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=-3.8 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS, URIBL_BLOCKED autolearn=no 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 DCB3EC48BE6 for ; Wed, 16 Jun 2021 20:57:33 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id B9EF2613D8 for ; Wed, 16 Jun 2021 20:57:33 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S233807AbhFPU7h (ORCPT ); Wed, 16 Jun 2021 16:59:37 -0400 Received: from out03.mta.xmission.com ([166.70.13.233]:52468 "EHLO out03.mta.xmission.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S233698AbhFPU7f (ORCPT ); Wed, 16 Jun 2021 16:59:35 -0400 Received: from in01.mta.xmission.com ([166.70.13.51]) by out03.mta.xmission.com with esmtps (TLS1.3) tls TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384 (Exim 4.93) (envelope-from ) id 1ltcbG-00ATlW-UN; Wed, 16 Jun 2021 14:57:26 -0600 Received: from ip68-227-160-95.om.om.cox.net ([68.227.160.95] 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 1ltcbF-00HDkM-UZ; Wed, 16 Jun 2021 14:57:26 -0600 From: ebiederm@xmission.com (Eric W. Biederman) To: Linus Torvalds Cc: Michael Schmitz , linux-arch , Jens Axboe , Oleg Nesterov , Al Viro , Linux Kernel Mailing List , Richard Henderson , Ivan Kokshaysky , Matt Turner , alpha , Geert Uytterhoeven , linux-m68k , Arnd Bergmann , Ley Foon Tan , Tejun Heo , Kees Cook References: <87sg1p30a1.fsf@disp2133> <87pmwsytb3.fsf@disp2133> <87sg1lwhvm.fsf@disp2133> <6e47eff8-d0a4-8390-1222-e975bfbf3a65@gmail.com> <924ec53c-2fd9-2e1c-bbb1-3fda49809be4@gmail.com> <87eed4v2dc.fsf@disp2133> <5929e116-fa61-b211-342a-c706dcb834ca@gmail.com> <87fsxjorgs.fsf@disp2133> <87zgvqor7d.fsf_-_@disp2133> <87mtrpg47k.fsf@disp2133> <87pmwlek8d.fsf_-_@disp2133> <87k0mtek4n.fsf_-_@disp2133> Date: Wed, 16 Jun 2021 15:57:19 -0500 In-Reply-To: (Linus Torvalds's message of "Wed, 16 Jun 2021 13:37:29 -0700") Message-ID: <878s398r4g.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=1ltcbF-00HDkM-UZ;;;mid=<878s398r4g.fsf@disp2133>;;;hst=in01.mta.xmission.com;;;ip=68.227.160.95;;;frm=ebiederm@xmission.com;;;spf=neutral X-XM-AID: U2FsdGVkX1/LF91xWCnMbfZu757yY8QEIXk7UwFgnxc= X-SA-Exim-Connect-IP: 68.227.160.95 X-SA-Exim-Mail-From: ebiederm@xmission.com Subject: Re: [PATCH 1/2] alpha/ptrace: Record and handle the absence of switch_stack 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 Linus Torvalds writes: > On Wed, Jun 16, 2021 at 1:00 PM Linus Torvalds > wrote: >> >> And even for debugging, I think it would be both easier and cheaper to >> just add a magic word to the entry stack instead. > > IOW, just add a > > unsigned long magic; > > to "struct switch_stack", and then make the stack switch code push that value. > > That would be cheap enough to be just unconditional, but you could > make it depend on a debug config option too, of course. > > It helps if 'xyz' is some constant that is easyish to generate. It > might not be a constant - maybe it could be the address of that > 'magic' field itself, so you'd just generate it with > > stq $r,($r) > > and it would be equally easy to just validate at lookup for that WARN_ON_ONCE(): > > WARN_ON_ONCE(switch_stack->magic != (unsigned long)&switch_stack->magic); > > or whatever. > > It's for debugging, not security. So it doesn't have to be some kind > of super-great magic number, just something easy to generate and check > (that isn't a common value like "0" that trivially exist on the stack > anyway). Fair enough. I was thinking for a moment that do_sigreturn might have a problem with that but restore_sigcontext makes it clear that struct switch_stack is not exposed to userspace. Do you know if struct switch_stack or pt_regs is ever exposeed to usespace? They are both defined in arch/alpha/include/uapi/asm/ptrace.h which makes me think userspace must see those definitions somewhere. Eric