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=-3.9 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SIGNED_OFF_BY,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 A12CFC04AB2 for ; Thu, 9 May 2019 14:13:51 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 7A06F2085A for ; Thu, 9 May 2019 14:13:51 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726933AbfEIONu (ORCPT ); Thu, 9 May 2019 10:13:50 -0400 Received: from shadbolt.e.decadent.org.uk ([88.96.1.126]:46942 "EHLO shadbolt.e.decadent.org.uk" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726694AbfEIONL (ORCPT ); Thu, 9 May 2019 10:13:11 -0400 Received: from [192.168.4.242] (helo=deadeye) by shadbolt.decadent.org.uk with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.89) (envelope-from ) id 1hOjnI-000129-Lt; Thu, 09 May 2019 15:13:08 +0100 Received: from ben by deadeye with local (Exim 4.92) (envelope-from ) id 1hOjnI-0006MP-FI; Thu, 09 May 2019 15:13:08 +0100 Content-Type: text/plain; charset="UTF-8" Content-Disposition: inline Content-Transfer-Encoding: 8bit MIME-Version: 1.0 From: Ben Hutchings To: linux-kernel@vger.kernel.org, stable@vger.kernel.org CC: akpm@linux-foundation.org, Denis Kirjanov , "Tom Gundersen" , "Jann Horn" , "Linus Torvalds" , "David Herrmann" Date: Thu, 09 May 2019 15:08:17 +0100 Message-ID: X-Mailer: LinuxStableQueue (scripts by bwh) X-Patchwork-Hint: ignore Subject: [PATCH 3.16 08/10] fork: record start_time late In-Reply-To: X-SA-Exim-Connect-IP: 192.168.4.242 X-SA-Exim-Mail-From: ben@decadent.org.uk X-SA-Exim-Scanned: No (on shadbolt.decadent.org.uk); SAEximRunCond expanded to false Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 3.16.67-rc1 review patch. If anyone has any objections, please let me know. ------------------ From: David Herrmann commit 7b55851367136b1efd84d98fea81ba57a98304cf upstream. This changes the fork(2) syscall to record the process start_time after initializing the basic task structure but still before making the new process visible to user-space. Technically, we could record the start_time anytime during fork(2). But this might lead to scenarios where a start_time is recorded long before a process becomes visible to user-space. For instance, with userfaultfd(2) and TLS, user-space can delay the execution of fork(2) for an indefinite amount of time (and will, if this causes network access, or similar). By recording the start_time late, it much closer reflects the point in time where the process becomes live and can be observed by other processes. Lastly, this makes it much harder for user-space to predict and control the start_time they get assigned. Previously, user-space could fork a process and stall it in copy_thread_tls() before its pid is allocated, but after its start_time is recorded. This can be misused to later-on cycle through PIDs and resume the stalled fork(2) yielding a process that has the same pid and start_time as a process that existed before. This can be used to circumvent security systems that identify processes by their pid+start_time combination. Even though user-space was always aware that start_time recording is flaky (but several projects are known to still rely on start_time-based identification), changing the start_time to be recorded late will help mitigate existing attacks and make it much harder for user-space to control the start_time a process gets assigned. Reported-by: Jann Horn Signed-off-by: Tom Gundersen Signed-off-by: David Herrmann Signed-off-by: Linus Torvalds [bwh: Backported to 3.16: start_time initialisation code is different] Signed-off-by: Ben Hutchings --- kernel/fork.c | 13 +++++++++++-- 1 file changed, 11 insertions(+), 2 deletions(-) --- a/kernel/fork.c +++ b/kernel/fork.c @@ -1265,9 +1265,6 @@ static struct task_struct *copy_process( posix_cpu_timers_init(p); - do_posix_clock_monotonic_gettime(&p->start_time); - p->real_start_time = p->start_time; - monotonic_to_bootbased(&p->real_start_time); p->io_context = NULL; p->audit_context = NULL; if (clone_flags & CLONE_THREAD) @@ -1423,6 +1420,18 @@ static struct task_struct *copy_process( p->task_works = NULL; /* + * From this point on we must avoid any synchronous user-space + * communication until we take the tasklist-lock. In particular, we do + * not want user-space to be able to predict the process start-time by + * stalling fork(2) after we recorded the start_time but before it is + * visible to the system. + */ + + do_posix_clock_monotonic_gettime(&p->start_time); + p->real_start_time = p->start_time; + monotonic_to_bootbased(&p->real_start_time); + + /* * Make it visible to the rest of the system, but dont wake it up yet. * Need tasklist lock for parent etc handling! */