fs: Refuse uid/gid changes which don't map into s_user_ns

Add checks to inode_change_ok to verify that uid and gid changes
will map into the superblock's user namespace. If they do not
fail with -EOVERFLOW. This cannot be overriden with ATTR_FORCE.
Signed-off-by: Seth Forshee <seth.forshee@canonical.com>

fs/attr.c

@@ -42,6 +42,17 @@ int inode_change_ok(const struct inode *inode, struct iattr *attr)
 			return error;
 	}
 
+	/*
+	 * Verify that uid/gid changes are valid in the target namespace
+	 * of the superblock. This cannot be overriden using ATTR_FORCE.
+	 */
+	if (ia_valid & ATTR_UID &&
+	    from_kuid(inode->i_sb->s_user_ns, attr->ia_uid) == (uid_t)-1)
+		return -EOVERFLOW;
+	if (ia_valid & ATTR_GID &&
+	    from_kgid(inode->i_sb->s_user_ns, attr->ia_gid) == (gid_t)-1)
+		return -EOVERFLOW;
+
 	/* If force is set do it anyway. */
 	if (ia_valid & ATTR_FORCE)
 		return 0;