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=-17.4 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_CR_TRAILER, INCLUDES_PATCH,MAILING_LIST_MULTI,NICE_REPLY_A,SPF_HELO_NONE,SPF_PASS, URIBL_BLOCKED,USER_AGENT_SANE_1 autolearn=ham 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 39A74C433E0 for ; Fri, 12 Feb 2021 13:15:59 +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 E0A1164DE3 for ; Fri, 12 Feb 2021 13:15:58 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org E0A1164DE3 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.84263.157958 (Exim 4.92) (envelope-from ) id 1lAYIU-0003Wa-Fc; Fri, 12 Feb 2021 13:15:46 +0000 X-Outflank-Mailman: Message body and most headers restored to incoming version Received: by outflank-mailman (output) from mailman id 84263.157958; Fri, 12 Feb 2021 13:15:46 +0000 Received: from localhost ([127.0.0.1] helo=lists.xenproject.org) by lists.xenproject.org with esmtp (Exim 4.92) (envelope-from ) id 1lAYIU-0003WT-CH; Fri, 12 Feb 2021 13:15:46 +0000 Received: by outflank-mailman (input) for mailman id 84263; Fri, 12 Feb 2021 13:15:45 +0000 Received: from all-amaz-eas1.inumbo.com ([34.197.232.57] helo=us1-amaz-eas2.inumbo.com) by lists.xenproject.org with esmtp (Exim 4.92) (envelope-from ) id 1lAYIT-0003WN-Iz for xen-devel@lists.xenproject.org; Fri, 12 Feb 2021 13:15:45 +0000 Received: from mx2.suse.de (unknown [195.135.220.15]) by us1-amaz-eas2.inumbo.com (Halon) with ESMTPS id 42ccefeb-8b8b-4181-9233-f87b4f09eaad; Fri, 12 Feb 2021 13:15:44 +0000 (UTC) Received: from relay2.suse.de (unknown [195.135.221.27]) by mx2.suse.de (Postfix) with ESMTP id C3362AC90; Fri, 12 Feb 2021 13:15:43 +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: 42ccefeb-8b8b-4181-9233-f87b4f09eaad 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=1613135743; 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=MedWXEGnoEv2qf7wr3/ey/o1lNQKGa50cAPo9p6v26k=; b=VhzsJugKsQ0gaYJ3hqKqTNlEruYn3LAWHXlBKWrapSvYeTF8VYFGdBcmJ/hLhHBJD5utsz LRi3caHomWKNd0X+/7vU4rIsjAJCn/wAX1VsQUPP2KZDx1EzpMbhuDKxpNJg52Xj4DAdC0 pd6tSyIwTDd1gxhMnRqzWFrhsb5ALRM= Subject: Re: [PATCH 04/17] x86/PV: harden guest memory accesses against speculative abuse 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> <5da0c123-3b90-97e8-e1e5-10286be38ce7@suse.com> From: Jan Beulich Message-ID: <9f680f3e-78a5-e2d2-b1f1-8426ebc9613e@suse.com> Date: Fri, 12 Feb 2021 14:15:45 +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 12.02.2021 14:02, Roger Pau Monné wrote: > On Fri, Feb 12, 2021 at 01:48:43PM +0100, Jan Beulich wrote: >> On 12.02.2021 11:41, Roger Pau Monné wrote: >>> On Thu, Jan 14, 2021 at 04:04:57PM +0100, Jan Beulich wrote: >>>> --- a/xen/include/asm-x86/asm-defns.h >>>> +++ b/xen/include/asm-x86/asm-defns.h >>>> @@ -44,3 +44,16 @@ >>>> .macro INDIRECT_JMP arg:req >>>> INDIRECT_BRANCH jmp \arg >>>> .endm >>>> + >>>> +.macro guest_access_mask_ptr ptr:req, scratch1:req, scratch2:req >>>> +#if defined(CONFIG_SPECULATIVE_HARDEN_GUEST_ACCESS) >>>> + mov $(HYPERVISOR_VIRT_END - 1), \scratch1 >>>> + mov $~0, \scratch2 >>>> + cmp \ptr, \scratch1 >>>> + rcr $1, \scratch2 >>>> + and \scratch2, \ptr >>> >>> If my understanding is correct, that's equivalent to: >>> >>> ptr &= ~0ull >> (ptr < HYPERVISOR_VIRT_END); >>> >>> It might be helpful to add this as a comment, to clarify the indented >>> functionality of the assembly bit. >>> >>> I wonder if the C code above can generate any jumps? As you pointed >>> out, we already use something similar in array_index_mask_nospec and >>> that's fine to do in C. >> >> Note how array_index_mask_nospec() gets away without any use of >> relational operators. They're what poses the risk of getting >> translated to branches. (Quite likely the compiler wouldn't use >> any in the case here, as the code can easily get away without, >> but we don't want to chance it. Afaict it would instead use a >> 3rd scratch register, so register pressure might still lead to >> using a branch instead in some exceptional case.) > > I see, it's not easy to build such construct without using any > relational operator. Would you mind adding the C equivalent next to > assembly chunk? Sure: /* * Here we want * * ptr &= ~0ull >> (ptr < HYPERVISOR_VIRT_END); * * but guaranteed without any conditional branches (hence in assembly). */ > I don't think I have further comments: > > Reviewed-by: Roger Pau Monné Thanks much! Jan