linux-kernel - Re: [PATCH RFC v2 4/6] proc: support mounting private procfs instances inside same pid namespace

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Date:   Tue, 2 May 2017 09:33:46 -0700
From:   Andy Lutomirski <luto@...nel.org>
To:     Djalal Harouni <tixxdz@...il.com>,
        "Eric W. Biederman" <ebiederm@...ssion.com>
Cc:     Andy Lutomirski <luto@...nel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Kees Cook <keescook@...omium.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linux FS Devel <linux-fsdevel@...r.kernel.org>,
        "kernel-hardening@...ts.openwall.com" 
        <kernel-hardening@...ts.openwall.com>,
        LSM List <linux-security-module@...r.kernel.org>,
        Linux API <linux-api@...r.kernel.org>,
        Dongsu Park <dpark@...teo.net>,
        Casey Schaufler <casey@...aufler-ca.com>,
        James Morris <james.l.morris@...cle.com>,
        "Serge E. Hallyn" <serge@...lyn.com>,
        Jeff Layton <jlayton@...chiereds.net>,
        "J. Bruce Fields" <bfields@...ldses.org>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Alexey Dobriyan <adobriyan@...il.com>,
        Ingo Molnar <mingo@...nel.org>,
        Oleg Nesterov <oleg@...hat.com>,
        Michal Hocko <mhocko@...e.com>,
        Jonathan Corbet <corbet@....net>
Subject: Re: [PATCH RFC v2 4/6] proc: support mounting private procfs
 instances inside same pid namespace

On Tue, May 2, 2017 at 7:29 AM, Djalal Harouni <tixxdz@...il.com> wrote:
> On Thu, Apr 27, 2017 at 12:13 AM, Andy Lutomirski <luto@...nel.org> wrote:
>> On Tue, Apr 25, 2017 at 5:23 AM, Djalal Harouni <tixxdz@...il.com> wrote:
> [...]
>>> We have to align procfs and modernize it to have a per mount context
>>> where at least the mount option do not propagate to all other mounts,
>>> then maybe we can continue to implement new features. One example is to
>>> require CAP_SYS_ADMIN in the init user namespace on some /proc/* which are
>>> not pids and which are are not virtualized by design, or CAP_NET_ADMIN
>>> inside userns on the net bits that are virtualized, etc.
>>> These mount options won't propagate to previous mounts, and the system
>>> will continue to be usable.
>>>
>>> Ths patch introduces the new 'limit_pids' mount option as it was also
>>> suggesed by Andy Lutomirski [1]. When this option is passed we
>>> automatically create a private procfs instance. This is not the default
>>> behaviour since we do not want to break userspace and we do not want to
>>> provide different devices IDs by default, please see [1] for why.
>>
>> I think that calling the option to make a separate instance
>> "limit_pids" is extremely counterintuitive.
>
> Ok.
>
>> My strong preference would be to make proc *always* make a separate
>> instance (unless it's a bind mount) and to make it work.  If that
>> means fudging stat() output, so be it.
>
> I also agree, but as said if we change stat(), userspace won't be able
> to notice if these two proc instances are really separated, the device
> ID is the only indication here.

I re-read all the threads and I'm still not convinced I see why we
need new_instance to be non-default.  It's true that the device
numbers of /proc/ns/* matter, but if you look (with stat -L, for
example), they're *already* not tied to the procfs instance.

I'm okay with adding new_instance to be on the safe side, but I'd like
it to be done in a way that we could make it become the default some
day without breaking anything.  This means that we need to be rather
careful about how new_instance and hidepid interact.