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=-7.3 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,NICE_REPLY_A,SPF_HELO_NONE,SPF_PASS,UNPARSEABLE_RELAY, USER_AGENT_SANE_1 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 E3191C5519F for ; Tue, 17 Nov 2020 17:00:46 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 91F4724654 for ; Tue, 17 Nov 2020 17:00:46 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=oracle.com header.i=@oracle.com header.b="ES2GUMrP" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728583AbgKQRAY (ORCPT ); Tue, 17 Nov 2020 12:00:24 -0500 Received: from userp2120.oracle.com ([156.151.31.85]:53872 "EHLO userp2120.oracle.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727231AbgKQRAX (ORCPT ); Tue, 17 Nov 2020 12:00:23 -0500 Received: from pps.filterd (userp2120.oracle.com [127.0.0.1]) by userp2120.oracle.com (8.16.0.42/8.16.0.42) with SMTP id 0AHGxbOP183116; Tue, 17 Nov 2020 16:59:37 GMT DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=oracle.com; h=subject : to : cc : references : from : message-id : date : mime-version : in-reply-to : content-type : content-transfer-encoding; s=corp-2020-01-29; bh=FCltDmIr+jXbG0Ys594WyQPGOm5F5LIyw1Gp1h4Kol4=; b=ES2GUMrPSa5Wdd1dlSxPc5NkJenJvcV5VJvwbONDrNJkh9ISgjc/V5lc/SO4pBEj0AI0 tM8glPlIVypW6pJGQb9pduqJ3zpXfb2xltfoBm4T19HfwpO0CnI+4yE/+n4yb7/lNQIf 5wuuZm9cwv2apFOHaHXsWkj74yHNKWzDl8C2ENYtm4QAPkCOZ/SROqCDXOI37dMSWR2X UEDQV5n5buYD9MzNKmEO/A072h2MWR0FiQSlxsU2tNOJCT5neMhkbvhuS16/K3/w/J9C GSVNvQnBLfdRn0CI9h7Q54Ahu6toFIRD0EjDoU8w6hyF78DJcA7adzmFkM8PITJOI+jI dA== Received: from userp3020.oracle.com (userp3020.oracle.com [156.151.31.79]) by userp2120.oracle.com with ESMTP id 34t7vn3m12-1 (version=TLSv1.2 cipher=ECDHE-RSA-AES256-GCM-SHA384 bits=256 verify=FAIL); Tue, 17 Nov 2020 16:59:37 +0000 Received: from pps.filterd (userp3020.oracle.com [127.0.0.1]) by userp3020.oracle.com (8.16.0.42/8.16.0.42) with SMTP id 0AHGtT4i002703; Tue, 17 Nov 2020 16:59:35 GMT Received: from aserv0122.oracle.com (aserv0122.oracle.com [141.146.126.236]) by userp3020.oracle.com with ESMTP id 34ts0r5jm3-1 (version=TLSv1.2 cipher=ECDHE-RSA-AES256-GCM-SHA384 bits=256 verify=OK); Tue, 17 Nov 2020 16:59:35 +0000 Received: from abhmp0007.oracle.com (abhmp0007.oracle.com [141.146.116.13]) by aserv0122.oracle.com (8.14.4/8.14.4) with ESMTP id 0AHGxWD9023272; Tue, 17 Nov 2020 16:59:32 GMT Received: from linux.home (/92.157.91.83) by default (Oracle Beehive Gateway v4.0) with ESMTP ; Tue, 17 Nov 2020 08:59:32 -0800 Subject: Re: [RFC][PATCH v2 12/21] x86/pti: Use PTI stack instead of trampoline stack To: Andy Lutomirski Cc: Thomas Gleixner , Ingo Molnar , Borislav Petkov , "H. Peter Anvin" , X86 ML , Dave Hansen , Peter Zijlstra , LKML , Tom Lendacky , Joerg Roedel , Konrad Rzeszutek Wilk , jan.setjeeilers@oracle.com, Junaid Shahid , oweisse@google.com, Mike Rapoport , Alexander Graf , mgross@linux.intel.com, kuzuno@gmail.com References: <20201116144757.1920077-1-alexandre.chartre@oracle.com> <20201116144757.1920077-13-alexandre.chartre@oracle.com> From: Alexandre Chartre Message-ID: <6f513efb-cde8-50f4-7872-13a18a10c4a6@oracle.com> Date: Tue, 17 Nov 2020 18:01:52 +0100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.12.0 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 7bit X-Proofpoint-Virus-Version: vendor=nai engine=6000 definitions=9808 signatures=668682 X-Proofpoint-Spam-Details: rule=notspam policy=default score=0 mlxlogscore=999 adultscore=0 bulkscore=0 suspectscore=0 spamscore=0 malwarescore=0 phishscore=0 mlxscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.12.0-2009150000 definitions=main-2011170121 X-Proofpoint-Virus-Version: vendor=nai engine=6000 definitions=9808 signatures=668682 X-Proofpoint-Spam-Details: rule=notspam policy=default score=0 mlxlogscore=999 suspectscore=0 malwarescore=0 bulkscore=0 impostorscore=0 lowpriorityscore=0 