linux-kernel - Re: Potential issues (security and otherwise) with the current cgroup-bpf API

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Message-ID: <CALCETrUc-iDUT8isKc43PPZ3xz31Sz+QTU+_SQQTAsWhH+zkLw@mail.gmail.com>
Date:   Mon, 19 Dec 2016 17:40:53 -0800
From:   Andy Lutomirski <luto@...capital.net>
To:     David Miller <davem@...emloft.net>
Cc:     Alexei Starovoitov <alexei.starovoitov@...il.com>,
        Andrew Lutomirski <luto@...nel.org>,
        Daniel Mack <daniel@...que.org>,
        Mickaël Salaün <mic@...ikod.net>,
        Kees Cook <keescook@...omium.org>, Jann Horn <jann@...jh.net>,
        Tejun Heo <tj@...nel.org>, David Ahern <dsahern@...il.com>,
        Thomas Graf <tgraf@...g.ch>,
        Michael Kerrisk <mtk.manpages@...il.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Linux API <linux-api@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Network Development <netdev@...r.kernel.org>
Subject: Re: Potential issues (security and otherwise) with the current
 cgroup-bpf API

On Mon, Dec 19, 2016 at 5:34 PM, David Miller <davem@...emloft.net> wrote:
> From: Alexei Starovoitov <alexei.starovoitov@...il.com>
> Date: Mon, 19 Dec 2016 16:02:56 -0800
>
>> huh? 'not right api' because it's using bpf syscall instead
>> of cgroup control-file? I think the opposite is the truth.
>
> I completely agree with Alexei on this.

So what happens when someone adds another type of filter?  Let's say
there's a simple, no-privilege-required list of allowed address
families that can hook up to the socket creation hook for a cgroup.
Does BPF_PROG_DETACH still detach it?  Or would both the bpf *and* the
list of allowed address families be in force?  If the latter, why
wouldn't two BPF programs on the same hook be allowed?

Concretely:

# mkdir /cgroup/a
# set_up_bpf_socket_rule /cgroup/a
# set_up_list_of_address_families /cgroup/a
# cat /cgroup/a/some_new_file [what gets displayed?]
# BPF_PROG_DETACH: what happens

By the way, even if Alexei is right, the BPF_PROG_DETACH API doesn't
even take a reference to a BPF program as an argument.  What is it
supposed to do if this mechanism ever gets extended?

--Andy