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From:   KP Singh <kpsingh@chromium.org>
Date:   Tue, 24 Mar 2020 15:42:14 +0100
To:     Stephen Smalley <stephen.smalley.work@gmail.com>
Cc:     Casey Schaufler <casey@schaufler-ca.com>,
        linux-kernel@vger.kernel.org, bpf@vger.kernel.org,
        LSM List <linux-security-module@vger.kernel.org>,
        Brendan Jackman <jackmanb@google.com>,
        Florent Revest <revest@google.com>,
        Alexei Starovoitov <ast@kernel.org>,
        Daniel Borkmann <daniel@iogearbox.net>,
        James Morris <jmorris@namei.org>,
        Kees Cook <keescook@chromium.org>,
        Paul Turner <pjt@google.com>, Jann Horn <jannh@google.com>,
        Florent Revest <revest@chromium.org>,
        Brendan Jackman <jackmanb@chromium.org>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Subject: Re: [PATCH bpf-next v5 5/7] bpf: lsm: Initialize the BPF LSM hooks
Message-ID: <20200324144214.GA1040@chromium.org>
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 <20200324015217.GA28487@chromium.org>
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On 24-M?r 10:37, Stephen Smalley wrote:
> On Mon, Mar 23, 2020 at 9:52 PM KP Singh <kpsingh@chromium.org> wrote:
> >
> > On 23-M?r 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