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

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Date:   Tue, 24 Mar 2020 10:51:32 -0400
From:   Stephen Smalley <stephen.smalley.work@...il.com>
To:     KP Singh <kpsingh@...omium.org>
Cc:     Casey Schaufler <casey@...aufler-ca.com>,
        linux-kernel@...r.kernel.org, bpf@...r.kernel.org,
        LSM List <linux-security-module@...r.kernel.org>,
        Brendan Jackman <jackmanb@...gle.com>,
        Florent Revest <revest@...gle.com>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        James Morris <jmorris@...ei.org>,
        Kees Cook <keescook@...omium.org>,
        Paul Turner <pjt@...gle.com>, Jann Horn <jannh@...gle.com>,
        Florent Revest <revest@...omium.org>,
        Brendan Jackman <jackmanb@...omium.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Subject: Re: [PATCH bpf-next v5 5/7] bpf: lsm: Initialize the BPF LSM hooks

On Tue, Mar 24, 2020 at 10:42 AM KP Singh <kpsingh@...omium.org> wrote:
>
> On 24-Mär 10:37, Stephen Smalley wrote:
> > On Mon, Mar 23, 2020 at 9:52 PM KP Singh <kpsingh@...omium.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...