linux-kernel - Re: [RFC PATCH 1/2] devices cgroup: allow can_attach() if ns

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Date:	Tue, 23 Jul 2013 14:04:26 -0500
From:	Serge Hallyn <serge.hallyn@...ntu.com>
To:	Tejun Heo <tj@...nel.org>
Cc:	"Eric W. Biederman" <ebiederm@...ssion.com>,
	lkml <linux-kernel@...r.kernel.org>,
	Containers <containers@...ts.linux-foundation.org>
Subject: Re: [RFC PATCH 1/2] devices cgroup: allow can_attach() if ns_capable

Quoting Tejun Heo (tj@...nel.org):
> On Tue, Jul 23, 2013 at 2:38 PM, Serge Hallyn <serge.hallyn@...ntu.com> wrote:
> > This doesn't delegate it into the container.  It allows me, on the host,
> > to set the cgroup for a container.
> 
> Hmmm? I'm a bit confused. Isn't the description saying that the patch
> allows pseudo-root in userns to change cgroup membership even if it
> isn't actually root?

If task A is uid 1000 on the host, and creates task B as uid X in a new
user namespace, then task A, still being uid 1000 on the host, is
privileged with respect to B and his namespace - i.e.
ns_capable(B->userns, CAP_SYS_ADMIN) is true.

> Besides, I find the whole check rather bogus and would actually much
> prefer just nuking the check and just follow the standard permission
> checks.

I'd be ok with that - but there's one case I'm not sure about:  If PAM
sets me up with /sys/fs/cgroup/devices/serge owned by me, then if I'm
thinking right, removing can_attach would mean I could move init into
/sys/fs/cgroup/devices/serge...

Is there something else stopping that from happening?

-serge
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