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

On 08/21/18 07:08, Alex Williamson wrote:
On Sun, 19 Aug 2018 22:25:19 +0800
Zhi Wang <zhi a wang 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.

Yes. The vendor driver should be the last keeper to nak a migration. It should be implemented inside the vendor driver.

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?

For now, we only depends on mdev_type, after the discussion of vendor id or device id.

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
Yes. If we could validate it earlier then better since, we don't need to wait until the DST machine start the VM and try to load the 1st states.

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,
What a good point!

I dig the kernel module security case, which seems similar with this case. The security of loading kernel module relies on root privilege and signature.

For root privilege, QEMU could run as non root in libvirtd. So this wouldn't be an option.

For signature, I am wondering if there is any similar cases in other kernel components, like KVM or another modules which provides ioctls to userspace. Maybe they don't even load some binary from userspace, but they could suffer from DDOS flood from userspace. Maybe some ioctls or interfaces in kernel should only allow signed/trusted userspace application to call. (previously it's "allow signed kernel module to load")



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