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" <ebiederm-aS9lmoZGLiVWk0Htik3J/w@public.gmane.org>
---
 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;
+}

+
 #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