Re: [Qemu-devel] [PATCH] migration: add capability to bypass the shared memory

[Stefan Hajnoczi]

On Sat, Mar 31, 2018 at 04:45:00PM +0800, Lai Jiangshan wrote:
> a) feature: qemu-local-migration, qemu-live-update
> Set the mem-path on the tmpfs and set share=on for it when
> start the vm. example:
> -object \
> memory-backend-file,id=mem,size=128M,mem-path=/dev/shm/memory,share=on \
> -numa node,nodeid=0,cpus=0-7,memdev=mem
> 
> when you want to migrate the vm locally (after fixed a security bug
> of the qemu-binary, or other reason), you can start a new qemu with
> the same command line and -incoming, then you can migrate the
> vm from the old qemu to the new qemu with the migration capability
> 'bypass-shared-memory' set. The migration will migrate the device-state
> *ONLY*, the memory is the origin memory backed by tmpfs file.

Marcelo, Andrea, Paolo: There was a more complex local migration
approach in 2013 with fd passing and vmsplice.  They specifically
avoided the approach proposed in this patch, but I don't remember why.

The closest to an explanation I've found is this message from Marcelo:

  Another possibility is to use memory that is not anonymous for guest
  RAM, such as hugetlbfs or tmpfs.

  IIRC ksm and thp have limitations wrt tmpfs.

https://www.spinics.net/lists/linux-mm/msg67437.html

Have the limitations been been solved since then?

> c)  feature: vm-template, vm-fast-live-clone
> the template vm is started as 1), and paused when the guest reaches
> the template point(example: the guest app is ready), then the template
> vm is saved. (the qemu process of the template can be killed now, because
> we need only the memory and the device state files (in tmpfs)).
> 
> Then we can launch one or multiple VMs base on the template vm states,
> the new VMs are started without the “share=on”, all the new VMs share
> the initial memory from the memory file, they save a lot of memory.
> all the new VMs start from the template point, the guest app can go to
> work quickly.
> 
> The new VM booted from template vm can’t become template again,
> if you need this unusual chained-template feature, you can write
> a cloneable-tmpfs kernel module for it.
> 
> The libvirt toolkit can’t manage vm-template currently, in the
> hyperhq/runv, we use qemu wrapper script to do it. I hope someone add
> “libvrit managed template” feature to libvirt.

This feature has been discussed multiple times in the past and probably
the reason why it's not in libvirt yet is that no one wants it badly
enough that they have solved the security issues.

RAM and disk contain secrets like address-space layout randomization,
random number generator state, cryptographic keys, etc.  Both the kernel
and userspace handle secrets, making it hard to isolate all secrets and
wipe them when cloning.

Risks:
1. If one cloned VM is exploited then all other VMs are more likely to
   be exploitable (e.g. kernel address space layout randomization).
2. If you give VMs cloned from the same template to untrusted users,
   they may be able to determine the secrets other users' VMs.

How are you wiping secrets and re-randomizing cloned VMs?  Security is a
major factor for using Kata, so it's important not to leak secrets
between cloned VMs.

Stefan

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