linux-kernel - Re: CLONE_NEWUSER|CLONE

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Date:	Wed, 13 Mar 2013 18:48:23 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	"Eric W. Biederman" <ebiederm@...ssion.com>
CC:	Kees Cook <keescook@...omium.org>,
	containers@...ts.linux-foundation.org,
	Sebastian Krahmer <krahmer@...e.de>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Oleg Nesterov <oleg@...hat.com>
Subject: Re: CLONE_NEWUSER|CLONE_FS root exploit

On 03/13/2013 11:35 AM, Eric W. Biederman wrote:
> Kees Cook <keescook-F7+t8E8rja9g9hUCZPvPmw@...lic.gmane.org> writes:
> 
>> Hi,
>>
>> It seem like we should block (at least) this combination. On 3.9, this
>> exploit works once uidmapping is added.
>>
>> http://www.openwall.com/lists/oss-security/2013/03/13/10
> 
> Yes.  That is a bad combination.  It let's chroot confuse privileged
> processes.
> 
> Now to figure out if this is easier to squash by adding a user_namespace
> to fs_struct or by just forbidding this combination.

It's worth making sure that setns(2) doesn't have similar issues.

Looking through other shared-but-not-a-namespace things, there are:

fs_struct: Buggy as noted.

files_struct: Probably harmless -- SCM_RIGHTS can emulate it

signal_struct: This interacts with the tty code.  Is it okay?

sighand_struct: Looks safe.  Famous last words.

FWIW, I've been alarmed in the past that struct path (e.g. the root
directory) implies an mnt_namespace (hidden in struct mount), and it's
entirely possible for the root directory's mnt_namespace not to match
nsproxy->mnt_namespace.  I'm not sure what the implications are, but
this doesn't seem healthy.

--Andy
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