Re: [PATCH bpf-next v5 5/7] bpf: lsm: Initialize the BPF LSM hooks

KP Singh
2020-03-24T14:42:20Z

On 24-Mär 10:37, Stephen Smalley wrote:
> On Mon, Mar 23, 2020 at 9:52 PM KP Singh <kpsingh@chromium.org> wrote:
> >
> > On 23-Mär 18:13, Casey Schaufler wrote:
> > > On 3/23/2020 9:44 AM, KP Singh wrote:
> > > > From: KP Singh <kpsingh@google.com>
> > > >
> > > > The bpf_lsm_ nops are initialized into the LSM framework like any other
> > > > LSM. Some LSM hooks do not have 0 as their default return value. The
> > > > __weak symbol for these hooks is overridden by a corresponding
> > > > definition in security/bpf/hooks.c
> > > >
> > > > + return 0;
> >
> > [...]
> >
> > > > +}
> > > > +
> > > > +DEFINE_LSM(bpf) = {
> > > > + .name = "bpf",
> > > > + .init = bpf_lsm_init,
> > >
> > > Have you given up on the "BPF must be last" requirement?
> >
> > Yes, we dropped it for as the BPF programs require CAP_SYS_ADMIN
> > anwyays so the position ~shouldn't~ matter. (based on some of the
> > discussions we had on the BPF_MODIFY_RETURN patches).
> >
> > However, This can be added later (in a separate patch) if really
> > deemed necessary.
>
> It matters for SELinux, as I previously explained. A process that has
> CAP_SYS_ADMIN is not assumed to be able to circumvent MAC policy.
> And executing prior to SELinux allows the bpf program to access and
> potentially leak to userspace information that wouldn't be visible to
> the
> process itself. However, I thought you were handling the order issue
> by putting it last in the list of lsms?

We can still do that if it does not work for SELinux.

Would it be okay to add bpf as LSM_ORDER_LAST?

LSMs like Landlock can then add LSM_ORDER_UNPRIVILEGED to even end up
after bpf?

- KP