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From:   Jann Horn <jannh@google.com>
Date:   Tue, 11 Feb 2020 19:44:05 +0100
Message-ID: <CAG48ez25mW+_oCxgCtbiGMX07g_ph79UOJa07h=o_6B6+Q-u5g@mail.gmail.com>
Subject: BPF LSM and fexit [was: [PATCH bpf-next v3 04/10] bpf: lsm: Add
 mutable hooks list for the BPF LSM]
To:     Alexei Starovoitov <alexei.starovoitov@gmail.com>,
        KP Singh <kpsingh@chromium.org>
Cc:     kernel list <linux-kernel@vger.kernel.org>, bpf@vger.kernel.org,
        linux-security-module <linux-security-module@vger.kernel.org>,
        Brendan Jackman <jackmanb@google.com>,
        Florent Revest <revest@google.com>,
        Thomas Garnier <thgarnie@google.com>,
        Alexei Starovoitov <ast@kernel.org>,
        Daniel Borkmann <daniel@iogearbox.net>,
        James Morris <jmorris@namei.org>,
        Kees Cook <keescook@chromium.org>,
        Thomas Garnier <thgarnie@chromium.org>,
        Michael Halcrow <mhalcrow@google.com>,
        Paul Turner <pjt@google.com>,
        Brendan Gregg <brendan.d.gregg@gmail.com>,
        Matthew Garrett <mjg59@google.com>,
        Christian Brauner <christian@brauner.io>,
        =?UTF-8?B?TWlja2HDq2wgU2FsYcO8bg==?= <mic@digikod.net>,
        Florent Revest <revest@chromium.org>,
        Brendan Jackman <jackmanb@chromium.org>,
        "Serge E. Hallyn" <serge@hallyn.com>,
        Mauro Carvalho Chehab <mchehab+samsung@kernel.org>,
        "David S. Miller" <davem@davemloft.net>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Kernel Team <kernel-team@fb.com>
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On Tue, Feb 11, 2020 at 6:58 PM Alexei Starovoitov
<alexei.starovoitov@gmail.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?