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

Stephen Smalley
2020-03-24T14:50:26Z

On Tue, Mar 24, 2020 at 10:42 AM KP Singh <kpsingh@chromium.org> wrote:
>
> 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:
> > > > 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?

I guess the question is whether we need an explicit LSM_ORDER_LAST or
can just handle it via the default
values for the lsm= parameter, where you are already placing bpf last
IIUC? If someone can mess with the kernel boot
parameters, they already have options to mess with SELinux, so it is no worse...