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 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 4DBB5C433EF for ; Tue, 2 Nov 2021 11:35:41 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 3607960F36 for ; Tue, 2 Nov 2021 11:35:41 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S230128AbhKBLiP (ORCPT ); Tue, 2 Nov 2021 07:38:15 -0400 Received: from us-smtp-delivery-124.mimecast.com ([216.205.24.124]:22133 "EHLO us-smtp-delivery-124.mimecast.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229778AbhKBLiN (ORCPT ); Tue, 2 Nov 2021 07:38:13 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1635852939; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=LmMvfElX6M+0WYCPcYmQMWdPiGm8k07IOEcDvgd1SxY=; b=UmhKxuQ+q4gKOZSX6rAiOeZQ5fMp4qqstKeZdwa7QKlSQGMQUR6GS2TGN/XLohjXfAMQty eP3uS9+lbWmKGRlxe614JzMgRa+liFGEp8hdhGpJtyvrvKp445usbGXzntpQ/WnzB0Hnem uvzLWywB9gubFclQm2sg1U7arJMwRBY= Received: from mail-wm1-f72.google.com (mail-wm1-f72.google.com [209.85.128.72]) (Using TLS) by relay.mimecast.com with ESMTP id us-mta-231-Y_o2DdGuOZasxzJLq8LDAw-1; Tue, 02 Nov 2021 07:35:38 -0400 X-MC-Unique: Y_o2DdGuOZasxzJLq8LDAw-1 Received: by mail-wm1-f72.google.com with SMTP id o22-20020a1c7516000000b0030d6f9c7f5fso6825390wmc.1 for ; Tue, 02 Nov 2021 04:35:37 -0700 (PDT) X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20210112; h=x-gm-message-state:date:from:to:cc:subject:message-id:references :mime-version:content-disposition:in-reply-to; bh=LmMvfElX6M+0WYCPcYmQMWdPiGm8k07IOEcDvgd1SxY=; b=ZzMa9U/qu4iSGPUH2Ro1WNdquRSRB8JYxzIWtUpkmRx+cAAUI5OVx/m4qOGkSq9oBR 6LF3EV8toRiuMvGHXewFeM4CcvFlCzF6qGNzrEe2Uo0IFNnF13grSjn74MQ11ojLdjd3 qaFJy8bjkVNbE0JLrfSaElVorxqiQAE8bG8PA/BXuTUy1c8uAV79EPEaAmgxQ7vynZrb JAqjofeBMYhk5YQrbiYpsWtBQ/XSUWr96pvmt0ETGnK/0Auy5lX8IxN4ZjVQZacEBQjd p76tVpl2V7TYLiH5Mb49av35Pt5Bn85LruYUH1tujEYCKSkaw2MQP6TuDbE4UwKJJKg3 PHrQ== X-Gm-Message-State: AOAM531MoI2xuW1N6jFua9E+yRk/I6/h40WCPSz5L3Je6Xg/ZZXYR4qA f+CYBRsm53ejG3xowIpMGxReSdEo7nw93CEUnXbWj80WL5J9hSGpfB9XJ9E2Qf9VAZG+1uKUpuU njxeo5dGm9vbJ X-Received: by 2002:a5d:4e81:: with SMTP id e1mr47330391wru.242.1635852936892; Tue, 02 Nov 2021 04:35:36 -0700 (PDT) X-Google-Smtp-Source: ABdhPJzPGl4SyFSPG9ZM0E/Oj2KRRGHWrzU20lxdKAhKtxgtI9RAzppH5ex8ctP1VZa3BxTTX/bRIQ== X-Received: by 2002:a5d:4e81:: with SMTP id e1mr47330357wru.242.1635852936676; Tue, 02 Nov 2021 04:35:36 -0700 (PDT) Received: from redhat.com ([2a03:c5c0:207e:c1:107d:c1da:65:fcb8]) by smtp.gmail.com with ESMTPSA id o20sm2145924wmq.47.2021.11.02.04.35.33 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 02 Nov 2021 04:35:36 -0700 (PDT) Date: Tue, 2 Nov 2021 07:35:31 -0400 From: "Michael S. Tsirkin" To: David Hildenbrand Cc: qemu-devel@nongnu.org, Paolo Bonzini , Eduardo Habkost , Marcel Apfelbaum , Igor Mammedov , Ani Sinha , Peter Xu , "Dr . David Alan Gilbert" , Stefan Hajnoczi , Richard Henderson , Philippe =?iso-8859-1?Q?Mathieu-Daud=E9?= , Hui Zhu , Sebastien Boeuf , kvm@vger.kernel.org Subject: Re: [PATCH v1 00/12] virtio-mem: Expose device memory via multiple memslots Message-ID: <20211102072843-mutt-send-email-mst@kernel.org> References: <20211027124531.57561-1-david@redhat.com> <20211101181352-mutt-send-email-mst@kernel.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Precedence: bulk List-ID: X-Mailing-List: kvm@vger.kernel.org On Tue, Nov 02, 2021 at 09:33:55AM +0100, David Hildenbrand wrote: > On 01.11.21 23:15, Michael S. Tsirkin wrote: > > On Wed, Oct 27, 2021 at 02:45:19PM +0200, David Hildenbrand wrote: > >> This is the follow-up of [1], dropping auto-detection and vhost-user > >> changes from the initial RFC. > >> > >> Based-on: 20211011175346.15499-1-david@redhat.com > >> > >> A virtio-mem device is represented by a single large RAM memory region > >> backed by a single large mmap. > >> > >> Right now, we map that complete memory region into guest physical addres > >> space, resulting in a very large memory mapping, KVM memory