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=-7.5 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI, NICE_REPLY_A,SPF_HELO_NONE,SPF_PASS,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 3501BC433E0 for ; Tue, 9 Feb 2021 13:15:30 +0000 (UTC) Received: from lists.xenproject.org (lists.xenproject.org [192.237.175.120]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPS id EAC9E64E92 for ; Tue, 9 Feb 2021 13:15:29 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org EAC9E64E92 Authentication-Results: mail.kernel.org; dmarc=fail (p=quarantine dis=none) header.from=suse.com Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=xen-devel-bounces@lists.xenproject.org Received: from list by lists.xenproject.org with outflank-mailman.83243.154416 (Exim 4.92) (envelope-from ) id 1l9SrQ-0002Cx-Nr; Tue, 09 Feb 2021 13:15:20 +0000 X-Outflank-Mailman: Message body and most headers restored to incoming version Received: by outflank-mailman (output) from mailman id 83243.154416; Tue, 09 Feb 2021 13:15:20 +0000 Received: from localhost ([127.0.0.1] helo=lists.xenproject.org) by lists.xenproject.org with esmtp (Exim 4.92) (envelope-from ) id 1l9SrQ-0002Cq-Kq; Tue, 09 Feb 2021 13:15:20 +0000 Received: by outflank-mailman (input) for mailman id 83243; Tue, 09 Feb 2021 13:15:19 +0000 Received: from us1-rack-iad1.inumbo.com ([172.99.69.81]) by lists.xenproject.org with esmtp (Exim 4.92) (envelope-from ) id 1l9SrP-0002Cl-Hy for xen-devel@lists.xenproject.org; Tue, 09 Feb 2021 13:15:19 +0000 Received: from mx2.suse.de (unknown [195.135.220.15]) by us1-rack-iad1.inumbo.com (Halon) with ESMTPS id 58693e7a-4c06-4031-bdf7-c96fc95b1f18; Tue, 09 Feb 2021 13:15:18 +0000 (UTC) Received: from relay2.suse.de (unknown [195.135.221.27]) by mx2.suse.de (Postfix) with ESMTP id A748CAFE2; Tue, 9 Feb 2021 13:15:17 +0000 (UTC) X-BeenThere: xen-devel@lists.xenproject.org List-Id: Xen developer discussion List-Unsubscribe: , List-Post: List-Help: List-Subscribe: , Errors-To: xen-devel-bounces@lists.xenproject.org Precedence: list Sender: "Xen-devel" X-Inumbo-ID: 58693e7a-4c06-4031-bdf7-c96fc95b1f18 X-Virus-Scanned: by amavisd-new at test-mx.suse.de DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=suse.com; s=susede1; t=1612876517; h=from:from:reply-to:date:date:message-id:message-id:to:to:cc:cc: mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=Nu/y5EI4TivFC/CQT7MSTKDOQzBsp2j+1XANDFk47X8=; b=VUO8T87PEzQgCDiAHrtiYBqe0tePaHFnbiQZpymt0Nr/OmCvMwCxYpAjRLfkxRZWuI1CIj O0PrsLNAgDJhbSkTEg/EQaFveZMrq+xprv2omT4IVxS47f+UmcOVt/OZzJ83sc6Em+cvDj biprcAQIV7pdfrGdOGDRf/B4IcX4Z5g= Subject: Re: [PATCH 02/17] x86: split __{get,put}_user() into "guest" and "unsafe" variants To: =?UTF-8?Q?Roger_Pau_Monn=c3=a9?= Cc: "xen-devel@lists.xenproject.org" , Andrew Cooper , Wei Liu , Tim Deegan , George Dunlap References: <4f1975a9-bdd9-f556-9db5-eb6c428f258f@suse.com> <13d1d621-21db-0e59-6603-2b22b6a9d180@suse.com> <199d2681-9704-8804-d3c3-d8ad24fca137@suse.com> From: Jan Beulich Message-ID: <1cf476b9-4ac1-9a12-7fdb-c898f02532f7@suse.com> Date: Tue, 9 Feb 2021 14:15:18 +0100 User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:78.0) Gecko/20100101 Thunderbird/78.7.1 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 8bit On 09.02.2021 14:07, Roger Pau Monné wrote: > On Fri, Feb 05, 2021 at 05:26:33PM +0100, Jan Beulich wrote: >> On 05.02.2021 17:18, Roger Pau Monné wrote: >>> On Fri, Feb 05, 2021 at 05:13:22PM +0100, Jan Beulich wrote: >>>> On 05.02.2021 16:43, Roger Pau Monné wrote: >>>>> On Thu, Jan 14, 2021 at 04:04:11PM +0100, Jan Beulich wrote: >>>>>> The "guest" variants are intended to work with (potentially) fully guest >>>>>> controlled addresses, while the "unsafe" variants are not. >>>>> >>>>> Just to clarify, both work against user addresses, but guest variants >>>>> need to be more careful because the guest provided address can also be >>>>> modified? >>>>> >>>>> I'm trying to understand the difference between "fully guest >>>>> controlled" and "guest controlled". >>>> >>>> Not exactly, not. "unsafe" means access to anything which may >>>> fault, guest controlled or not. do_invalid_op()'s reading of >>>> the insn stream is a good example - the faulting insn there >>>> isn't guest controlled at all, but we still want to be careful >>>> when trying to read these bytes, as we don't want to fully >>>> trust %rip there. > > Oh, I see. It's possible that %rip points to an unmapped address > there, and we need to be careful when reading, even if the value of > %rip cannot be controlled by the guest and can legitimacy point to > Xen's address space. > >>> Would it make sense to threat everything as 'guest' accesses for the >>> sake of not having this difference? >> >> That's what we've been doing until now. It is the purpose of >> this change to allow the two to behave differently. >> >>> I think having two accessors it's likely to cause confusion and could >>> possibly lead to the wrong one being used in unexpected contexts. Does >>> it add a too big performance penalty to always use the most >>> restrictive one? >> >> The problem is the most restrictive one is going to be too >> restrictive - we wouldn't be able to access Xen space anymore >> e.g. from the place pointed at above as example. This is >> because for guest accesses (but not for "unsafe" ones) we're >> going to divert them into non-canonical space (and hence make >> speculation impossible, as such an access would fault) if it >> would touch Xen space. > > Yes, I understand now. I think it would have been helpful (for me) to > have the first sentence as: > > The "guest" variants are intended to work with (potentially) fully guest > controlled addresses, while the "unsafe" variants are expected to be > used in order to access addresses not under the guest control, but > that could trigger faults anyway (like accessing the instruction > stream in do_invalid_op). I can use some of this, but in particular "access addresses not under the guest control" isn't entirely correct. The question isn't whether there's a guest control aspect, but which part of the address space the addresses are in. See specifically x86/PV: use get_unsafe() instead of copy_from_unsafe()" for two pretty good examples. The address within the linear page tables are - in a way at least - still somewhat guest controlled, but we're deliberately accessing Xen virtual addresses there. Jan