userns: Don't allow setgroups until a gid mapping has been setablished

commit 273d2c67c3e179adb1e74f403d1e9a06e3f841b5 upstream.

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.

Reviewed-by: Andy Lutomirski <luto@amacapital.net>
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/include/linux/user_namespace.h b/include/linux/user_namespace.h
index 14105c26a83618da5d91ae09331fb83ba8f8ad6b..648cf88c5d6d5b66c2c246be4820a8672f6ddc5e 100644 (file)
--- a/include/linux/user_namespace.h
+++ b/include/linux/user_namespace.h
@@ -59,6 +59,7 @@ extern struct seq_operations proc_projid_seq_operations;
 extern ssize_t proc_uid_map_write(struct file *, const char __user *, size_t, loff_t *);
 extern ssize_t proc_gid_map_write(struct file *, const char __user *, size_t, loff_t *);
 extern ssize_t proc_projid_map_write(struct file *, const char __user *, size_t, loff_t *);
+extern bool userns_may_setgroups(const struct user_namespace *ns);
 #else
 
 static inline struct user_namespace *get_user_ns(struct user_namespace *ns)
@@ -83,6 +84,10 @@ static inline void put_user_ns(struct user_namespace *ns)
 {
 }
 
+static inline bool userns_may_setgroups(const struct user_namespace *ns)
+{
+       return true;
+}
 #endif
 
 void update_mnt_policy(struct user_namespace *userns);

diff --git a/kernel/groups.c b/kernel/groups.c
index 984bb629c68cfc1687bc2f84295016c170e7aeb0..67b4ba30475fbc2b902e5a6226aeac2d2aaa5803 100644 (file)
--- 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 */
@@ -227,7 +228,8 @@ bool may_setgroups(void)
 {
        struct user_namespace *user_ns = current_user_ns();
 
-       return ns_capable(user_ns, CAP_SETGID);
+       return ns_capable(user_ns, CAP_SETGID) &&
+               userns_may_setgroups(user_ns);
 }
 
 /*

diff --git a/kernel/user_namespace.c b/kernel/user_namespace.c
index 59790206ca2efaa243853675e5161f0ad4bcba54..b083fa233daa63f494e6d4d2f41e8fb8caad7829 100644 (file)
--- a/kernel/user_namespace.c
+++ b/kernel/user_namespace.c
@@ -832,6 +832,20 @@ static bool new_idmap_permitted(const struct file *file,
        return false;
 }
 
+bool userns_may_setgroups(const struct user_namespace *ns)
+{
+       bool allowed;
+
+       mutex_lock(&id_map_mutex);
+       /* It is not safe to use setgroups until a gid mapping in
+        * the user namespace has been established.
+        */
+       allowed = ns->gid_map.nr_extents != 0;
+       mutex_unlock(&id_map_mutex);
+
+       return allowed;
+}
+
 static void *userns_get(struct task_struct *task)
 {
        struct user_namespace *user_ns;