[libvirt] Matching the type of mediated devices in the migration

[Alex Williamson]

On Sun, 19 Aug 2018 22:25:19 +0800
Zhi Wang <zhi.a.wang at intel.com> wrote:

> Share some updates of my work on this topic recently:
> 
> Thanks for Erik's guide and advices. Now my PoC patches almost works. 
> Will send the RFC soon.
> 
> Mostly the ideas are based on Alex's idea: a match between a device 
> state version and a minimum required version
> 
> 
> "Match of versions" in Libvirt
> 
> Initialization stage:
> 
> - Libvirt would detect if there is any device state version in a 
> "mdev_type" of a mediated device when creating a mdev node in node 
> device tree.
> 	- If the "mdev_type" of a mediated device *has* a device state version, 
> then this mediated device supports migration.
> 	- If not, (compatibility case, mostly for old vendor drivers which 
> don't support migration), this mediated device doesn't support migration
> 
> Migration stage:
> 
> - Libvirt would put the mdev information inside cookies and send them 
> between src machine and dst machine. So a new type of cookie would be 
> added here.
> 
> There are different versions of migration protocols in libvirt. Each of 
> them starts to send cookies in different sequence. The idea here is to 
> let the match happens as early as possible. Looks like QEMU driver in 
> libvirt only support V2/V3 proto.
> 
> 
> V2 proto:
> 
> - The match would happen in SRC machine after the DST machine transfers 
> the cookies with mdev information back to the SRC machine during the 
> "preparation" stage. The disadvantage is the DST virtual machine has 
> already been created in "preparation" stage. If the match fails, the 
> virtual machine in DST machine has to be killed as well, which would 
> waste some time.
> 
> V3 proto:
> 
> - The match would happen in DST machine after the SRC machine transfers 
> the cookies to the DST machine during the "begin" stage. As the DST 
> machine hasn't entered into "preparation" stage at this time, the 
> virtual machine hasn't been created in DST machine at this point. No 
> extra VM destroy is needed if the match fails. This would be the ideal 
> place for a match.
> 
> "Match of version" in QEMU level
> 
> As there are several different types of migration in libvirt. In a 
> migration with hypervisor native transport, the target machine could 
> even not have libvirtd, the migration happens between device models 
> directly. So we need a match in QEMU level as well. We might still need 
> Kirti's approach as the last level match.

The kernel and vendor driver will always have a last opportunity to nak
a migration, the purpose of making certain information readily
available to libvirt is only to allow userspace some insight into where
a migration is likely to be successful.  Even if we expose these things
to userspace, it's the kernel's responsibility to validate the
migration data.  In fact, pushing state information for a device into
the kernel would seem to be a massive security target.  For instance
how many vulnerabilities might a malicious user be able to exploit in
the code that parses the device specific state information?  How do we
even detect non-malicious user errors, like trying to migrate GVTg
device state to an NVIDIA vGPU?

The latter at least suggests that the kernel needs to perform the same
set of validation that we're trying to enable userspace to do.
Cornelia also mentioned that some mdev devices are more or less shells
within which a device is configured, such as ccw and likely the crypto
ap devices.  In those cases the mdev type might not be sufficient meta
data about what we're dealing with.  This might suggest some sort of
header within the migration region parsed by common code for basic
validation.

Are there any suggestions how we can deal with security issues?
Allowing userspace to provide a data stream representing the internal
state of a virtual device model living within the kernel seems
troublesome.  If we need to trust the data stream, do we need to
somehow make the operation more privileged than what a vfio user might
have otherwise?  Does the data stream need to be somehow signed and how
might we do that?  How can we build in protection against an untrusted
restore image?  Thanks,

Alex