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 5DE03C433DB for ; Thu, 18 Feb 2021 13:10:28 +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 0319664E76 for ; Thu, 18 Feb 2021 13:10:27 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 0319664E76 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.86644.162770 (Exim 4.92) (envelope-from ) id 1lCj4U-0001Vo-D9; Thu, 18 Feb 2021 13:10:18 +0000 X-Outflank-Mailman: Message body and most headers restored to incoming version Received: by outflank-mailman (output) from mailman id 86644.162770; Thu, 18 Feb 2021 13:10:18 +0000 Received: from localhost ([127.0.0.1] helo=lists.xenproject.org) by lists.xenproject.org with esmtp (Exim 4.92) (envelope-from ) id 1lCj4U-0001Vh-9s; Thu, 18 Feb 2021 13:10:18 +0000 Received: by outflank-mailman (input) for mailman id 86644; Thu, 18 Feb 2021 13:10:16 +0000 Received: from us1-rack-iad1.inumbo.com ([172.99.69.81]) by lists.xenproject.org with esmtp (Exim 4.92) (envelope-from ) id 1lCj4S-0001Vb-Sx for xen-devel@lists.xenproject.org; Thu, 18 Feb 2021 13:10:16 +0000 Received: from mx2.suse.de (unknown [195.135.220.15]) by us1-rack-iad1.inumbo.com (Halon) with ESMTPS id 46dfe39e-c5e1-4c95-9f5f-2bf023dfdbb4; Thu, 18 Feb 2021 13:10:15 +0000 (UTC) Received: from relay2.suse.de (unknown [195.135.221.27]) by mx2.suse.de (Postfix) with ESMTP id A8497ACD4; Thu, 18 Feb 2021 13:10:14 +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: 46dfe39e-c5e1-4c95-9f5f-2bf023dfdbb4 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=1613653814; 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=DkihpiD9twC+qInFw8JwaUWcXXcm86I8D9b69rHendI=; b=TY5M1aQRVrfuw1ZbCSWOGvPOehwQcjJ+Hoa9lttFpaeget+rCwL4SeuR/up9puyMui/ssh WF1me6v+1KU+WjwQClWVyVobdooR2yFMLtCo/OguN5weAMUEc8jvzBGdZedCcD4krP27Fm /tGXfvh5BU+EtFtw/nO6oeuQ8He+JLk= 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> From: Jan Beulich Message-ID: Date: Thu, 18 Feb 2021 14:10:14 +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: <51618338-daff-5b9a-5214-e0788d95992b@xen.org> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit 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. Jan