From mboxrd@z Thu Jan 1 00:00:00 1970 From: ebiederm-aS9lmoZGLiVWk0Htik3J/w@public.gmane.org (Eric W. Biederman) Subject: Re: [CFT][PATCH 2/7] userns: Don't allow setgroups until a gid mapping has been setablished Date: Mon, 08 Dec 2014 16:39:30 -0600 Message-ID: <874mt5ojfh.fsf@x220.int.ebiederm.org> References: <52e0643bd47b1e5c65921d6e00aea1f724bb510a.1417281801.git.luto@amacapital.net> <87h9xez20g.fsf@x220.int.ebiederm.org> <87mw75ygwp.fsf@x220.int.ebiederm.org> <87fvcxyf28.fsf_-_@x220.int.ebiederm.org> <874mtdyexp.fsf_-_@x220.int.ebiederm.org> <87a935u3nj.fsf@x220.int.ebiederm.org> <87388xodlj.fsf@x220.int.ebiederm.org> <87h9x5re41.fsf_-_@x220.int.ebiederm.org> <87bnndre2h.fsf_-_@x220.int.ebiederm.org> <5486237D.4060304@nod.at> <548625E3.6020400@nod.at> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <548625E3.6020400-/L3Ra7n9ekc@public.gmane.org> (Richard Weinberger's message of "Mon, 08 Dec 2014 23:27:47 +0100") List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: containers-bounces-cunTk1MwBs9QetFLy7KEm3xJsTq8ys+cHZ5vskTnxNA@public.gmane.org Errors-To: containers-bounces-cunTk1MwBs9QetFLy7KEm3xJsTq8ys+cHZ5vskTnxNA@public.gmane.org To: Richard Weinberger Cc: linux-man , Kees Cook , Linux API , Linux Containers , Josh Triplett , stable , Andy Lutomirski , Kenton Varda , LSM , Michael Kerrisk-manpages , Casey Schaufler , Andrew Morton , "linux-kernel-u79uwXL29TY76Z2rM5mHXA@public.gmane.org" List-Id: containers.vger.kernel.org Richard Weinberger writes: > Am 08.12.2014 um 23:25 schrieb Andy Lutomirski: >> On Mon, Dec 8, 2014 at 2:17 PM, Richard Weinberger 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-u79uwXL29TY76Z2rM5mHXA@public.gmane.org >>>> Signed-off-by: "Eric W. Biederman" >>>> --- >>>> 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. Read Documentation/atomic_ops.txt a plain u32 is atomic by definiton. Which is a little bit convoluted. However that is part of the of the gid mapping path and I optimized that as far as I humanly could so that calls like stat don't take a noticable slow donw. On this path we don't particularly care except that I am using an the existing data structure. Eric