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

Kees Cook
2020-03-24T17:57:50Z

On Tue, Mar 24, 2020 at 03:51:55PM +0100, KP Singh wrote:
> On 24-Mär 10:51, Stephen Smalley wrote:
> > 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...
>
> Yeah, we do add BPF as the last LSM in the default list. So, I will
> avoid adding LSM_ORDER_LAST for now.

FWIW, this is my preference as well. If there ends up being a stronger
need, then we can implement LSM_ORDER_LAST at that time.

--
Kees Cook