linux-kernel - Re: CLONE_PARENT after setns(CLONE

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Message-ID: <20131106193311.GA18720@redhat.com>
Date:	Wed, 6 Nov 2013 20:33:11 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Serge Hallyn <serge.hallyn@...ntu.com>,
	Andy Lutomirski <luto@...capital.net>,
	Brad Spengler <spender@...ecurity.net>
Cc:	Christian Seiler <christian@...kd.de>,
	lkml <linux-kernel@...r.kernel.org>,
	Andy Whitcroft <apw@...onical.com>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Lxc development list <lxc-devel@...ts.sourceforge.net>
Subject: Re: CLONE_PARENT after setns(CLONE_NEWPID)

Hi Serge,

On 11/06, Serge Hallyn wrote:
>
> Hi Oleg,
>
> commit 40a0d32d1eaffe6aac7324ca92604b6b3977eb0e :
> "fork: unify and tighten up CLONE_NEWUSER/CLONE_NEWPID checks"
> breaks lxc-attach in 3.12.  That code forks a child which does
> setns() and then does a clone(CLONE_PARENT).  That way the
> grandchild can be in the right namespaces (which the child was
> not) and be a child of the original task, which is the monitor.

Thanks...

Yes, this is what 40a0d32d1ea explicitly tries to disallow.

> Is there a real danger in allowing CLONE_PARENT
> when current->nsproxy->pidns_for_children is not our pidns,
> or was this done out of an "over-abundance of caution"?

I am not sure... This all was based on the long discussion, and
it was decided that the CLONE_PARENT check should be consistent
wrt CLONE_NEWPID and pidns_for_children != task_active_pid_ns().

> Can we
> safely revert that new extra check?

Well, usually we do not break user-space, but I am not sure about
this case...

Eric, Andy, what do you think?

And if we allow CLONE_PARENT when ->pidns_for_children was changed,
should we also allow, say, CLONE_NEWPID && CLONE_PARENT ?

Oleg.

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