linux-kernel - Re: [CFT][PATCH 2/7] userns: Don't allow setgroups until a gid mapping has been setablished

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Message-ID: <548625E3.6020400@nod.at>
Date:	Mon, 08 Dec 2014 23:27:47 +0100
From:	Richard Weinberger <richard@....at>
To:	Andy Lutomirski <luto@...capital.net>
CC:	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Linux Containers <containers@...ts.linux-foundation.org>,
	Josh Triplett <josh@...htriplett.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Kees Cook <keescook@...omium.org>,
	Michael Kerrisk-manpages <mtk.manpages@...il.com>,
	Linux API <linux-api@...r.kernel.org>,
	linux-man <linux-man@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	LSM <linux-security-module@...r.kernel.org>,
	Casey Schaufler <casey@...aufler-ca.com>,
	"Serge E. Hallyn" <serge@...lyn.com>,
	Kenton Varda <kenton@...dstorm.io>,
	stable <stable@...r.kernel.org>
Subject: Re: [CFT][PATCH 2/7] userns: Don't allow setgroups until a gid mapping
 has been setablished

Am 08.12.2014 um 23:25 schrieb Andy Lutomirski:
> On Mon, Dec 8, 2014 at 2:17 PM, Richard Weinberger <richard@....at> wrote:
>> Am 08.12.2014 um 23:07 schrieb Eric W. Biederman:
>>>
>>> setgroups is unique in not needing a valid mapping before it can be called,
>>> in the case of setgroups(0, NULL) which drops all supplemental groups.
>>>
>>> The design of the user namespace assumes that CAP_SETGID can not actually
>>> be used until a gid mapping is established.  Therefore add a helper function
>>> to see if the user namespace gid mapping has been established and call
>>> that function in the setgroups permission check.
>>>
>>> This is part of the fix for CVE-2014-8989, being able to drop groups
>>> without privilege using user namespaces.
>>>
>>> Cc: stable@...r.kernel.org
>>> Signed-off-by: "Eric W. Biederman" <ebiederm@...ssion.com>
>>> ---
>>>  include/linux/user_namespace.h | 9 +++++++++
>>>  kernel/groups.c                | 7 ++++++-
>>>  2 files changed, 15 insertions(+), 1 deletion(-)
>>>
>>> diff --git a/include/linux/user_namespace.h b/include/linux/user_namespace.h
>>> index e95372654f09..41cc26e5a350 100644
>>> --- a/include/linux/user_namespace.h
>>> +++ b/include/linux/user_namespace.h
>>> @@ -37,6 +37,15 @@ struct user_namespace {
>>>
>>>  extern struct user_namespace init_user_ns;
>>>
>>> +static inline bool userns_gid_mappings_established(const struct user_namespace *ns)
>>> +{
>>> +     bool established;
>>> +     smp_mb__before_atomic();
>>> +     established = ACCESS_ONCE(ns->gid_map.nr_extents) != 0;
>>> +     smp_mb__after_atomic();
>>> +     return established;
>>> +}
>>> +
>>
>> Maybe this is a stupid question, but why do we need all this magic
>> around established =  ... ?
>> The purpose of this code is to check whether ns->gid_map.nr_extents != 0
>> in a lock-free manner?
>>
> 
> See my other comment -- the ordering will matter at the end of the series.

But ns->gid_map.nr_extents is not atomic, it is a plain u32.
This confuses me.

Thanks,
//richard
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