Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755913AbaLHWLx (ORCPT ); Mon, 8 Dec 2014 17:11:53 -0500 Received: from mail-lb0-f172.google.com ([209.85.217.172]:38957 "EHLO mail-lb0-f172.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755434AbaLHWLt (ORCPT ); Mon, 8 Dec 2014 17:11:49 -0500 MIME-Version: 1.0 In-Reply-To: <87bnndre2h.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> From: Andy Lutomirski Date: Mon, 8 Dec 2014 14:11:27 -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: 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" , Richard Weinberger , 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 2:07 PM, Eric W. Biederman 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@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; > +} I don't think this works on all platforms. ACCESS_ONCE is not atomic in the smp_mb__before_atomic sense. > + > #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 > #include > #include > +#include > #include > > /* 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 -- 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/