spamscore=0 adultscore=0 mlxscore=0 priorityscore=1501 phishscore=0 clxscore=1015 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.12.0-2009150000 definitions=main-2011170122 Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 11/17/20 4:52 PM, Andy Lutomirski wrote: > On Tue, Nov 17, 2020 at 7:07 AM Alexandre Chartre > wrote: >> >> >> >> On 11/16/20 7:34 PM, Andy Lutomirski wrote: >>> On Mon, Nov 16, 2020 at 10:10 AM Alexandre Chartre >>> wrote: >>>> >>>> >>>> On 11/16/20 5:57 PM, Andy Lutomirski wrote: >>>>> On Mon, Nov 16, 2020 at 6:47 AM Alexandre Chartre >>>>> wrote: >>>>>> >>>>>> When entering the kernel from userland, use the per-task PTI stack >>>>>> instead of the per-cpu trampoline stack. Like the trampoline stack, >>>>>> the PTI stack is mapped both in the kernel and in the user page-table. >>>>>> Using a per-task stack which is mapped into the kernel and the user >>>>>> page-table instead of a per-cpu stack will allow executing more code >>>>>> before switching to the kernel stack and to the kernel page-table. >>>>> >>>>> Why? >>>> >>>> When executing more code in the kernel, we are likely to reach a point >>>> where we need to sleep while we are using the user page-table, so we need >>>> to be using a per-thread stack. >>>> >>>>> I can't immediately evaluate how nasty the page table setup is because >>>>> it's not in this patch. >>>> >>>> The page-table is the regular page-table as introduced by PTI. It is just >>>> augmented with a few additional mapping which are in patch 11 (x86/pti: >>>> Extend PTI user mappings). >>>> >>>>> But AFAICS the only thing that this enables is sleeping with user pagetables. >>>> >>>> That's precisely the point, it allows to sleep with the user page-table. >>>> >>>>> Do we really need to do that? >>>> >>>> Actually, probably not with this particular patchset, because I do the page-table >>>> switch at the very beginning and end of the C handler. I had some code where I >>>> moved the page-table switch deeper in the kernel handler where you definitively >>>> can sleep (for example, if you switch back to the user page-table before >>>> exit_to_user_mode_prepare()). >>>> >>>> So a first step should probably be to not introduce the per-task PTI trampoline stack, >>>> and stick with the existing trampoline stack. The per-task PTI trampoline stack can >>>> be introduced later when the page-table switch is moved deeper in the C handler and >>>> we can effectively sleep while using the user page-table. >>> >>> Seems reasonable. >>> >> >> I finally remember why I have introduced a per-task PTI trampoline stack right now: >> that's to be able to move the CR3 switch anywhere in the C handler. To do so, we need >> a per-task stack to enter (and return) from the C handler as the handler can potentially >> go to sleep. >> >> Without a per-task trampoline stack, we would be limited to call the switch CR3 functions >> from the assembly entry code before and after calling the C function handler (also called >> from assembly). > > The noinstr part of the C entry code won't sleep. > But the noinstr part of the handler can sleep, and if it does we will need to preserve the trampoline stack (even if we switch to the per-task kernel stack to execute the noinstr part). Example: #define DEFINE_IDTENTRY(func) \ static __always_inline void __##func(struct pt_regs *regs); \ \ __visible noinstr void func(struct pt_regs *regs) \ { \ irqentry_state_t state; -+ \ | \ user_pagetable_escape(regs); | use trampoline stack (1) state = irqentry_enter(regs); | \ instrumentation_begin(); -+ \ run_idt(__##func, regs); |===| run __func() on kernel stack (this can sleep) instrumentation_end(); -+ \ irqentry_exit(regs, state); | use trampoline stack (2) user_pagetable_return(regs); -+ \ } Between (1) and (2) we need to preserve and use the same trampoline stack in case __func() went sleeping. alex.