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

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Date:   Sat, 6 Apr 2019 10:02:58 -0700
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Kees Cook <keescook@...omium.org>
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 06, 2019 at 09:43:50AM -0700, Kees Cook wrote:
> On Fri, Apr 5, 2019 at 12:36 PM Andrey Ignatov <rdna@...com> wrote:
> >
> > v2->v3:
> > - simplify C based selftests by relying on variable offset stack access.
> >
> > v1->v2:
> > - add fs/proc/proc_sysctl.c mainteners to Cc:.
> >
> > The patch set introduces new BPF hook for sysctl.
> >
> > It adds new program type BPF_PROG_TYPE_CGROUP_SYSCTL and attach type
> > BPF_CGROUP_SYSCTL.
> >
> > 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.
> >
> > The hook has access to sysctl name, current sysctl value and (on write
> > only) to new sysctl value via corresponding helpers. New sysctl value can
> > be overridden by program. Both name and values (current/new) are
> > represented as strings same way they're visible in /proc/sys/. It is up to
> > program to parse these strings.
> >
> > To help with parsing the most common kind of sysctl value, vector of
> > integers, two new helpers are provided: bpf_strtol and bpf_strtoul with
> > semantic similar to user space strtol(3) and strtoul(3).
> >
> > The hook also provides bpf_sysctl context with two fields:
> > * @write indicates whether sysctl is being read (= 0) or written (= 1);
> > * @file_pos is sysctl file position to read from or write to, can be
> >   overridden.
> >
> > The hook allows to make better isolation for containerized applications
> > that are run as root so that one container can't change a sysctl and affect
> > all other containers on a host, make changes to allowed sysctl in a safer
> > way and simplify sysctl tracing for cgroups.
> 
> This sounds more like an LSM than BPF. 

not at all. the key difference is being cgroup scoped.
essentially for different containers.

> So sysctls can get blocked when
> new BPF is added to a cgroup? 

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

> 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