> > > > > > > > On Mon, Jan 14, 2019 at 02:15:40AM -0500, Pankaj Gupta wrote: > > > > > > > > > > > > Until you have images (and hence host page cache) shared between > > > > > > > multiple guests. People will want to do this, because it means > > > > > > > they > > > > > > > only need a single set of pages in host memory for executable > > > > > > > binaries rather than a set of pages per guest. Then you have > > > > > > > multiple guests being able to detect residency of the same set of > > > > > > > pages. If the guests can then, in any way, control eviction of > > > > > > > the > > > > > > > pages from the host cache, then we have a guest-to-guest > > > > > > > information > > > > > > > leak channel. > > > > > > > > > > > > I don't think we should ever be considering something that would > > > > > > allow a > > > > > > guest to evict page's from the host's pagecache [1]. The guest > > > > > > should > > > > > > be able to kick its own references to the host's pagecache out of > > > > > > its > > > > > > own pagecache, but not be able to influence whether the host or > > > > > > another > > > > > > guest has a read-only mapping cached. > > > > > > > > > > > > [1] Unless the guest is allowed to modify the host's file; > > > > > > obviously > > > > > > truncation, holepunching, etc are going to evict pages from the > > > > > > host's > > > > > > page cache. > > > > > > > > > > This is so correct. Guest does not not evict host page cache pages > > > > > directly. > > > > > > > > They don't right now. > > > > > > > > But someone is going to end up asking for discard to work so that > > > > the guest can free unused space in the underlying spares image (i.e. > > > > make use of fstrim or mount -o discard) because they have workloads > > > > that have bursts of space usage and they need to trim the image > > > > files afterwards to keep their overall space usage under control. > > > > > > > > And then.... > > > > > > ...we reject / push back on that patch citing the above concern. > > > > So at what point do we draw the line? > > > > We're allowing writable DAX mappings, but as I've pointed out that > > means we are going to be allowing a potential information leak via > > files with shared extents to be directly mapped and written to. > > > > But we won't allow useful admin operations that allow better > > management of host side storage space similar to how normal image > > files are used by guests because it's an information leak vector? > > > > That's splitting some really fine hairs there... > > May I summarize that th security implications need to > be documented? > > In fact that would make a fine security implications section > in the device specification. This is a very good suggestion. I will document the security implications in details in device specification with details of what all filesystem features we don't support and why. Best regards, Pankaj > > > > > > > > > > In case of virtio-pmem & DAX, guest clears guest page cache > > > > > exceptional entries. > > > > > Its solely decision of host to take action on the host page cache > > > > > pages. > > > > > > > > > > In case of virtio-pmem, guest does not modify host file directly i.e > > > > > don't > > > > > perform hole punch & truncation operation directly on host file. > > > > > > > > ... this will no longer be true, and the nuclear landmine in this > > > > driver interface will have been armed.... > > > > > > I agree with the need to be careful when / if explicit cache control > > > is added, but that's not the case today. > > > > "if"? > > > > I expect it to be "when", not if. Expect the worst, plan for it now. > > > > Cheers, > > > > Dave. > > -- > > Dave Chinner > > david@xxxxxxxxxxxxx > >
