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=-10.5 required=3.0 tests=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SIGNED_OFF_BY, SPF_PASS,URIBL_BLOCKED,USER_AGENT_GIT 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 328F4C43387 for ; Fri, 11 Jan 2019 14:42:42 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 0243F21848 for ; Fri, 11 Jan 2019 14:42:42 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1547217762; bh=+0th/WukIC0QQpvb+Lp3RvNBIw+8cl4B0gw3J/JOL4s=; h=From:To:Cc:Subject:Date:In-Reply-To:References:List-ID:From; b=1+qrfGn3vbV9Lo19QlY1MDjz8lyC15/FmVeQTu5JO1ixkfhDNOxqWZEhMH5UOUiNL 19W5QtXufYeo910bWdqtRjj1AIuaChyrCPOXIMbWLadhdkvkgSip60D7w2E8nYQNVp vWFUWZM8Gy/Z87SYwMtMn/OONf1SMpFG3ffK1RX0= Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S2404166AbfAKOmk (ORCPT ); Fri, 11 Jan 2019 09:42:40 -0500 Received: from mail.kernel.org ([198.145.29.99]:35434 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S2404208AbfAKOmk (ORCPT ); Fri, 11 Jan 2019 09:42:40 -0500 Received: from localhost (5356596B.cm-6-7b.dynamic.ziggo.nl [83.86.89.107]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPSA id DD0212063F; Fri, 11 Jan 2019 14:42:38 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1547217759; bh=+0th/WukIC0QQpvb+Lp3RvNBIw+8cl4B0gw3J/JOL4s=; h=From:To:Cc:Subject:Date:In-Reply-To:References:From; b=P2fEPQxEQEuKohIZ+IPfqPrgmZvFuJ5H2erUdRDWukJcWtBfxm4CIik0kVl1P0voT MTg9+0r6yV6HfCitJLWxfSQ1QGVmJBCFH4gcFV9ofdIypxOuNbkkcnIFOZd9KE/JcE 2dPlHwLVC+eKWYbMv2QJZpkSriE0L75+4c8TuWFo= From: Greg Kroah-Hartman To: linux-kernel@vger.kernel.org Cc: Greg Kroah-Hartman , stable@vger.kernel.org, Jann Horn , Tom Gundersen , David Herrmann , Linus Torvalds Subject: [PATCH 4.20 03/65] fork: record start_time late Date: Fri, 11 Jan 2019 15:14:49 +0100 Message-Id: <20190111131056.544951319@linuxfoundation.org> X-Mailer: git-send-email 2.20.1 In-Reply-To: <20190111131055.331350141@linuxfoundation.org> References: <20190111131055.331350141@linuxfoundation.org> User-Agent: quilt/0.65 X-stable: review X-Patchwork-Hint: ignore MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit Sender: stable-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: stable@vger.kernel.org 4.20-stable 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 Signed-off-by: Greg Kroah-Hartman --- kernel/fork.c | 13 +++++++++++-- 1 file changed, 11 insertions(+), 2 deletions(-) --- a/kernel/fork.c +++ b/kernel/fork.c @@ -1837,8 +1837,6 @@ static __latent_entropy struct task_stru posix_cpu_timers_init(p); - p->start_time = ktime_get_ns(); - p->real_start_time = ktime_get_boot_ns(); p->io_context = NULL; audit_set_context(p, NULL); cgroup_fork(p); @@ -2005,6 +2003,17 @@ static __latent_entropy struct task_stru goto bad_fork_free_pid; /* + * 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. + */ + + p->start_time = ktime_get_ns(); + p->real_start_time = ktime_get_boot_ns(); + + /* * Make it visible to the rest of the system, but dont wake it up yet. * Need tasklist lock for parent etc handling! */