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=-2.3 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,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 C89FAC433E0 for ; Tue, 19 May 2020 14:49:28 +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 9F56A2072C for ; Tue, 19 May 2020 14:49:28 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 9F56A2072C Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=suse.com Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=xen-devel-bounces@lists.xenproject.org Received: from localhost ([127.0.0.1] helo=lists.xenproject.org) by lists.xenproject.org with esmtp (Exim 4.92) (envelope-from ) id 1jb3YB-0004hI-1Y; Tue, 19 May 2020 14:48:59 +0000 Received: from us1-rack-iad1.inumbo.com ([172.99.69.81]) by lists.xenproject.org with esmtp (Exim 4.92) (envelope-from ) id 1jb3YA-0004hD-C3 for xen-devel@lists.xenproject.org; Tue, 19 May 2020 14:48:58 +0000 X-Inumbo-ID: de2745cc-99df-11ea-b07b-bc764e2007e4 Received: from mx2.suse.de (unknown [195.135.220.15]) by us1-rack-iad1.inumbo.com (Halon) with ESMTPS id de2745cc-99df-11ea-b07b-bc764e2007e4; Tue, 19 May 2020 14:48:57 +0000 (UTC) X-Virus-Scanned: by amavisd-new at test-mx.suse.de Received: from relay2.suse.de (unknown [195.135.220.254]) by mx2.suse.de (Postfix) with ESMTP id 300FDADF7; Tue, 19 May 2020 14:48:59 +0000 (UTC) Subject: Re: [PATCH] x86/traps: Rework #PF[Rsvd] bit handling To: Andrew Cooper References: <20200518153820.18170-1-andrew.cooper3@citrix.com> <2783ddc5-9919-3c97-ba52-2f734e7d72d5@suse.com> <62d4999b-7db3-bac6-28ed-bb636347df38@citrix.com> From: Jan Beulich Message-ID: <3088e420-a72a-1b2d-144f-115610488418@suse.com> Date: Tue, 19 May 2020 16:48:55 +0200 User-Agent: Mozilla/5.0 (Windows NT 6.1; WOW64; rv:68.0) Gecko/20100101 Thunderbird/68.8.0 MIME-Version: 1.0 In-Reply-To: <62d4999b-7db3-bac6-28ed-bb636347df38@citrix.com> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 8bit X-BeenThere: xen-devel@lists.xenproject.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: Xen developer discussion List-Unsubscribe: , List-Post: List-Help: List-Subscribe: , Cc: Xen-devel , Wei Liu , =?UTF-8?Q?Roger_Pau_Monn=c3=a9?= Errors-To: xen-devel-bounces@lists.xenproject.org Sender: "Xen-devel" On 19.05.2020 16:11, Andrew Cooper wrote: > On 19/05/2020 09:34, Jan Beulich wrote: >> On 18.05.2020 17:38, Andrew Cooper wrote: >>> @@ -1439,6 +1418,18 @@ void do_page_fault(struct cpu_user_regs *regs) >>> if ( unlikely(fixup_page_fault(addr, regs) != 0) ) >>> return; >>> >>> + /* >>> + * Xen have reserved bits in its pagetables, nor do we permit PV guests to >>> + * write any. Such entries would be vulnerable to the L1TF sidechannel. >>> + * >>> + * The only logic which intentionally sets reserved bits is the shadow >>> + * MMIO fastpath (SH_L1E_MMIO_*), which is careful not to be >>> + * L1TF-vulnerable, and handled via the VMExit #PF intercept path, rather >>> + * than here. >> What about SH_L1E_MAGIC and sh_l1e_gnp()? The latter gets used by >> _sh_propagate() without visible restriction to HVM. > > SH_L1E_MAGIC looks to be redundant with SH_L1E_MMIO_MAGIC.  > sh_l1e_mmio() is the only path which ever creates an entry like that. > > sh_l1e_gnp() is a very well hidden use of reserved bits, but surely > can't be used for PV guests, as there doesn't appear to be anything to > turn the resulting fault back into a plain not-present. Well, in this case the implied question remains: How does this fit with what _sh_propagate() does? >> And of course every time I look at this code I wonder how we can >> get away with (quoting a comment) "We store 28 bits of GFN in >> bits 4:32 of the entry." Do we have a hidden restriction >> somewhere guaranteeing that guests won't have (emulated MMIO) >> GFNs above 1Tb when run in shadow mode? > > I've raised that several times before.  Its broken. > > Given that shadow frames are limited to 44 bits anyway (and not yet > levelled safely in the migration stream), my suggestion for fixing this > was just to use one extra nibble for the extra 4 bits and call it done. Would you remind(?) me of where this 44-bit restriction is coming from? Jan