 5, 2020 at 10:54 AM KP Singh <kpsingh at chromium.org> wrote:
>
> On 05-Mar 08:51, Stephen Smalley wrote:
> > IIUC you've switched from a model where the BPF program would be
> > invoked after the original function logic
> > and the BPF program is skipped if the original function logic returns
> > non-zero to a model where the BPF program is invoked first and
> > the original function logic is skipped if the BPF program returns
> > non-zero.  I'm not keen on that for userspace-loaded code attached
>
> We do want to continue the KRSI series and the effort to implement a
> proper BPF LSM. In the meantime, the tracing + error injection
> solution helps us to:
>
>   * Provide better debug capabilities.
>   * And parallelize the effort to come up with the right helpers
>     for our LSM work and work on sleepable BPF which is also essential
>     for some of the helpers.
>
> As you noted, in the KRSI v4 series, we mentioned that we would like
> to have the user-space loaded BPF programs be unable to override the
> decision made by the in-kernel logic/LSMs, but this got shot down:
>
>    https://lore.kernel.org/bpf/00c216e1-bcfd-b7b1-5444-2a2dfa69190b@schaufler-ca.com
>
> I would like to continue this discussion when we post the v5 series
> for KRSI as to what the correct precedence order should be for the
> BPF_PROG_TYPE_LSM and would appreciate if you also bring it up there.

That's fine but I guess I don't see why you or anyone else would
bother with introducing a BPF_PROG_TYPE_LSM
if BPF_PROG_MODIFY_RETURN is accepted and is allowed to attach to the
LSM hooks.  What's the benefit to you
if you can achieve your goals directly with MODIFY_RETURN?

> > to LSM hooks; it means that userspace BPF programs can run even if
> > SELinux would have denied access and SELinux hooks get
> > skipped entirely if the BPF program returns an error.  I think Casey
> > may have wrongly pointed you in this direction on the grounds
> > it can already happen with the base DAC checking logic.  But that's
>
> What we can do for this tracing/modify_ret series, is to remove
> the special casing for "security_" functions in the BPF code and add
> ALLOW_ERROR_INJECTION calls to the security hooks. This way, if
> someone needs to disable the BPF programs being able to modify
> security hooks, they can disable error injection. If that's okay, we
> can send a patch.

Realistically distros tend to enable lots of developer-friendly
options including error injection, and most users don't build their
own kernels
and distros won't support them when they do. So telling users they can
just rebuild their kernel without error injection if they care about
BPF programs being able to modify security hooks isn't really viable.
The security modules need a way to veto it based on their policies.
That's why I suggested a security hook here.