linux-kernel - BPF LSM and fexit [was: [PATCH bpf-next v3 04/10] bpf: lsm: Add mutable hooks list for the BPF LSM]

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Message-ID: <CAG48ez25mW+_oCxgCtbiGMX07g_ph79UOJa07h=o_6B6+Q-u5g@mail.gmail.com>
Date:   Tue, 11 Feb 2020 19:44:05 +0100
From:   Jann Horn <jannh@...gle.com>
To:     Alexei Starovoitov <alexei.starovoitov@...il.com>,
        KP Singh <kpsingh@...omium.org>
Cc:     kernel list <linux-kernel@...r.kernel.org>, bpf@...r.kernel.org,
        linux-security-module <linux-security-module@...r.kernel.org>,
        Brendan Jackman <jackmanb@...gle.com>,
        Florent Revest <revest@...gle.com>,
        Thomas Garnier <thgarnie@...gle.com>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        James Morris <jmorris@...ei.org>,
        Kees Cook <keescook@...omium.org>,
        Thomas Garnier <thgarnie@...omium.org>,
        Michael Halcrow <mhalcrow@...gle.com>,
        Paul Turner <pjt@...gle.com>,
        Brendan Gregg <brendan.d.gregg@...il.com>,
        Matthew Garrett <mjg59@...gle.com>,
        Christian Brauner <christian@...uner.io>,
        Mickaël Salaün <mic@...ikod.net>,
        Florent Revest <revest@...omium.org>,
        Brendan Jackman <jackmanb@...omium.org>,
        "Serge E. Hallyn" <serge@...lyn.com>,
        Mauro Carvalho Chehab <mchehab+samsung@...nel.org>,
        "David S. Miller" <davem@...emloft.net>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Kernel Team <kernel-team@...com>
Subject: BPF LSM and fexit [was: [PATCH bpf-next v3 04/10] bpf: lsm: Add
 mutable hooks list for the BPF LSM]

On Tue, Feb 11, 2020 at 6:58 PM Alexei Starovoitov
<alexei.starovoitov@...il.com> wrote:
> On Tue, Feb 11, 2020 at 01:43:34PM +0100, KP Singh wrote:
[...]
> > * When using the semantic provided by fexit, the BPF LSM program will
> >   always be executed and will be able to override / clobber the
> >   decision of LSMs which appear before it in the ordered list. This
> >   semantic is very different from what we currently have (i.e. the BPF
> >   LSM hook is only called if all the other LSMs allow the action) and
> >   seems to be bypassing the LSM framework.
>
> It that's a concern it's trivial to add 'if (RC == 0)' check to fexit
> trampoline generator specific to lsm progs.
[...]
> Using fexit mechanism and bpf_sk_storage generalization is
> all that is needed. None of it should touch security/*.

If I understand your suggestion correctly, that seems like a terrible
idea to me from the perspective of inspectability and debuggability.
If at runtime, a function can branch off elsewhere to modify its
decision, I want to see that in the source code. If someone e.g.
changes the parameters or the locking rules around a security hook,
how are they supposed to understand the implications if that happens
through some magic fexit trampoline that is injected at runtime?