userns: Don't let unprivileged users trick privileged users into setting the id_map

When we require privilege for setting /proc/<pid>/uid_map or
/proc/<pid>/gid_map no longer allow an unprivileged user to
open the file and pass it to a privileged program to write
to the file.

Instead when privilege is required require both the opener and the
writer to have the necessary capabilities.

I have tested this code and verified that setting /proc/<pid>/uid_map
fails when an unprivileged user opens the file and a privielged user
attempts to set the mapping, that unprivileged users can still map
their own id, and that a privileged users can still setup an arbitrary
mapping.
Reported-by: Andy Lutomirski <luto@amacapital.net>
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
Signed-off-by: Andy Lutomirski <luto@amacapital.net>

kernel/user_namespace.c

@@ -25,7 +25,8 @@
 
 static struct kmem_cache *user_ns_cachep __read_mostly;
 
-static bool new_idmap_permitted(struct user_namespace *ns, int cap_setid,
+static bool new_idmap_permitted(const struct file *file,
+				struct user_namespace *ns, int cap_setid,
 				struct uid_gid_map *map);
 
 static void set_cred_user_ns(struct cred *cred, struct user_namespace *user_ns)
@@ -700,7 +701,7 @@ static ssize_t map_write(struct file *file, const char __user *buf,
 
 	ret = -EPERM;
 	/* Validate the user is allowed to use user id's mapped to. */
-	if (!new_idmap_permitted(ns, cap_setid, &new_map))
+	if (!new_idmap_permitted(file, ns, cap_setid, &new_map))
 		goto out;
 
 	/* Map the lower ids from the parent user namespace to the
@@ -787,7 +788,8 @@ ssize_t proc_projid_map_write(struct file *file, const char __user *buf, size_t
 			 &ns->projid_map, &ns->parent->projid_map);
 }
 
-static bool new_idmap_permitted(struct user_namespace *ns, int cap_setid,
+static bool new_idmap_permitted(const struct file *file, 
+				struct user_namespace *ns, int cap_setid,
 				struct uid_gid_map *new_map)
 {
 	/* Allow mapping to your own filesystem ids */
@@ -811,8 +813,10 @@ static bool new_idmap_permitted(struct user_namespace *ns, int cap_setid,
 
 	/* Allow the specified ids if we have the appropriate capability
 	 * (CAP_SETUID or CAP_SETGID) over the parent user namespace.
+	 * And the opener of the id file also had the approprpiate capability.
 	 */
-	if (ns_capable(ns->parent, cap_setid))
+	if (ns_capable(ns->parent, cap_setid) &&
+	    file_ns_capable(file, ns->parent, cap_setid))
 		return true;
 
 	return false;