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 35E89C4332F for ; Thu, 28 Oct 2021 15:56:39 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 11F3C610CA for ; Thu, 28 Oct 2021 15:56:39 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S230183AbhJ1P7F (ORCPT ); Thu, 28 Oct 2021 11:59:05 -0400 Received: from out01.mta.xmission.com ([166.70.13.231]:60400 "EHLO out01.mta.xmission.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229752AbhJ1P7D (ORCPT ); Thu, 28 Oct 2021 11:59:03 -0400 Received: from in01.mta.xmission.com ([166.70.13.51]:54984) by out01.mta.xmission.com with esmtps (TLS1.3) tls TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384 (Exim 4.93) (envelope-from ) id 1mg7lb-00HVH7-MW; Thu, 28 Oct 2021 09:56:35 -0600 Received: from ip68-227-160-95.om.om.cox.net ([68.227.160.95]:42000 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 1mg7la-00BNln-Mc; Thu, 28 Oct 2021 09:56:35 -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 In-Reply-To: <7c99f791-4a87-ae52-bee7-cb794b0741d2@de.ibm.com> (Christian Borntraeger's message of "Tue, 26 Oct 2021 11:38:44 +0200") References: <87y26nmwkb.fsf@disp2133> <20211020174406.17889-11-ebiederm@xmission.com> <7c99f791-4a87-ae52-bee7-cb794b0741d2@de.ibm.com> User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/26.1 (gnu/linux) Date: Thu, 28 Oct 2021 10:56:24 -0500 Message-ID: <87y26dp2hj.fsf@disp2133> MIME-Version: 1.0 Content-Type: text/plain X-XM-SPF: eid=1mg7la-00BNln-Mc;;;mid=<87y26dp2hj.fsf@disp2133>;;;hst=in01.mta.xmission.com;;;ip=68.227.160.95;;;frm=ebiederm@xmission.com;;;spf=neutral X-XM-AID: U2FsdGVkX18rKKOlSmtxyq2C51ggZJ/yg2hgTayDimU= 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 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. In principle it isn't too hard to close that race, and I was hoping to be able to tell you that I had sorted by the time I replied. Unfortunately it looks like it will take another week or two so will probably not be ready by the merge window. I am definitely going to close that race. 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"); >> } >>