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 C7522C48BE6 for ; Wed, 16 Jun 2021 20:43:05 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 9BB216128B for ; Wed, 16 Jun 2021 20:43:05 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S233658AbhFPUpK (ORCPT ); Wed, 16 Jun 2021 16:45:10 -0400 Received: from out02.mta.xmission.com ([166.70.13.232]:56806 "EHLO out02.mta.xmission.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S233639AbhFPUpJ (ORCPT ); Wed, 16 Jun 2021 16:45:09 -0400 Received: from in02.mta.xmission.com ([166.70.13.52]) by out02.mta.xmission.com with esmtps (TLS1.3) tls TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384 (Exim 4.93) (envelope-from ) id 1ltcND-000wZU-Bn; Wed, 16 Jun 2021 14:42:56 -0600 Received: from ip68-227-160-95.om.om.cox.net ([68.227.160.95] helo=email.xmission.com) by in02.mta.xmission.com with esmtpsa (TLS1.3) tls TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384 (Exim 4.93) (envelope-from ) id 1ltcNC-001U5E-8W; Wed, 16 Jun 2021 14:42:54 -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 15:42:05 -0500 In-Reply-To: (Linus Torvalds's message of "Wed, 16 Jun 2021 13:00:52 -0700") Message-ID: <87czsla6ea.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=1ltcNC-001U5E-8W;;;mid=<87czsla6ea.fsf@disp2133>;;;hst=in02.mta.xmission.com;;;ip=68.227.160.95;;;frm=ebiederm@xmission.com;;;spf=neutral X-XM-AID: U2FsdGVkX19cp4yLb9sESwklHZupra9p6ENsPqWdcfE= 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 in02.mta.xmission.com) Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Linus Torvalds writes: > On Wed, Jun 16, 2021 at 11:32 AM Eric W. Biederman > wrote: >> >> Prevent security holes by recording when all of the registers are >> available so generic code changes do not result in security holes >> on alpha. > > Please no, not this way. ldl/stc is extremely expensive on some alpha cpus. > > I really think thatTIF_ALLREGS_SAVED bit isn't worth it, except > perhaps for debugging. > > And even for debugging, I think it would be both easier and cheaper to > just add a magic word to the entry stack instead. I think I can do something like that. Looking at arch/alpha/asm/cache.h it looks like alpha had either 32byte or 64bit cachelines. Which makes struct switch_stack a full 10 or 5 cachelines in size. So pushing something extra might hit an extra cacheline. However it looks like struct pt_regs is 16 bytes short of a full cache line so struct switch_stack isn't going to be cacheline aligned. Adding an extra 8 bytes of magic number will hopefully be in the noise. If I can I would like to find something that is cheap enough that I can always leave on. Mostly because there is little enough testing that a bug that allows anyone to stomp the kernel stack has existed for 17 years without being noticed. If you want it to be a debug option only I can certainly make that happen. I am still going "Eek! Arbitrary stack smash!" in my head. Eric