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=-18.2 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,INCLUDES_CR_TRAILER,INCLUDES_PATCH, MAILING_LIST_MULTI,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 39289C433B4 for ; Fri, 14 May 2021 01:05:01 +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 E5A106142E for ; Fri, 14 May 2021 01:05:00 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org E5A106142E Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=kernel.org 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.127137.238871 (Exim 4.92) (envelope-from ) id 1lhMG0-0005ON-1y; Fri, 14 May 2021 01:04:48 +0000 X-Outflank-Mailman: Message body and most headers restored to incoming version Received: by outflank-mailman (output) from mailman id 127137.238871; Fri, 14 May 2021 01:04:48 +0000 Received: from localhost ([127.0.0.1] helo=lists.xenproject.org) by lists.xenproject.org with esmtp (Exim 4.92) (envelope-from ) id 1lhMFz-0005OG-Ui; Fri, 14 May 2021 01:04:47 +0000 Received: by outflank-mailman (input) for mailman id 127137; Fri, 14 May 2021 01:04:46 +0000 Received: from us1-rack-iad1.inumbo.com ([172.99.69.81]) by lists.xenproject.org with esmtp (Exim 4.92) (envelope-from ) id 1lhMFy-0005OA-KC for xen-devel@lists.xenproject.org; Fri, 14 May 2021 01:04:46 +0000 Received: from mail.kernel.org (unknown [198.145.29.99]) by us1-rack-iad1.inumbo.com (Halon) with ESMTPS id f2364f62-8199-4894-a393-ec3e80e73e7a; Fri, 14 May 2021 01:04:46 +0000 (UTC) Received: by mail.kernel.org (Postfix) with ESMTPSA id 08824613B5; Fri, 14 May 2021 01:04:44 +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: f2364f62-8199-4894-a393-ec3e80e73e7a DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1620954285; bh=h8ak1LFNpEuvvrnEcyltA6NYwXM3pDzQJjkY5k1zegM=; h=Date:From:To:cc:Subject:In-Reply-To:References:From; b=pAWeeTTmkOE7ea830IPQIJN+SBeYpUarDv5wIeY1DmCdaSw0O85yDIUSYfmeP7DQT Vo29uoYNHQNZD1dyinBccn/fVUttftU9ivrf8zRuJTOaVwd41rTt5EevpbywmtiTr+ eyKomCdFpN/L9N+ZvYyo3FMln03dduTzWCOXwLj6hGDqswg+puRh82FQNy6HU+bfX+ X0MkAU66eTKidHnrcFOK50e44ASsNrfpl6PmaZvviatrqK9XbPmAAV0skBc3klBSTS T7uEBjgoYlOSLTyG9gl9brl9zAgHJvxbL4ggl7OPtnnGqpoWCGx0DRbMff/PCndPuw 6xcL+83MyXaKw== Date: Thu, 13 May 2021 18:04:44 -0700 (PDT) From: Stefano Stabellini X-X-Sender: sstabellini@sstabellini-ThinkPad-T480s To: Julien Grall cc: Stefano Stabellini , xen-devel , Wei Chen , Henry.Wang@arm.com, Penny.Zheng@arm.com, Bertrand Marquis , Julien Grall , Volodymyr Babchuk Subject: Re: [PATCH RFCv2 08/15] xen/arm32: mm: Check if the virtual address is shared before updating it In-Reply-To: Message-ID: References: <20210425201318.15447-1-julien@xen.org> <20210425201318.15447-9-julien@xen.org> User-Agent: Alpine 2.21 (DEB 202 2017-01-01) MIME-Version: 1.0 Content-Type: multipart/mixed; BOUNDARY="8323329-835495777-1620954177=:5018" Content-ID: This message is in MIME format. The first part should be readable text, while the remaining parts are likely unreadable without MIME-aware tools. --8323329-835495777-1620954177=:5018 Content-Type: text/plain; CHARSET=UTF-8 Content-Transfer-Encoding: 8BIT Content-ID: On Thu, 13 May 2021, Julien Grall wrote: > On Thu, 13 May 2021, 23:32 Stefano Stabellini, wrote: > On Wed, 12 May 2021, Julien Grall wrote: > > Hi Stefano, > > > > On 12/05/2021 23:00, Stefano Stabellini wrote: > > > On Sun, 25 Apr 2021, Julien Grall wrote: > > > > From: Julien Grall > > > > > > > > Only the first 2GB of the virtual address space is shared between all > > > > the page-tables on Arm32. > > > > > > > > There is a long outstanding TODO in xen_pt_update() stating that the > > > > function is can only work with shared mapping. Nobody