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

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Date:	Mon, 8 Dec 2014 14:33:02 -0800
From:	Andy Lutomirski <luto@...capital.net>
To:	"Eric W. Biederman" <ebiederm@...ssion.com>
Cc:	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>,
	Richard Weinberger <richard@....at>,
	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

On Mon, Dec 8, 2014 at 2:26 PM, Eric W. Biederman <ebiederm@...ssion.com> wrote:
> Andy Lutomirski <luto@...capital.net> writes:
>
>> On Mon, Dec 8, 2014 at 2:07 PM, Eric W. Biederman <ebiederm@...ssion.com> wrote:
>>>
>>> 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;
>>> +}
>>
>> I don't think this works on all platforms.  ACCESS_ONCE is not atomic
>> in the smp_mb__before_atomic sense.
>
> Documentation/atomic_ops.txt documents ACCESS_ONCE as being equivalent
> to atomic_read() and atomic_set().  smp_mb__before_atomic and
> smp_mb__after_atomic() are Documented as working with atomic_read and
> atomic_set.  Maybe it is a stretch to use them but it doesn't seem like
> much of a stretch.

I don't fully understand the design there.  I think this is an attempt
to work around the fact that test_bit is fully atomic on x86 but not
elsewhere.

>
> Further at this point I don't know that any barriers are strictly
> needed, beyond the ACCESS_ONCE.  However since x86 does all of the
> ordering in hardware that I need I am not going to find any bugs that
> don't require a barrier.
>
> All I really want is the same level of barriers I would get if I used a
> spin-lock protected data structure so I don't need to worry about
> crazy smp issues that happen when the hardware decides it is safe to
> reorder things.

Use smp_rmb(), I think.  It'll be obviously correct, and the
performance impact really doesn't matter.

Also, on platforms where this stuff matters, the barrier in
smp_mb__whatever will be a full fence, whereas smp_rmb may be lighter
weight.

--Andy

>
> Eric
>
>
>>> +
>>>  #ifdef CONFIG_USER_NS
>>>
>>>  static inline struct user_namespace *get_user_ns(struct user_namespace *ns)
>>> diff --git a/kernel/groups.c b/kernel/groups.c
>>> index 02d8a251c476..e0335e44f76a 100644
>>> --- a/kernel/groups.c
>>> +++ b/kernel/groups.c
>>> @@ -6,6 +6,7 @@
>>>  #include <linux/slab.h>
>>>  #include <linux/security.h>
>>>  #include <linux/syscalls.h>
>>> +#include <linux/user_namespace.h>
>>>  #include <asm/uaccess.h>
>>>
>>>  /* init to 2 - one for init_task, one to ensure it is never freed */
>>> @@ -217,7 +218,11 @@ bool may_setgroups(void)
>>>  {
>>>         struct user_namespace *user_ns = current_user_ns();
>>>
>>> -       return ns_capable(user_ns, CAP_SETGID);
>>> +       /* It is not safe to use setgroups until a gid mapping in
>>> +        * the user namespace has been established.
>>> +        */
>>> +       return userns_gid_mappings_established(user_ns) &&
>>> +               ns_capable(user_ns, CAP_SETGID);
>>>  }
>>>
>>>  /*
>>> --
>>> 1.9.1
>>>
>>
>> --Andy



-- 
Andy Lutomirski
AMA Capital Management, LLC
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