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 E7E87C43387 for ; Tue, 18 Dec 2018 15:17:44 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id BA344218A2 for ; Tue, 18 Dec 2018 15:17:44 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726710AbeLRPRo (ORCPT ); Tue, 18 Dec 2018 10:17:44 -0500 Received: from smtp.ctxuk.citrix.com ([185.25.65.24]:35891 "EHLO SMTP.EU.CITRIX.COM" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726555AbeLRPRo (ORCPT ); Tue, 18 Dec 2018 10:17:44 -0500 X-IronPort-AV: E=Sophos;i="5.56,368,1539648000"; d="scan'208";a="83498840" Date: Tue, 18 Dec 2018 16:13:31 +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: <20181218151331.iwkq3roqwigf3e2r@mac> References: <1545094538-31426-1-git-send-email-dongli.zhang@oracle.com> <20181218093300.qfslnloxooqnu2im@mac> <51ea8086-e5fd-b1db-f935-101160ca9359@oracle.com> MIME-Version: 1.0 Content-Type: text/plain; charset="iso-8859-1" Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <51ea8086-e5fd-b1db-f935-101160ca9359@oracle.com> User-Agent: NeoMutt/20180716 X-ClientProxiedBy: AMSPEX02CAS02.citrite.net (10.69.22.113) To AMSPEX02CL01.citrite.net (10.69.22.125) 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 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 :). > + ring_page_order = xenbus_read_unsigned(dev->otherend, > + "ring-page-order", 0); > + > + if (ring_page_order > xen_blkif_max_ring_order) { > + err = -EINVAL; > + xenbus_dev_fatal(dev, err, > + "requested ring page order %d exceed max:%d", > + ring_page_order, > + xen_blkif_max_ring_order); > + return err; > + } > + > + be->blkif->nr_ring_pages = 1 << ring_page_order; > > > For the rest, I would do something like: > > + err = xenbus_scanf(XBT_NIL, dir, ring_ref_name, > + "%u", &ring_ref[i]); > + > + if (err != 1 && nr_grefs > 1) { > + xenbus_dev_fatal(dev, err, "reading %s/%s", > + dir, ring_ref_name); > + return -EINVAL; > + } > > > Thank you very much! Thanks!