has ever called > > >             ^ remove > > > > > > > the function with private mapping, however as we add more callers > > > > there is a risk to mess things up. > > > > > > > > Introduce a new define to mark the ened of the shared mappings and use > > >                                       ^end > > > > > > > it in xen_pt_update() to verify if the address is correct. > > > > > > > > Note that on Arm64, all the mappings are shared. Some compiler may > > > > complain about an always true check, so the new define is not introduced > > > > for arm64 and the code is protected with an #ifdef. > > >   On arm64 we could maybe define SHARED_VIRT_END to an arbitrarely large > > > value, such as: > > > > > > #define SHARED_VIRT_END (1UL<<48) > > > > > > or: > > > > > > #define SHARED_VIRT_END DIRECTMAP_VIRT_END > > > > > > ? > > > > I thought about it but I didn't want to define to a random value... I felt not > > define it was better. > > Yeah, I see your point: any restrictions in addressing (e.g. 48bits) > are physical address restrictions. Here we are talking about virtual > address restriction, and I don't think there are actually any > restrictions there?  We could validly map something at > 0xffff_ffff_ffff_ffff. So even (1<<48) which makes sense at the physical > level, doesn't make sense in terms of virtual addresses. > > > The limit for the virtual address is 2^64. > > > > > > > Signed-off-by: Julien Grall > > > > > > > > --- > > > >      Changes in v2: > > > >          - New patch > > > > --- > > > >   xen/arch/arm/mm.c            | 11 +++++++++-- > > > >   xen/include/asm-arm/config.h |  4 ++++ > > > >   2 files changed, 13 insertions(+), 2 deletions(-) > > > > > > > > diff --git a/xen/arch/arm/mm.c b/xen/arch/arm/mm.c > > > > index 8fac24d80086..5c17cafff847 100644 > > > > --- a/xen/arch/arm/mm.c > > > > +++ b/xen/arch/arm/mm.c > > > > @@ -1275,11 +1275,18 @@ static int xen_pt_update(unsigned long virt, > > > >        * For arm32, page-tables are different on each CPUs. Yet, they > > > > share > > > >        * some common mappings. It is assumed that only common mappings > > > >        * will be modified with this function. > > > > -     * > > > > -     * XXX: Add a check. > > > >        */ > > > >       const mfn_t root = virt_to_mfn(THIS_CPU_PGTABLE); > > > >   +#ifdef SHARED_VIRT_END > > > > +    if ( virt > SHARED_VIRT_END || > > > > +         (SHARED_VIRT_END - virt) < nr_mfns ) > > > > > > The following would be sufficient, right? > > > > > >      if ( virt + nr_mfns > SHARED_VIRT_END ) > > > > This would not protect against an overflow. So I think it is best if we keep > > my version. > > But there can be no overflow with the way SHARED_VIRT_END is defined. > > Even if SHARED_VIRT_END was defined at (1<<48) there can be no overflow. > Only if we defined SHARED_VIRT_END as 0xffff_ffff_ffff_ffff we would > have an overflow, but you wrote above that your preference is not to do > that. > > > You can have an overflow regardless the value of SHARED_VIRT_END. > > Imagine virt = 2^64 - 1 and nr_mfs = 1. The addition would result to 0. > > As a consequence the check would pass when it should not. Yes you are right, I don't know how I missed it! > One can argue that the caller will always provide sane values. However given the simplicity of the check, this is not worth the trouble if > a caller is buggy. > > Now, the problem with SHARED_VIRT_END equals to 2^64 - 1 is not the overflow but the compiler that may throw an error/warning for always > true check. Hence the reason of not defining SHARED_VIRT_END on arm64. OK, all checks out. Reviewed-by: Stefano Stabellini --8323329-835495777-1620954177=:5018--