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=-8.9 required=3.0 tests=FROM_EXCESS_BASE64, HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_PATCH,MAILING_LIST_MULTI,SIGNED_OFF_BY, SPF_PASS,USER_AGENT_NEOMUTT 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 854E6C43387 for ; Wed, 19 Dec 2018 09:09:16 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 5E3C620874 for ; Wed, 19 Dec 2018 09:09:16 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728313AbeLSJJP (ORCPT ); Wed, 19 Dec 2018 04:09:15 -0500 Received: from smtp.eu.citrix.com ([185.25.65.24]:55326 "EHLO SMTP.EU.CITRIX.COM" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1728282AbeLSJJP (ORCPT ); Wed, 19 Dec 2018 04:09:15 -0500 X-IronPort-AV: E=Sophos;i="5.56,372,1539648000"; d="scan'208";a="83540675" Date: Wed, 19 Dec 2018 10:09:06 +0100 From: Roger Pau =?utf-8?B?TW9ubsOp?= To: Dongli Zhang CC: , , , , , Subject: Re: [Xen-devel] [PATCH v2 1/1] xen/blkback: rework connect_ring() to avoid inconsistent xenstore 'ring-page-order' set by malicious blkfront Message-ID: <20181219090906.y4ouwhroydie7zk5@mac> References: <1545094538-31426-1-git-send-email-dongli.zhang@oracle.com> <20181218093300.qfslnloxooqnu2im@mac> <51ea8086-e5fd-b1db-f935-101160ca9359@oracle.com> <20181218151331.iwkq3roqwigf3e2r@mac> MIME-Version: 1.0 Content-Type: text/plain; charset="iso-8859-1" Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: User-Agent: NeoMutt/20180716 X-ClientProxiedBy: AMSPEX02CAS01.citrite.net (10.69.22.112) To AMSPEX02CL02.citrite.net (10.69.22.126) Sender: linux-block-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-block@vger.kernel.org On Tue, Dec 18, 2018 at 11:29:16PM +0800, Dongli Zhang wrote: > > > On 12/18/2018 11:13 PM, Roger Pau Monné wrote: > > On Tue, Dec 18, 2018 at 07:31:59PM +0800, Dongli Zhang wrote: > >> Hi Roger, > >> > >> On 12/18/2018 05:33 PM, Roger Pau Monné wrote: > >>> On Tue, Dec 18, 2018 at 08:55:38AM +0800, Dongli Zhang wrote: > >>>> The xenstore 'ring-page-order' is used globally for each blkback queue and > >>>> therefore should be read from xenstore only once. However, it is obtained > >>>> in read_per_ring_refs() which might be called multiple times during the > >>>> initialization of each blkback queue. > >>>> > >>>> If the blkfront is malicious and the 'ring-page-order' is set in different > >>>> value by blkfront every time before blkback reads it, this may end up at > >>>> the "WARN_ON(i != (XEN_BLKIF_REQS_PER_PAGE * blkif->nr_ring_pages));" in > >>>> xen_blkif_disconnect() when frontend is destroyed. > >>>> > >>>> This patch reworks connect_ring() to read xenstore 'ring-page-order' only > >>>> once. > >>>> > >>>> Signed-off-by: Dongli Zhang > >>>> --- > >>>> Changed since v1: > >>>> * change the order of xenstore read in read_per_ring_refs(suggested by Roger Pau Monne) > >>>> * use xenbus_read_unsigned() in connect_ring() (suggested by Roger Pau Monne) > >>>> > >>>> drivers/block/xen-blkback/xenbus.c | 70 ++++++++++++++++++++++---------------- > >>>> 1 file changed, 40 insertions(+), 30 deletions(-) > >>>> > >>>> diff --git a/drivers/block/xen-blkback/xenbus.c b/drivers/block/xen-blkback/xenbus.c > >>>> index a4bc74e..7178f0f 100644 > >>>> --- a/drivers/block/xen-blkback/xenbus.c > >>>> +++ b/drivers/block/xen-blkback/xenbus.c > >>>> @@ -926,7 +926,7 @@ static int read_per_ring_refs(struct xen_blkif_ring *ring, const char *dir) > >>>> int err, i, j; > >>>> struct xen_blkif *blkif = ring->blkif; > >>>> struct xenbus_device *dev = blkif->be->dev; > >>>> - unsigned int ring_page_order, nr_grefs, evtchn; > >>>> + unsigned int nr_grefs, evtchn; > >>>> > >>>> err = xenbus_scanf(XBT_NIL, dir, "event-channel", "%u", > >>>> &evtchn); > >>>> @@ -936,43 +936,38 @@ static int read_per_ring_refs(struct xen_blkif_ring *ring, const char *dir) > >>>> return err; > >>>> } > >>>> > >>>> - err = xenbus_scanf(XBT_NIL, dev->otherend, "ring-page-order", "%u", > >>>> - &ring_page_order); > >>>> - if (err != 1) { > >>>> - err = xenbus_scanf(XBT_NIL, dir, "ring-ref", "%u", &ring_ref[0]); > >>>> - if (err != 1) { > >>>> + nr_grefs = blkif->nr_ring_pages; > >>>> + WARN_ON(!nr_grefs); > >>>> + > >>>> + for (i = 0; i < nr_grefs; i++) { > >>>> + char ring_ref_name[RINGREF_NAME_LEN]; > >>>> + > >>>> + snprintf(ring_ref_name, RINGREF_NAME_LEN, "ring-ref%u", i); > >>>> + err = xenbus_scanf(XBT_NIL, dir, ring_ref_name, > >>>> + "%u", &ring_ref[i]); > >>>> + > >>>> + if (err != 1 && (i || (!i && nr_grefs > 1))) { > >>> > >>> AFAICT the above condition can be simplified as "err != 1 && > >>> nr_grefs". > >>> > >>>> err = -EINVAL; > >>> > >>> There's no point in setting err here... > >>> > >>>> - xenbus_dev_fatal(dev, err, "reading %s/ring-ref", dir); > >>>> + xenbus_dev_fatal(dev, err, "reading %s/%s", > >>>> + dir, ring_ref_name); > >>>> return err; > >>> > >>> ...since you can just return -EINVAL (same applies to the other > >>> instance below). > >> > >> I would like to confirm if I would keep the err = -EINVAL in below because most > >> of the below code is copied from original implementation without modification. > >> > >> There is no err set by xenbus_read_unsigned(). > > > > Right, but instead of doing: > > > > err = -EINVAL; > > return err; > > > > You can just do: > > > > return -EINVAL; > > > > Which is one line shorter :). > > However, for the "ring-page-order" case, the err used in xenbus_dev_fatal() is > not set as xenbus_read_unsigned() does not return any err? > > For "ring-page-order", I would still need to set err = -EINVAL with extra one > line of code? Given this, I don't have a strong opinion, so do as you please. Thanks, Roger.