linux-kernel - Re: CLONE_NEWUSER|CLONE

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Date:	Thu, 14 Mar 2013 13:29:10 -0700
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	Andy Lutomirski <luto@...capital.net>
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

Andy Lutomirski <luto@...capital.net> writes:

> 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.

setns(2) and unshare(2) are done and merged.  See commit.

commit e66eded8309ebf679d3d3c1f5820d1f2ca332c71
Author: Eric W. Biederman <ebiederm@...ssion.com>
Date:   Wed Mar 13 11:51:49 2013 -0700

    userns: Don't allow CLONE_NEWUSER | CLONE_FS


> 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?

It should be.  The tty code is heavily pid based, and CLONE_NEWPID
requires !CLONE_VM (which implies !CLONE_SIGHAND and !CLONE_VM).

> 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.

The calls to check_mnt prevent abuse of the files found with fs_struct
not matching the current mount namespace.

static inline int check_mnt(struct mount *mnt)
{
	return mnt->mnt_ns == current->nsproxy->mnt_ns;
}

Thanks for looking I know I did the same double take and wondered if I
had missed anything else by accident when this bug showed up.

So far even just looking it all over again I can't see anything.  But I
have clearly been blind before.

Eric


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