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,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 8ED69C433E0 for ; Fri, 19 Feb 2021 08:56:48 +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 0791864E76 for ; Fri, 19 Feb 2021 08:56:47 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 0791864E76 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.86809.163236 (Exim 4.92) (envelope-from ) id 1lD1aV-0002Jb-9K; Fri, 19 Feb 2021 08:56:35 +0000 X-Outflank-Mailman: Message body and most headers restored to incoming version Received: by outflank-mailman (output) from mailman id 86809.163236; Fri, 19 Feb 2021 08:56:35 +0000 Received: from localhost ([127.0.0.1] helo=lists.xenproject.org) by lists.xenproject.org with esmtp (Exim 4.92) (envelope-from ) id 1lD1aV-0002JU-6Q; Fri, 19 Feb 2021 08:56:35 +0000 Received: by outflank-mailman (input) for mailman id 86809; Fri, 19 Feb 2021 08:56:33 +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 1lD1aT-0002JP-QJ for xen-devel@lists.xenproject.org; Fri, 19 Feb 2021 08:56:33 +0000 Received: from mx2.suse.de (unknown [195.135.220.15]) by us1-amaz-eas2.inumbo.com (Halon) with ESMTPS id 91cd361b-6633-4316-9f5f-11fa1442adcf; Fri, 19 Feb 2021 08:56:32 +0000 (UTC) Received: from relay2.suse.de (unknown [195.135.221.27]) by mx2.suse.de (Postfix) with ESMTP id CB986AED2; Fri, 19 Feb 2021 08:56:31 +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: 91cd361b-6633-4316-9f5f-11fa1442adcf 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=1613724991; 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=ITrIOhijD+0MgNfyXd0YvcHKDVayblTBb3Txi7ML4fk=; b=rR4bkhKpFpDpi3jQc1XlnSqgfuMMYh/hfv1rZduUKPP0uHqtEzRaCWRty3Sy+CwxoRJdJo KjxnJCnAhy+Sn+IaCFJQAhgC0SJ2w5Kh+Aw89QOXujU6CNMscne+gzimXidt+itmwN3SZt JqSYM2ET5xdtcxiRj+eqj0TPGX0wpF4= Subject: Re: [for-4.15][PATCH v3 2/3] xen/x86: iommu: Ignore IOMMU mapping requests when a domain is dying To: paul@xen.org Cc: hongyxia@amazon.co.uk, iwj@xenproject.org, Julien Grall , Andrew Cooper , Kevin Tian , xen-devel@lists.xenproject.org, Julien Grall References: <20210217142458.3769-1-julien@xen.org> <20210217142458.3769-3-julien@xen.org> <20f68b12-a767-b1d3-a3dd-9f92172def5f@suse.com> <268f1991-bacb-c888-04c0-0e4cf670b6b5@suse.com> <14f21eac-7d5d-9dda-18d2-206614e91339@xen.org> From: Jan Beulich Message-ID: Date: Fri, 19 Feb 2021 09:56:32 +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: <14f21eac-7d5d-9dda-18d2-206614e91339@xen.org> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit On 18.02.2021 15:00, Paul Durrant wrote: > On 18/02/2021 13:05, Jan Beulich wrote: >> On 17.02.2021 17:07, Julien Grall wrote: >>> On 17/02/2021 15:01, Jan Beulich wrote: >>>> On 17.02.2021 15:24, Julien Grall wrote: >>>>> From: Julien Grall >>>>> >>>>> The new x86 IOMMU page-tables allocator will release the pages when >>>>> relinquishing the domain resources. However, this is not sufficient >>>>> when the domain is dying because nothing prevents page-table to be >>>>> allocated. >>>>> >>>>> Currently page-table allocations can only happen from iommu_map(). As >>>>> the domain is dying, there is no good reason to continue to modify the >>>>> IOMMU page-tables. >>>> >>>> While I agree this to be the case right now, I'm not sure it is a >>>> good idea to build on it (in that you leave the unmap paths >>>> untouched). >>> >>> I don't build on that assumption. See next patch. >> >> Yet as said there that patch makes unmapping perhaps more fragile, >> by introducing a latent error source into the path. >> >>>> Imo there's a fair chance this would be overlooked at >>>> the point super page mappings get introduced (which has been long >>>> overdue), and I thought prior discussion had lead to a possible >>>> approach without risking use-after-free due to squashed unmap >>>> requests. >>> >>> I know you suggested to zap the root page-tables... However, I don't >>> think this is 4.15 material and you agree with this (you were the one >>> pointed out that out). >> >> Paul - do you have any thoughts here? Outright zapping isn't >> going to work, we'd need to switch to quarantine page tables at >> the very least to prevent the issue with babbling devices. But >> that still seems better to me than the introduction of a latent >> issue on the unmap paths. >> > > I've not really been following this. AFAICT clear_root_pgtable() does > not actually mess with any context entries so the device view of the > tables won't change, will it? Correct. Elsewhere I said that "zapping" here has two meanings, one is what Julien does and the other is what I mean and what I understand you also refer to - to actually replace the mappings referenced by a context entry as soon as we no longer guarantee to update them correctly. My concern with not touching the unmap paths is that if there was any "best effort" unmap left anywhere in the tree, we may still end up with a use-after-free when late unmaps are now made possibly fail by failing late allocation attempts. Jan