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 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 8680FC4332F for ; Fri, 29 Oct 2021 19:34:30 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 6B97460F93 for ; Fri, 29 Oct 2021 19:34:30 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S230334AbhJ2Tg6 (ORCPT ); Fri, 29 Oct 2021 15:36:58 -0400 Received: from out03.mta.xmission.com ([166.70.13.233]:37482 "EHLO out03.mta.xmission.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229474AbhJ2Tg4 (ORCPT ); Fri, 29 Oct 2021 15:36:56 -0400 Received: from in01.mta.xmission.com ([166.70.13.51]:57118) by out03.mta.xmission.com with esmtps (TLS1.3) tls TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384 (Exim 4.93) (envelope-from ) id 1mgXdx-007QLV-Jn; Fri, 29 Oct 2021 13:34:25 -0600 Received: from ip68-227-160-95.om.om.cox.net ([68.227.160.95]:32942 helo=email.xmission.com) by in01.mta.xmission.com with esmtpsa (TLS1.3) tls TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384 (Exim 4.93) (envelope-from ) id 1mgXdw-00EJtM-9Y; Fri, 29 Oct 2021 13:34:24 -0600 From: ebiederm@xmission.com (Eric W. Biederman) To: Christian Borntraeger Cc: linux-kernel@vger.kernel.org, linux-arch@vger.kernel.org, Linus Torvalds , Oleg Nesterov , Al Viro , Kees Cook , Heiko Carstens , Vasily Gorbik , linux-s390@vger.kernel.org References: <87y26nmwkb.fsf@disp2133> <20211020174406.17889-11-ebiederm@xmission.com> <7c99f791-4a87-ae52-bee7-cb794b0741d2@de.ibm.com> <87y26dp2hj.fsf@disp2133> Date: Fri, 29 Oct 2021 14:32:36 -0500 In-Reply-To: <87y26dp2hj.fsf@disp2133> (Eric W. Biederman's message of "Thu, 28 Oct 2021 10:56:24 -0500") Message-ID: <87v91fhbjf.fsf@disp2133> User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/26.1 (gnu/linux) MIME-Version: 1.0 Content-Type: text/plain X-XM-SPF: eid=1mgXdw-00EJtM-9Y;;;mid=<87v91fhbjf.fsf@disp2133>;;;hst=in01.mta.xmission.com;;;ip=68.227.160.95;;;frm=ebiederm@xmission.com;;;spf=neutral X-XM-AID: U2FsdGVkX1+IQGABJNds1K89FHzqTI6D9ZKRfirmdT4= X-SA-Exim-Connect-IP: 68.227.160.95 X-SA-Exim-Mail-From: ebiederm@xmission.com Subject: Re: [PATCH 11/20] signal/s390: Use force_sigsegv in default_trap_handler X-SA-Exim-Version: 4.2.1 (built Sat, 08 Feb 2020 21:53:50 +0000) X-SA-Exim-Scanned: Yes (on in01.mta.xmission.com) Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org ebiederm@xmission.com (Eric W. Biederman) writes: > Christian Borntraeger writes: > >> Am 20.10.21 um 19:43 schrieb Eric W. Biederman: >>> Reading the history it is unclear why default_trap_handler calls >>> do_exit. It is not even menthioned in the commit where the change >>> happened. My best guess is that because it is unknown why the >>> exception happened it was desired to guarantee the process never >>> returned to userspace. >>> >>> Using do_exit(SIGSEGV) has the problem that it will only terminate one >>> thread of a process, leaving the process in an undefined state. >>> >>> Use force_sigsegv(SIGSEGV) instead which effectively has the same >>> behavior except that is uses the ordinary signal mechanism and >>> terminates all threads of a process and is generally well defined. >> >> Do I get that right, that programs can not block SIGSEGV from force_sigsegv >> with a signal handler? Thats how I read the code. If this is true >> then >> >> Reviewed-by: Christian Borntraeger > > 99% true, and it is what force_sigsegv(SIGSEGV) intends to do. > > Andy Lutormorski pointed at a race where a thread can call sigaction > and change the signal handler after force_sigsegv has run but before > the process dequeues the SIGSEGV. I now have a simple patch that closes the sigaction vs force_sig race, that I am adding to this set of changes. So now I can say programs can not block force_sigsegv(SIGSEGV) with a signal handler or any other method. Eric >>> Cc: Heiko Carstens >>> Cc: Vasily Gorbik >>> Cc: Christian Borntraeger >>> Cc: linux-s390@vger.kernel.org >>> Fixes: ca2ab03237ec ("[PATCH] s390: core changes") >>> History Tree: https://git.kernel.org/pub/scm/linux/kernel/git/tglx/history.git >>> Signed-off-by: "Eric W. Biederman" >>> --- >>> arch/s390/kernel/traps.c | 2 +- >>> 1 file changed, 1 insertion(+), 1 deletion(-) >>> >>> diff --git a/arch/s390/kernel/traps.c b/arch/s390/kernel/traps.c >>> index bcefc2173de4..51729ea2cf8e 100644 >>> --- a/arch/s390/kernel/traps.c >>> +++ b/arch/s390/kernel/traps.c >>> @@ -84,7 +84,7 @@ static void default_trap_handler(struct pt_regs *regs) >>> { >>> if (user_mode(regs)) { >>> report_user_fault(regs, SIGSEGV, 0); >>> - do_exit(SIGSEGV); >>> + force_sigsegv(SIGSEGV); >>> } else >>> die(regs, "Unknown program exception"); >>> } >>>