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=-12.3 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_PATCH, MAILING_LIST_MULTI,NICE_REPLY_A,SPF_HELO_NONE,SPF_PASS,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 8A5B1C433E0 for ; Fri, 19 Feb 2021 08:47:00 +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 1C9CD64D9F for ; Fri, 19 Feb 2021 08:47:00 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 1C9CD64D9F 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.86806.163213 (Exim 4.92) (envelope-from ) id 1lD1Qx-0001Ab-4c; Fri, 19 Feb 2021 08:46:43 +0000 X-Outflank-Mailman: Message body and most headers restored to incoming version Received: by outflank-mailman (output) from mailman id 86806.163213; Fri, 19 Feb 2021 08:46:43 +0000 Received: from localhost ([127.0.0.1] helo=lists.xenproject.org) by lists.xenproject.org with esmtp (Exim 4.92) (envelope-from ) id 1lD1Qx-0001AU-0s; Fri, 19 Feb 2021 08:46:43 +0000 Received: by outflank-mailman (input) for mailman id 86806; Fri, 19 Feb 2021 08:46:42 +0000 Received: from us1-rack-iad1.inumbo.com ([172.99.69.81]) by lists.xenproject.org with esmtp (Exim 4.92) (envelope-from ) id 1lD1Qv-0001AP-Vx for xen-devel@lists.xenproject.org; Fri, 19 Feb 2021 08:46:42 +0000 Received: from mx2.suse.de (unknown [195.135.220.15]) by us1-rack-iad1.inumbo.com (Halon) with ESMTPS id be2a6f6e-ec37-43c3-92e6-fb57a8345150; Fri, 19 Feb 2021 08:46:40 +0000 (UTC) Received: from relay2.suse.de (unknown [195.135.221.27]) by mx2.suse.de (Postfix) with ESMTP id 18929ACCF; Fri, 19 Feb 2021 08:46:40 +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: be2a6f6e-ec37-43c3-92e6-fb57a8345150 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=1613724400; 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=r3RyBAYQCeDYzG/Exeg8ZGxCTU7G67wdtJL3rkXfd9g=; b=BPxy0O2Pk5cCHPNBxhDbSu95YAqmx+wnOPtTyDhfvAj81AEA8r9T9b53xx6DiWoKoCUPI1 fnyeq4O9Ngk0832No9vOxBKgrqoWmj/7W5CxMuCWIp6emOYwpibOeJTarK+zFgu68rNYlt VOhrAnVZQ4wTq4M9YZwjoZ5rSBCbmjo= Subject: Re: [for-4.15][PATCH v3 3/3] xen/iommu: x86: Harden the IOMMU page-table allocator To: Julien Grall Cc: hongyxia@amazon.co.uk, iwj@xenproject.org, Julien Grall , Paul Durrant , xen-devel@lists.xenproject.org References: <20210217142458.3769-1-julien@xen.org> <20210217142458.3769-4-julien@xen.org> <51618338-daff-5b9a-5214-e0788d95992b@xen.org> <96971bbb-05ec-7df0-a8d7-931cc0b41a77@xen.org> <141ea545-3725-5305-d352-057ff7c70c4f@suse.com> <6e467ed0-34f1-498d-a9ce-7e0f2e606033@xen.org> From: Jan Beulich Message-ID: Date: Fri, 19 Feb 2021 09:46:40 +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: <6e467ed0-34f1-498d-a9ce-7e0f2e606033@xen.org> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit On 18.02.2021 18:41, Julien Grall wrote: > > > On 18/02/2021 17:04, Jan Beulich wrote: >> On 18.02.2021 14:19, Julien Grall wrote: >>> >>> >>> On 18/02/2021 13:10, Jan Beulich wrote: >>>> On 17.02.2021 17:29, Julien Grall wrote: >>>>> On 17/02/2021 15:13, Jan Beulich wrote: >>>>>> On 17.02.2021 15:24, Julien Grall wrote:> --- a/xen/drivers/passthrough/x86/iommu.c> +++ b/xen/drivers/passthrough/x86/iommu.c> @@ -149,6 +149,13 @@ int arch_iommu_domain_init(struct domain *d)> > void arch_iommu_domain_destroy(struct domain *d)> {> + /*> + * There should be not page-tables left allocated by the time the >>>>>> Nit: s/not/no/ ? >>>>>> >>>>>>> + * domain is destroyed. Note that arch_iommu_domain_destroy() is >>>>>>> + * called unconditionally, so pgtables may be unitialized. >>>>>>> + */ >>>>>>> + ASSERT(dom_iommu(d)->platform_ops == NULL || >>>>>>> + page_list_empty(&dom_iommu(d)->arch.pgtables.list)); >>>>>>> } >>>>>>> >>>>>>> static bool __hwdom_init hwdom_iommu_map(const struct domain *d, >>>>>>> @@ -279,6 +286,9 @@ int iommu_free_pgtables(struct domain *d) >>>>>>> */ >>>>>>> hd->platform_ops->clear_root_pgtable(d); >>>>>>> >>>>>>> + /* After this barrier no new page allocations can occur. */ >>>>>>> + spin_barrier(&hd->arch.pgtables.lock); >>>>>> >>>>>> Didn't patch 2 utilize the call to ->clear_root_pgtable() itself as >>>>>> the barrier? Why introduce another one (with a similar comment) >>>>>> explicitly now? >>>>> The barriers act differently, one will get against any IOMMU page-tables >>>>> modification. The other one will gate against allocation. >>>>> >>>>> There is no guarantee that the former will prevent the latter. >>>> >>>> Oh, right - different locks. I got confused here because in both >>>> cases the goal is to prevent allocations. >>>> >>>>>>> @@ -315,9 +326,29 @@ struct page_info *iommu_alloc_pgtable(struct domain *d) >>>>>>> unmap_domain_page(p); >>>>>>> >>>>>>> spin_lock(&hd->arch.pgtables.lock); >>>>>>> - page_list_add(pg, &hd->arch.pgtables.list); >>>>>>> + /* >>>>>>> + * The IOMMU page-tables are freed when relinquishing the domain, but >>>>>>> + * nothing prevent allocation to happen afterwards. There is no valid >>>>>>> + * reasons to continue to update the IOMMU page-tables while the >>>>>>> + * domain is dying. >>>>>>> + * >>>>>>> + * So prevent page-table allocation when the domain is dying. >>>>>>> + * >>>>>>> + * We relying on &hd->arch.pgtables.lock to synchronize d->is_dying. >>>>>>> + */ >>>>>>> + if ( likely(!d->is_dying) ) >>>>>>> + { >>>>>>> + alive = true; >>>>>>> + page_list_add(pg, &hd->arch.pgtables.list); >>>>>>> + } >>>>>>> spin_unlock(&hd->arch.pgtables.lock); >>>>>>> >>>>>>> + if ( unlikely(!alive) ) >>>>>>> + { >>>>>>> + free_domheap_page(pg); >>>>>>> + pg = NULL; >>>>>>> + } >>>>>>> + >>>>>>> return pg; >>>>>>> } >>>>>> >>>>>> As before I'm concerned of this forcing error paths to be taken >>>>>> elsewhere, in case an allocation still happens (e.g. from unmap >>>>>> once super page mappings are supported). Considering some of the >>>>>> error handling in the IOMMU code is to invoke domain_crash(), it >>>>>> would be quite unfortunate if we ended up crashing a domain >>>>>> while it is being cleaned up after. >>>>> >>>>> It is unfortunate, but I think this is better than having to leak page >>>>> tables. >>>>> >>>>>> >>>>>> Additionally, the (at present still hypothetical) unmap case, if >>>>>> failing because of the change here, would then again chance to >>>>>> leave mappings in place while the underlying pages get freed. As >>>>>> this would likely require an XSA, the change doesn't feel like >>>>>> "hardening" to me. >>>>> >>>>> I would agree with this if memory allocations could never fail. That's >>>>> not that case and will become worse as we use IOMMU pool. >>>>> >>>>> Do you have callers in mind that doesn't check the returns of iommu_unmap()? >>>> >>>> The function is marked __must_check, so there won't be any direct >>>> callers ignoring errors (albeit I may be wrong here - we used to >>>> have cases where we simply suppressed the resulting compiler >>>> diagnostic, without really handling errors; not sure if all of >>>> these are gone by now). Risks might be elsewhere. >>> >>> But this is not a new risk. So I don't understand why you think my patch >>> is the one that may lead to an XSA in the future. >> >> I didn't mean to imply it would _lead_ to an XSA (you're >> right that the problem was there already before), but the term >> "harden" suggests to me that the patch aims at eliminating >> possible conditions. > > It elimitates the risk that someone inadvertently call > iommu_alloc_pgtable() when the domain is dying. If this is happening > after the page tables have been freed, then we would end up to leak memory. > >> IOW the result here looks to me as if it >> would yield a false sense of safety. > > So you are concerned about the wording rather than the code itself. Is > that correct? In a way, yes. First of all I'd like us to settle on what to do with late unmap requests, for 4.15 (and if need be longer term). Jan > If so, how about "xen/iommu: Make the IOMMU page-table allocator > slightly firmer"? > > Cheers, >