Return-Path: Received: from mail-ot1-f68.google.com ([209.85.210.68]:35694 "EHLO mail-ot1-f68.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726553AbfAMBni (ORCPT ); Sat, 12 Jan 2019 20:43:38 -0500 Received: by mail-ot1-f68.google.com with SMTP id 81so16507277otj.2 for ; Sat, 12 Jan 2019 17:43:37 -0800 (PST) MIME-Version: 1.0 References: <20190109144736.17452-1-pagupta@redhat.com> <20190110012617.GA4205@dastard> <20190110101757.GC15790@quack2.suse.cz> <1354249849.63357171.1547343519970.JavaMail.zimbra@redhat.com> In-Reply-To: <1354249849.63357171.1547343519970.JavaMail.zimbra@redhat.com> From: Dan Williams Date: Sat, 12 Jan 2019 17:43:26 -0800 Message-ID: Subject: Re: [PATCH v3 0/5] kvm "virtio pmem" device To: Pankaj Gupta Cc: Jan Kara , Dave Chinner , Linux Kernel Mailing List , KVM list , Qemu Developers , linux-nvdimm , linux-fsdevel , virtualization@lists.linux-foundation.org, Linux ACPI , linux-ext4 , linux-xfs , Stefan Hajnoczi , Rik van Riel , Nitesh Narayan Lal , Kevin Wolf , Paolo Bonzini , Ross Zwisler , vishal l verma , dave jiang , David Hildenbrand , jmoyer , xiaoguangrong eric , Christoph Hellwig , "Michael S. Tsirkin" , Jason Wang , lcapitulino@redhat.com, Igor Mammedov , Eric Blake , Matthew Wilcox , "Theodore Ts'o" , adilger kernel , darrick wong , "Rafael J. Wysocki" Content-Type: text/plain; charset="UTF-8" Sender: linux-ext4-owner@vger.kernel.org List-ID: On Sat, Jan 12, 2019 at 5:38 PM Pankaj Gupta wrote: > > > > > > > On Thu 10-01-19 12:26:17, Dave Chinner wrote: > > > On Wed, Jan 09, 2019 at 08:17:31PM +0530, Pankaj Gupta wrote: > > > > This patch series has implementation for "virtio pmem". > > > > "virtio pmem" is fake persistent memory(nvdimm) in guest > > > > which allows to bypass the guest page cache. This also > > > > implements a VIRTIO based asynchronous flush mechanism. > > > > > > Hmmmm. Sharing the host page cache direct into the guest VM. Sounds > > > like a good idea, but..... > > > > > > This means the guest VM can now run timing attacks to observe host > > > side page cache residency, and depending on the implementation I'm > > > guessing that the guest will be able to control host side page > > > cache eviction, too (e.g. via discard or hole punch operations). > > > > > > Which means this functionality looks to me like a new vector for > > > information leakage into and out of the guest VM via guest > > > controlled host page cache manipulation. > > > > > > https://arxiv.org/pdf/1901.01161 > > > > > > I might be wrong, but if I'm not we're going to have to be very > > > careful about how guest VMs can access and manipulate host side > > > resources like the page cache..... > > > > Right. Thinking about this I would be more concerned about the fact that > > guest can effectively pin amount of host's page cache upto size of the > > device/file passed to guest as PMEM, can't it Pankaj? Or is there some QEMU > > magic that avoids this? > > Yes, guest will pin these host page cache pages using 'get_user_pages' by > elevating the page reference count. But these pages can be reclaimed by host > at any time when there is memory pressure. Wait, how can the guest pin the host pages? I would expect this to happen only when using vfio and device assignment. Otherwise, no the host can't reclaim a pinned page, that's the whole point of a pin to prevent the mm from reclaiming ownership. > KVM does not permanently pin pages. vfio does that but we are not using > it here. Right, so I'm confused by your pin assertion above.