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 16:26:54 -0600 Message-ID: <87h9x5ok0h.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> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: (Andy Lutomirski's message of "Mon, 8 Dec 2014 14:11:27 -0800") 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: Andy Lutomirski Cc: linux-man , Kees Cook , Linux API , Linux Containers , Josh Triplett , stable , "linux-kernel-u79uwXL29TY76Z2rM5mHXA@public.gmane.org" , Kenton Varda , LSM , Michael Kerrisk-manpages , Richard Weinberger , Casey Schaufler , Andrew Morton List-Id: linux-man@vger.kernel.org Andy Lutomirski writes: > 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-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; >> +} > > 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. 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. 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 >> #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