userns: Convert user specfied uids and gids in chown into kuids and kgid

Acked-by: Serge Hallyn <[e-mail]>
Signed-off-by: Eric W. Biederman <[e-mail]>

fs/open.c

···
 	struct inode *inode = path->dentry->d_inode;
 	int error;
 	struct iattr newattrs;
+	kuid_t uid;
+	kgid_t gid;
+
+	uid = make_kuid(current_user_ns(), user);
+	gid = make_kgid(current_user_ns(), group);
 
 	newattrs.ia_valid =  ATTR_CTIME;
 	if (user != (uid_t) -1) {
+		if (!uid_valid(uid))
+			return -EINVAL;
 		newattrs.ia_valid |= ATTR_UID;
-		newattrs.ia_uid = user;
+		newattrs.ia_uid = uid;
 	}
 	if (group != (gid_t) -1) {
+		if (!gid_valid(gid))
+			return -EINVAL;
 		newattrs.ia_valid |= ATTR_GID;
-		newattrs.ia_gid = group;
+		newattrs.ia_gid = gid;
 	}
 	if (!S_ISDIR(inode->i_mode))
 		newattrs.ia_valid |=