linux-kernel - Re: [PATCHv3 0/2] capability controlled user-namespaces

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Message-ID: <CAF2d9jh+xMW7_5dwAPkoAuDyx=k=FNh3VfiMvDkCOBD2KF5npw@mail.gmail.com>
Date:   Tue, 9 Jan 2018 18:08:58 -0800
From:   Mahesh Bandewar (महेश बंडेवार) 
        <maheshb@...gle.com>
To:     "Serge E. Hallyn" <serge@...lyn.com>
Cc:     James Morris <james.l.morris@...cle.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Netdev <netdev@...r.kernel.org>,
        Kernel-hardening <kernel-hardening@...ts.openwall.com>,
        Linux API <linux-api@...r.kernel.org>,
        Kees Cook <keescook@...omium.org>,
        "Eric W . Biederman" <ebiederm@...ssion.com>,
        Eric Dumazet <edumazet@...gle.com>,
        David Miller <davem@...emloft.net>,
        Mahesh Bandewar <mahesh@...dewar.net>
Subject: Re: [PATCHv3 0/2] capability controlled user-namespaces

On Tue, Jan 9, 2018 at 2:28 PM, Serge E. Hallyn <serge@...lyn.com> wrote:
> Quoting Mahesh Bandewar (महेश बंडेवार) (maheshb@...gle.com):
>> On Mon, Jan 8, 2018 at 10:36 AM, Serge E. Hallyn <serge@...lyn.com> wrote:
>> > Quoting Mahesh Bandewar (महेश बंडेवार) (maheshb@...gle.com):
>> >> On Mon, Jan 8, 2018 at 10:11 AM, Serge E. Hallyn <serge@...lyn.com> wrote:
>> >> > Quoting Mahesh Bandewar (महेश बंडेवार) (maheshb@...gle.com):
>> >> >> On Mon, Jan 8, 2018 at 7:47 AM, Serge E. Hallyn <serge@...lyn.com> wrote:
>> >> >> > Quoting James Morris (james.l.morris@...cle.com):
>> >> >> >> On Mon, 8 Jan 2018, Serge E. Hallyn wrote:
>> >> >> >> I meant in terms of "marking" a user ns as "controlled" type -- it's
>> >> >> >> unnecessary jargon from an end user point of view.
>> >> >> >
>> >> >> > Ah, yes, that was my point in
>> >> >> >
>> >> >> > http://lkml.iu.edu/hypermail/linux/kernel/1711.1/01845.html
>> >> >> > and
>> >> >> > http://lkml.iu.edu/hypermail/linux/kernel/1711.1/02276.html
>> >> >> >
>> >> >> >> This may happen internally but don't make it a special case with a
>> >> >> >> different name and don't bother users with internal concepts: simply
>> >> >> >> implement capability whitelists with the default having equivalent
>> >> >
>> >> > So the challenge is to have unprivileged users be contained, while
>> >> > allowing trusted workloads in containers created by a root user to
>> >> > bypass the restriction.
>> >> >
>> >> > Now, the current proposal actually doesn't support a root user starting
>> >> > an application that it doesn't quite trust in such a way that it *is*
>> >> > subject to the whitelist.
>> >>
>> >> Well, this is not hard since root process can spawn another process
>> >> and loose privileges before creating user-ns to be controlled by the
>> >> whitelist.
>> >
>> > It would have to drop cap_sys_admin for the container to be marked as
>> > "controlled", which may prevent the container runtime from properly starting
>> > the container.
>> >
>> Yes, but that's a conflict of trusted operations (that requires
>> SYS_ADMIN) and untrusted processes it may spawn.
>
> Not sure I understand what you're saying, but
>
> I guess that in any case the task which is doing unshare(CLONE_NEWNS)
> can drop cap_sys_admin first.  Though that is harder if using clone,
> and it is awkward because it's not the container manager, but the user,
> who will judge whether the container workload should be restricted.
> So the container driver will add a flag like "run-controlled", and
> the driver will convert that to dropping a capability; which again
> is weird.  It would seem nicer to introduce a userns flag, 'caps-controlled'
> For an unprivileged userns, it is always set to 1, and root cannot
> change it.  For a root-created userns, it stays 0, but root can set it
> to 1 (using /proc file?).  In this way a either container runtime or just an
> admin script can say "no wait I want this container to still be controlled".
>
> Or we could instead add a second sysctl to decide whether all or only
> 'controlled' user namespaces should be controlled.  That's not pretty though.
>
Yes, I like your idea of a flag to clone() which will force the
user-ns to be controlled. This will have effect only on the root user
and any other user specifying is actually a NOP since those will be
controlled with or without that flag. But this is still an enhancement
to the current patch-set and I don't mind doing it as a follow-up
after this patch-series.

At this moment James has asked for Eric's input, which I believe
hasn't been recorded.