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

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: [email protected] Reviewed-by: Andy Lutomirski <[email protected]> Signed-off-by: "Eric W. Biederman" <[email protected]>
berg · Dec 9, 2014 · 273d2c6 · 273d2c6
1 parent 0542f17
commit 273d2c6
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Showing 3 changed files with 22 additions and 1 deletion.
diff --git a/include/linux/user_namespace.h b/include/linux/user_namespace.h
@@ -63,6 +63,7 @@ extern const 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)
@@ -87,6 +88,10 @@ static inline void put_user_ns(struct user_namespace *ns)
 {
 }
 
+static inline bool userns_may_setgroups(const struct user_namespace *ns)
+{
+	return true;
+}
 #endif
 
 #endif /* _LINUX_USER_H */
diff --git 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 +218,8 @@ 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
@@ -843,6 +843,20 @@ 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;