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=-2.3 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_PASS,USER_AGENT_MUTT autolearn=ham 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 E7C6BC004D3 for ; Mon, 22 Oct 2018 13:46:38 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id C887B2064C for ; Mon, 22 Oct 2018 13:46:36 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org C887B2064C Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=redhat.com Authentication-Results: mail.kernel.org; spf=none smtp.mailfrom=linux-security-module-owner@vger.kernel.org Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727587AbeJVWFR (ORCPT ); Mon, 22 Oct 2018 18:05:17 -0400 Received: from mx1.redhat.com ([209.132.183.28]:25872 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727210AbeJVWFR (ORCPT ); Mon, 22 Oct 2018 18:05:17 -0400 Received: from smtp.corp.redhat.com (int-mx04.intmail.prod.int.phx2.redhat.com [10.5.11.14]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id 412788830A; Mon, 22 Oct 2018 13:46:37 +0000 (UTC) Received: from dhcp-27-174.brq.redhat.com (unknown [10.43.17.106]) by smtp.corp.redhat.com (Postfix) with SMTP id BC27C5ED3A; Mon, 22 Oct 2018 13:46:34 +0000 (UTC) Received: by dhcp-27-174.brq.redhat.com (nbSMTP-1.00) for uid 1000 oleg@redhat.com; Mon, 22 Oct 2018 15:46:36 +0200 (CEST) Date: Mon, 22 Oct 2018 15:46:34 +0200 From: Oleg Nesterov To: Tetsuo Handa 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 Subject: Re: KASAN: use-after-free Read in task_is_descendant Message-ID: <20181022134634.GA7358@redhat.com> References: <0000000000004904df0578b7d3da@google.com> <20181022095439.GA4613@redhat.com> <76013c9e-0664-ef5e-b6c0-d48f6ce5db3c@i-love.sakura.ne.jp> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <76013c9e-0664-ef5e-b6c0-d48f6ce5db3c@i-love.sakura.ne.jp> User-Agent: Mutt/1.5.24 (2015-08-30) X-Scanned-By: MIMEDefang 2.79 on 10.5.11.14 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.28]); Mon, 22 Oct 2018 13:46:37 +0000 (UTC) Sender: owner-linux-security-module@vger.kernel.org Precedence: bulk List-ID: On 10/22, Tetsuo Handa wrote: > > > However, task_is_descendant() looks unnecessarily complicated, it could be > > > > static int task_is_descendant(struct task_struct *parent, > > struct task_struct *child) > > { > > int rc = 0; > > struct task_struct *walker; > > > > if (!parent || !child) > > return 0; > > > > rcu_read_lock(); > > for (walker = child; walker->pid; walker = rcu_dereference(walker->real_parent)) > > if (same_thread_group(parent, walker)) { > > rc = 1; > > break; > > } > > rcu_read_unlock(); > > > > return rc; > > } > > > > And again, I do not know how/if yama ensures that child is rcu-protected, perhaps > > task_is_descendant() needs to check pid_alive(child) right after rcu_read_lock() ? > > Since the caller (ptrace() path) called get_task_struct(child), child itself can't be > released. Do we still need pid_alive(child) ? get_task_struct(child) can only ensure that this task_struct can't be freed. Suppose that this child exits after get_task_struct(), then its real_parent exits too and calls call_rcu(delayed_put_task_struct). Now, when task_is_descendant() is called, rcu_read_lock() can happen after rcu gp, iow child->parent can be already freed/reused/unmapped. We need to ensure that child is still protected by RCU. Oleg.