On Thu, Dec 07, 2017 at 04:29:45PM +0000, Stefan Hajnoczi wrote: > On Thu, Dec 7, 2017 at 2:02 PM, Michael S. Tsirkin <m...@redhat.com> wrote: > > On Thu, Dec 07, 2017 at 01:08:04PM +0000, Stefan Hajnoczi wrote: > >> Instead of responding individually to these points, I hope this will > >> explain my perspective. Let me know if you do want individual > >> responses, I'm happy to talk more about the points above but I think > >> the biggest difference is our perspective on this: > >> > >> Existing vhost-user slave code should be able to run on top of > >> vhost-pci. For example, QEMU's > >> contrib/vhost-user-scsi/vhost-user-scsi.c should work inside the guest > >> with only minimal changes to the source file (i.e. today it explicitly > >> opens a UNIX domain socket and that should be done by libvhost-user > >> instead). It shouldn't be hard to add vhost-pci vfio support to > >> contrib/libvhost-user/ alongside the existing UNIX domain socket code. > >> > >> This seems pretty easy to achieve with the vhost-pci PCI adapter that > >> I've described but I'm not sure how to implement libvhost-user on top > >> of vhost-pci vfio if the device doesn't expose the vhost-user > >> protocol. > >> > >> I think this is a really important goal. Let's use a single > >> vhost-user software stack instead of creating a separate one for guest > >> code only. > >> > >> Do you agree that the vhost-user software stack should be shared > >> between host userspace and guest code as much as possible? > > > > > > > > The sharing you propose is not necessarily practical because the security > > goals > > of the two are different. > > > > It seems that the best motivation presentation is still the original rfc > > > > http://virtualization.linux-foundation.narkive.com/A7FkzAgp/rfc-vhost-user-enhancements-for-vm2vm-communication > > > > So comparing with vhost-user iotlb handling is different: > > > > With vhost-user guest trusts the vhost-user backend on the host. > > > > With vhost-pci we can strive to limit the trust to qemu only. > > The switch running within a VM does not have to be trusted. > > Can you give a concrete example? > > I have an idea about what you're saying but it may be wrong: > > Today the iotlb mechanism in vhost-user does not actually enforce > memory permissions. The vhost-user slave has full access to mmapped > memory regions even when iotlb is enabled. Currently the iotlb just > adds an indirection layer but no real security. (Is this correct?)
Not exactly. iotlb protects against malicious drivers within guest. But yes, not against a vhost-user driver on the host. > Are you saying the vhost-pci device code in QEMU should enforce iotlb > permissions so the vhost-user slave guest only has access to memory > regions that are allowed by the iotlb? Yes. > This is a weak way to enforce memory permissions. If the guest is > able to exploit a bug in QEMU then it has full memory access. That's par for the course though. We don't have many of these. If you assume qemu is insecure, using a theoretical kernel-based mechanism does not add much since kernel exploits are pretty common too :). > It's a > security problem waiting to happen It's better security than running the switch on host though. > and QEMU generally doesn't > implement things this way. Not sure what does this mean. > A stronger solution is for the vhost-user master to control memory > protection and to disallow the vhost-user slave from changing memory > protection. I think the kernel mechanism to support this does not > exist today. Such a mechanism would also make the vhost-user host > userspace use case secure. The kernel mechanism to do this would > definitely be useful outside of virtualization too. > > Stefan In theory, maybe. But I'm not up to implementing this, it is very far from trivial. We can do a QEMU based one and then add the kernel based one on top when it surfaces. Also I forgot - this has some performance advantages too. -- MST
