Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755796AbaLHWsQ (ORCPT ); Mon, 8 Dec 2014 17:48:16 -0500 Received: from mail-la0-f49.google.com ([209.85.215.49]:34704 "EHLO mail-la0-f49.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755698AbaLHWsN (ORCPT ); Mon, 8 Dec 2014 17:48:13 -0500 MIME-Version: 1.0 In-Reply-To: <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> <874mt5ojfh.fsf@x220.int.ebiederm.org> From: Andy Lutomirski Date: Mon, 8 Dec 2014 14:47:50 -0800 Message-ID: Subject: Re: [CFT][PATCH 2/7] userns: Don't allow setgroups until a gid mapping has been setablished To: "Eric W. Biederman" Cc: Richard Weinberger , Linux Containers , Josh Triplett , Andrew Morton , Kees Cook , Michael Kerrisk-manpages , Linux API , linux-man , "linux-kernel@vger.kernel.org" , LSM , Casey Schaufler , "Serge E. Hallyn" , Kenton Varda , stable Content-Type: text/plain; charset=UTF-8 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, Dec 8, 2014 at 2:39 PM, Eric W. Biederman wrote: > 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@vger.kernel.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. > I still don't understand why the helper changed to smp_mb__before_atomic. > 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. As an example, arm64 defines both smp_mb__before_atomic and smp_mb__after_atomic as smp_mb(), which is heavier then smp_rmb(), and there are two of them. So I still like the explicit smp_rmb() better. --Andy > > Eric > -- Andy Lutomirski AMA Capital Management, LLC -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/