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 10:37:51 -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 Mon, Mar 23, 2020 at 9:52 PM KP Singh <kpsingh@...omium.org> wrote:
>
> On 23-Mär 18:13, Casey Schaufler wrote:
> > On 3/23/2020 9:44 AM, KP Singh wrote:
> > > From: KP Singh <kpsingh@...gle.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?