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=-2.1 required=3.0 tests=DKIM_INVALID,DKIM_SIGNED, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS, USER_AGENT_SANE_1 autolearn=no 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 BCCDEC4332B for ; Mon, 23 Mar 2020 09:52:23 +0000 (UTC) Received: from lists.gnu.org (lists.gnu.org [209.51.188.17]) (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 85BC22072D for ; Mon, 23 Mar 2020 09:52:23 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=fail reason="signature verification failed" (1024-bit key) header.d=redhat.com header.i=@redhat.com header.b="hxpAkmem" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 85BC22072D Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=redhat.com Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Received: from localhost ([::1]:59412 helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1jGJks-0003EW-P7 for qemu-devel@archiver.kernel.org; Mon, 23 Mar 2020 05:52:22 -0400 Received: from eggs.gnu.org ([2001:470:142:3::10]:56565) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1jGJkF-0002kU-6R for qemu-devel@nongnu.org; Mon, 23 Mar 2020 05:51:44 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1jGJkE-0002hG-4Q for qemu-devel@nongnu.org; Mon, 23 Mar 2020 05:51:43 -0400 Received: from us-smtp-delivery-74.mimecast.com ([216.205.24.74]:25157) by eggs.gnu.org with esmtps (TLS1.0:DHE_RSA_AES_256_CBC_SHA1:32) (Exim 4.71) (envelope-from ) id 1jGJkE-0002h6-1U for qemu-devel@nongnu.org; Mon, 23 Mar 2020 05:51:42 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1584957101; h=from:from:reply-to:reply-to:subject:subject: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=CNa8iCJTHhnJbWOKq9OanBw15fWGGMIvyoguE/mi5EU=; b=hxpAkmemhidkpR22IA9bGvTn1e2GltyQ/CsgUtMp41hkmVprNdG2zJJ2KGfxok5hHE2Kav 1m7cpMc4muEhru6dSvXSIXWJsCv5YeAd2oIONUmny+wpfNuU+wjA4N0qPyLSa2nipQ8Wgu ZCFOmY01iXD6NcC+QSIiNnWrUiXkcsA= Received: from mimecast-mx01.redhat.com (mimecast-mx01.redhat.com [209.132.183.4]) (Using TLS) by relay.mimecast.com with ESMTP id us-mta-169-5P8WT0_xOsW1Gvuqfo5V6A-1; Mon, 23 Mar 2020 05:51:38 -0400 X-MC-Unique: 5P8WT0_xOsW1Gvuqfo5V6A-1 Received: from smtp.corp.redhat.com (int-mx01.intmail.prod.int.phx2.redhat.com [10.5.11.11]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id 2BE69107ACC7; Mon, 23 Mar 2020 09:51:37 +0000 (UTC) Received: from redhat.com (ovpn-112-208.ams2.redhat.com [10.36.112.208]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 57F5C9B930; Mon, 23 Mar 2020 09:51:35 +0000 (UTC) Date: Mon, 23 Mar 2020 09:51:32 +0000 From: Daniel =?utf-8?B?UC4gQmVycmFuZ8Op?= To: Eric Blake Subject: Re: discard and v2 qcow2 images Message-ID: <20200323095132.GB3379720@redhat.com> References: <20200320185848.GA5720@igalia.com> MIME-Version: 1.0 In-Reply-To: User-Agent: Mutt/1.13.3 (2020-01-12) X-Scanned-By: MIMEDefang 2.79 on 10.5.11.11 X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: quoted-printable Content-Disposition: inline X-detected-operating-system: by eggs.gnu.org: GNU/Linux 2.2.x-3.x [generic] [fuzzy] X-Received-From: 216.205.24.74 X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.23 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Reply-To: Daniel =?utf-8?B?UC4gQmVycmFuZ8Op?= Cc: Kevin Wolf , Alberto Garcia , qemu-devel@nongnu.org, qemu-block@nongnu.org, Max Reitz Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On Fri, Mar 20, 2020 at 02:35:44PM -0500, Eric Blake wrote: > On 3/20/20 1:58 PM, Alberto Garcia wrote: > > Hi, > >=20 > > when full_discard is false in discard_in_l2_slice() then the selected > > cluster should be deallocated and it should read back as zeroes. This > > is done by clearing the cluster offset field and setting OFLAG_ZERO in > > the L2 entry. > >=20 > > This flag is however only supported when qcow_version >=3D 3. In older > > images the cluster is simply deallocated, exposing any possible > > previous data from the backing file. >=20 > Discard is advisory, and has no requirements that discarded data read bac= k > as zero. However, if write zeroes uses discard under the hood, then THAT > usage must guarantee reading back as zero. >=20 > >=20 > > This can be trivially reproduced like this: > >=20 > > qemu-img create -f qcow2 backing.img 64k > > qemu-io -c 'write -P 0xff 0 64k' backing.img > > qemu-img create -f qcow2 -o compat=3D0.10 -b backing.img top.img > > qemu-io -c 'write -P 0x01 0 64k' top.img > >=20 > > After this, top.img is filled with 0x01. Now we issue a discard > > command: > >=20 > > qemu-io -c 'discard 0 64k' top.img > >=20 > > top.img should now read as zeroes, but instead you get the data from > > the backing file (0xff). If top.img was created with compat=3D1.1 > > instead (the default) then it would read as zeroes after the discard. >=20 > I'd argue that this is undesirable behavior, but not a bug. I think the ability to read old data from the backing file could potentially be considered a security flaw, depending on what the original data was in the backing file. Regards, Daniel --=20 |: https://berrange.com -o- https://www.flickr.com/photos/dberrange= :| |: https://libvirt.org -o- https://fstop138.berrange.com= :| |: https://entangle-photo.org -o- https://www.instagram.com/dberrange= :|