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Date: Wed, 7 Mar 2018 00:06:55 +0100
From: MickaĆ«l SalaĆ¼n <mic@...ikod.net>
To: Tycho Andersen <tycho@...ho.ws>, Andy Lutomirski <luto@...capital.net>
Cc: LKML <linux-kernel@...r.kernel.org>, Alexei Starovoitov <ast@...nel.org>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Casey Schaufler <casey@...aufler-ca.com>,
        Daniel Borkmann <daniel@...earbox.net>,
        David Drysdale
 <drysdale@...gle.com>,
        "David S . Miller" <davem@...emloft.net>,
        "Eric W . Biederman" <ebiederm@...ssion.com>,
        James Morris <james.l.morris@...cle.com>, Jann Horn <jann@...jh.net>,
        Jonathan Corbet <corbet@....net>,
        Michael Kerrisk <mtk.manpages@...il.com>,
        Kees Cook <keescook@...omium.org>, Paul Moore <paul@...l-moore.com>,
        Sargun Dhillon <sargun@...gun.me>,
        "Serge E . Hallyn" <serge@...lyn.com>, Shuah Khan <shuah@...nel.org>,
        Tejun Heo <tj@...nel.org>, Thomas Graf <tgraf@...g.ch>,
        Will Drewry <wad@...omium.org>,
        Kernel Hardening <kernel-hardening@...ts.openwall.com>,
        Linux API <linux-api@...r.kernel.org>,
        LSM List <linux-security-module@...r.kernel.org>,
        Network Development <netdev@...r.kernel.org>
Subject: Re: [PATCH bpf-next v8 00/11] Landlock LSM: Toward unprivileged
 sandboxing


On 06/03/2018 23:46, Tycho Andersen wrote:
> On Tue, Mar 06, 2018 at 10:33:17PM +0000, Andy Lutomirski wrote:
>>>> Suppose I'm writing a container manager.  I want to run "mount" in the
>>>> container, but I don't want to allow moun() in general and I want to
>>>> emulate certain mount() actions.  I can write a filter that catches
>>>> mount using seccomp and calls out to the container manager for help.
>>>> This isn't theoretical -- Tycho wants *exactly* this use case to be
>>>> supported.
>>>
>>> Well, I think this use case should be handled with something like
>>> LD_PRELOAD and a helper library. FYI, I did something like this:
>>> https://github.com/stemjail/stemshim
>>
>> I doubt that will work for containers.  Containers that use user
>> namespaces and, for example, setuid programs aren't going to honor
>> LD_PRELOAD.
> 
> Or anything that calls syscalls directly, like go programs.

That's why the vDSO-like approach. Enforcing an access control is not
the issue here, patching a buggy userland (without patching its code) is
the issue isn't it?

As far as I remember, the main problem is to handle file descriptors
while "emulating" the kernel behavior. This can be done with a "shim"
code mapped in every processes. Chrome used something like this (in a
previous sandbox mechanism) as a kind of emulation (with the current
seccomp-bpf ). I think it should be doable to replace the (userland)
emulation code with an IPC wrapper receiving file descriptors through
UNIX socket.



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