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Mon, 9 Mar 2020 15:44:21 +0000 (UTC) Received: from redhat.com (ovpn-112-61.ams2.redhat.com [10.36.112.61]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 93FAE19C69; Mon, 9 Mar 2020 15:44:15 +0000 (UTC) Date: Mon, 9 Mar 2020 15:44:12 +0000 From: Daniel =?utf-8?B?UC4gQmVycmFuZ8Op?= To: Eric Blake Subject: Re: [PATCH v3 1/4] block: Add trivial backing_fmt support to qcow, sheepdog, vmdk Message-ID: <20200309154412.GL3033513@redhat.com> References: <20200306225121.3199279-1-eblake@redhat.com> <20200306225121.3199279-2-eblake@redhat.com> <20200309152112.GC6478@linux.fritz.box> <7b7f12f8-ca03-12d4-b93d-2edefb51cb42@redhat.com> MIME-Version: 1.0 In-Reply-To: <7b7f12f8-ca03-12d4-b93d-2edefb51cb42@redhat.com> User-Agent: Mutt/1.13.3 (2020-01-12) X-Scanned-By: MIMEDefang 2.84 on 10.5.11.23 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: 207.211.31.81 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 , Fam Zheng , pkrempa@redhat.com, "open list:Sheepdog" , qemu-block@nongnu.org, libvir-list@redhat.com, Michael Tokarev , qemu-devel@nongnu.org, mreitz@redhat.com, "open list:Trivial patches" , Liu Yuan , Laurent Vivier Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On Mon, Mar 09, 2020 at 10:32:52AM -0500, Eric Blake wrote: > On 3/9/20 10:21 AM, Kevin Wolf wrote: > > Am 06.03.2020 um 23:51 hat Eric Blake geschrieben: > > > For qcow2 and qed, we want to encourage the use of -F always, as thes= e > > > formats can suffer from data corruption or security holes if backing > > > format is probed. But for other formats, the backing format cannot b= e > > > recorded. Making the user decide on a per-format basis whether to > > > supply a backing format string is awkward, better is to just blindly > > > accept a backing format argument even if it is ignored by the > > > contraints of the format at hand. > > >=20 > > > Signed-off-by: Eric Blake > >=20 > > I'm not sure if I agree with this reasoning. Accepting and silently > > ignoring -F could give users a false sense of security. If I specify a > > -F raw and QEMU later probes qcow2, that would be very surprising. >=20 > Do we know what formats qcow, sheepdog, and vmdk expect to probe? I'm > wondering if we can compromise by checking that the requested backing ima= ge > has the specified format, and error if it is not, rather than completely > ignoring it - but at the same time, the image formats have no where to > record a backing format. Consider the user creates an image with "-F raw". We can validate the backi= ng image is raw, and so our check succeeds. Later the malicious c= an write a qcow header into this raw file and QEMU will thereafter probe the image as qcow, not raw. IOW, in the case of "-F raw", even if we immediately check the format, we'r= e still not offering the protection promised by the "-F" flag, because that promise refers to the runtime behaviour of the QEMU emulator, not the immediate qemu-img cmd. We could support "-F ..." and validate any non-raw formats, while raising a runtime error in the case of "-F raw", as only the "raw" backing format has the probing security risk. Users who need to use qcow, with a backing file, without a format can just not pass "-F" and in doing so will be insecure. We could take this opportunity to deprecate 'qcow' perhaps, declare it a read-only format, restricted to qemu-img/qemu-io for purpose of data liberation ? For sheepdog, if it is something we genuinely still care about, then adding a backing file format record seems neccessary, unless we either forbid use of raw backing files, or forbid use of non-raw backing files, either way would be safe. > I'm guessing that qcow works with either raw or qcow as backing format (a= nd > anything else is odd - a qcow2 backing to a qcow is unusual, and would be > better to reject). I'm not sure if sheepdog can be backed by anything bu= t > another sheepdog, similarly, I'm not sure if a vmdk can be backed by > anything but another vmdk. If so, it should be simple enough to do a v4 = of > this patch which requires -F to be a known-acceptable probe type for thes= e > images. >=20 > Still, the point of this patch is that I want to add -F into all the > iotests, and without something along the lines of this patch, all of thos= e > iotests are broken for these image formats. Patch 2 is a lot harder to > write if we have to make our use of -F conditional on the image format in > question. >=20 > --=20 > Eric Blake, Principal Software Engineer > Red Hat, Inc. +1-919-301-3226 > Virtualization: qemu.org | libvirt.org >=20 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= :|