From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-0.8 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_PASS autolearn=unavailable autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id F0D70C46475 for ; Thu, 25 Oct 2018 13:15:25 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id BDB0720831 for ; Thu, 25 Oct 2018 13:15:25 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org BDB0720831 Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=i-love.sakura.ne.jp Authentication-Results: mail.kernel.org; spf=none smtp.mailfrom=linux-kernel-owner@vger.kernel.org Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727363AbeJYVsG (ORCPT ); Thu, 25 Oct 2018 17:48:06 -0400 Received: from www262.sakura.ne.jp ([202.181.97.72]:42204 "EHLO www262.sakura.ne.jp" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727125AbeJYVsG (ORCPT ); Thu, 25 Oct 2018 17:48:06 -0400 Received: from fsav404.sakura.ne.jp (fsav404.sakura.ne.jp [133.242.250.103]) by www262.sakura.ne.jp (8.15.2/8.15.2) with ESMTP id w9PDF4X2024317; Thu, 25 Oct 2018 22:15:04 +0900 (JST) (envelope-from penguin-kernel@i-love.sakura.ne.jp) Received: from www262.sakura.ne.jp (202.181.97.72) by fsav404.sakura.ne.jp (F-Secure/fsigk_smtp/530/fsav404.sakura.ne.jp); Thu, 25 Oct 2018 22:15:04 +0900 (JST) X-Virus-Status: clean(F-Secure/fsigk_smtp/530/fsav404.sakura.ne.jp) Received: from [192.168.1.8] (softbank060157065137.bbtec.net [60.157.65.137]) (authenticated bits=0) by www262.sakura.ne.jp (8.15.2/8.15.2) with ESMTPSA id w9PDExVI024230 (version=TLSv1.2 cipher=DHE-RSA-AES256-SHA bits=256 verify=NO); Thu, 25 Oct 2018 22:15:04 +0900 (JST) (envelope-from penguin-kernel@i-love.sakura.ne.jp) Subject: Re: KASAN: use-after-free Read in task_is_descendant To: Oleg Nesterov Cc: serge@hallyn.com, syzbot , jmorris@namei.org, keescook@chromium.org, linux-kernel@vger.kernel.org, linux-security-module@vger.kernel.org, syzkaller-bugs@googlegroups.com References: <76013c9e-0664-ef5e-b6c0-d48f6ce5db3c@i-love.sakura.ne.jp> <20181022134634.GA7358@redhat.com> <201810250215.w9P2Fm2M078167@www262.sakura.ne.jp> <20181025111355.GA3725@redhat.com> <20181025121709.GD3725@redhat.com> From: Tetsuo Handa Message-ID: Date: Thu, 25 Oct 2018 22:14:59 +0900 User-Agent: Mozilla/5.0 (Windows NT 6.3; WOW64; rv:52.0) Gecko/20100101 Thunderbird/52.9.1 MIME-Version: 1.0 In-Reply-To: <20181025121709.GD3725@redhat.com> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 2018/10/25 21:17, Oleg Nesterov wrote: >>> And yes, task_is_descendant() can hit the dead child, if nothing else it can >>> be killed. This can explain the kasan report. >> >> The kasan is reporting that child->real_parent (or maybe child->real_parent->real_parent >> or child->real_parent->real_parent->real_parent ...) was pointing to already freed memory, >> isn't it? > > Yes. and you know, I am all confused. I no longer can understand you :/ Why don't we need to check every time like shown below? Why checking only once is sufficient? --- a/security/yama/yama_lsm.c +++ b/security/yama/yama_lsm.c @@ -285,7 +285,7 @@ static int task_is_descendant(struct task_struct *parent, rcu_read_lock(); if (!thread_group_leader(parent)) parent = rcu_dereference(parent->group_leader); - while (walker->pid > 0) { + while (pid_alive(walker) && walker->pid > 0) { if (!thread_group_leader(walker)) walker = rcu_dereference(walker->group_leader); if (walker == parent) {