linux-kernel - Re: [PATCH v3 bpf-next 00/21] bpf: Sysctl hook

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Message-ID: <CAGXu5jJTCjE6FiqPgnSZbp1rzNs35RV-bXXg=+Nu5bO7v1Ac0g@mail.gmail.com>
Date:   Tue, 9 Apr 2019 09:50:50 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc:     Andrey Ignatov <rdna@...com>,
        Network Development <netdev@...r.kernel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Roman Gushchin <guro@...com>, kernel-team <kernel-team@...com>,
        Luis Chamberlain <mcgrof@...nel.org>,
        Alexey Dobriyan <adobriyan@...il.com>,
        LKML <linux-kernel@...r.kernel.org>,
        "linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
        linux-security-module <linux-security-module@...r.kernel.org>,
        Jann Horn <jannh@...gle.com>
Subject: Re: [PATCH v3 bpf-next 00/21] bpf: Sysctl hook

On Sat, Apr 6, 2019 at 10:03 AM Alexei Starovoitov
<alexei.starovoitov@...il.com> wrote:
>
> On Sat, Apr 06, 2019 at 09:43:50AM -0700, Kees Cook wrote:
> > On Fri, Apr 5, 2019 at 12:36 PM Andrey Ignatov <rdna@...com> wrote:
> > > BPF_CGROUP_SYSCTL hook is placed before calling to sysctl's proc_handler so
> > > that accesses (read/write) to sysctl can be controlled for specific cgroup
> > > and either allowed or denied, or traced.
> >
> > This sounds more like an LSM than BPF.
>
> not at all. the key difference is being cgroup scoped.
> essentially for different containers.

Okay, works for me. I was looking at it from the perspective of
something providing resource access control policy, which usually
falls into the LSM world.

> bpf prog is attached to this hook in a particular cgroup
> and executed for sysctls for tasks that belong to that cgroup.

So it's root limiting root-in-a-container? Nice to have some
boundaries there, for sure.

> > Can the BPF be removed (or rather,
> > what's the lifetime of such BPF?)
>
> same as all other cgroup-bpf hooks.
> Do you have a specific concern or just asking how life time of programs
> is managed?
> High level description of lifetime is here:
> https://facebookmicrosites.github.io/bpf/blog/2018/08/31/object-lifetime.html

I'm mostly curious about the access control stacking. i.e. can
in-container root add new eBPF to its own cgroup, and if so, can it
undo the restrictions already present? (I assume it can't, but figured
I'd ask...)

-- 
Kees Cook