slot, ... > >> although only a small amount of memory might actually be exposed to the VM. > >> > >> For example, when starting a VM with a 1 TiB virtio-mem device that only > >> exposes little device memory (e.g., 1 GiB) towards the VM initialliy, > >> in order to hotplug more memory later, we waste a lot of memory on metadata > >> for KVM memory slots (> 2 GiB!) and accompanied bitmaps. Although some > >> optimizations in KVM are being worked on to reduce this metadata overhead > >> on x86-64 in some cases, it remains a problem with nested VMs and there are > >> other reasons why we would want to reduce the total memory slot to a > >> reasonable minimum. > >> > >> We want to: > >> a) Reduce the metadata overhead, including bitmap sizes inside KVM but also > >> inside QEMU KVM code where possible. > >> b) Not always expose all device-memory to the VM, to reduce the attack > >> surface of malicious VMs without using userfaultfd. > > > > I'm confused by the mention of these security considerations, > > and I expect users will be just as confused. > > Malicious VMs wanting to consume more memory than desired is only > relevant when running untrusted VMs in some environments, and it can be > caught differently, for example, by carefully monitoring and limiting > the maximum memory consumption of a VM. We have the same issue already > when using virtio-balloon to logically unplug memory. For me, it's a > secondary concern ( optimizing a is much more important ). > > Some users showed interest in having QEMU disallow access to unplugged > memory, because coming up with a maximum memory consumption for a VM is > hard. This is one step into that direction without having to run with > uffd enabled all of the time. Sorry about missing the memo - is there a lot of overhead associated with uffd then? > ("security is somewhat the wrong word. we won't be able to steal any > information from the hypervisor.) Right. Let's just spell it out. Further, removing memory still requires guest cooperation. > > > So let's say user wants to not be exposed. What value for > > the option should be used? What if a lower option is used? > > Is there still some security advantage? > > My recommendation will be to use 1 memslot per gigabyte as default if > possible in the configuration. If we have a virtio-mem devices with a > maximum size of 128 GiB, the suggestion will be to use memslots=128. > Some setups will require less (e.g., vhost-user until adjusted, old > KVM), some setups can allow for more. I assume that most users will > later set "memslots=0", to enable auto-detection mode. > > > Assume we have a virtio-mem device with a maximum size of 1 TiB and we > hotplugged 1 GiB to the VM. With "memslots=1", the malicious VM could > actually access the whole 1 TiB. With "memslots=1024", the malicious VM > could only access additional ~ 1 GiB. With "memslots=512", ~ 2 GiB. > That's the reduced attack surface. > > Of course, it's different after we hotunplugged memory, before we have > VIRTIO_MEM_F_UNPLUGGED_INACCESSIBLE support in QEMU, because all memory > inside the usable region has to be accessible and we cannot "unplug" the > memslots. > > > Note: With upcoming VIRTIO_MEM_F_UNPLUGGED_INACCESSIBLE changes in QEMU, > one will be able to disallow any access for malicious VMs by setting the > memblock size just as big as the device block size. > > So with a 128 GiB virtio-mem device with memslots=128,block-size=1G, or > with memslots=1024,block-size=128M we could make it impossible for a > malicious VM to consume more memory than intended. But we lose > flexibility due to the block size and the limited number of available > memslots. > > But again, for "full protection against malicious VMs" I consider > userfaultfd protection more flexible. This approach here gives some > advantage, especially when having large virtio-mem devices that start > out small. > > -- > Thanks, > > David / dhildenb