Changelog
==========

2018-01-11: Original QSB published
2018-01-23: Updated mitigation plan to XPTI; added Xen package versions

[...]

(Proper) patching
==================

## Qubes 4.0

As explained above, almost all the VMs in Qubes 4.0 are
fully-virtualized by default (specifically, they are HVMs), which
mitigates the most severe issue, Meltdown. The only PV domains in Qubes
4.0 are stub domains, which we plan to eliminate by switching to PVH
where possible. This will be done in Qubes 4.0-rc4 and also released as
a normal update for existing Qubes 4.0 installations. The only remaining
PV stub domains will be those used for VMs with PCI devices. (In the
default configuration, these are sys-net and sys-usb.) To protect those
domains, we will provide the Xen page-table isolation (XPTI) patch, as
described in the following section on Qubes 3.2.

## Qubes 3.2

Previously, we had planned to release an update for Qubes 3.2 that would
have made almost all VMs run in PVH mode by backporting support for this
mode from Qubes 4.0. However, a much less drastic option has become
available sooner than we and the Xen Security Team anticipated: what the
Xen Security Team refers to as a "stage 1" implementation of the Xen
page-table isolation (XPTI) mitigation strategy [5]. This mitigation
will make the most sensitive memory regions (including all of physical
memory mapped into Xen address space) immune to the Meltdown attack. In
addition, this mitigation will work on systems that lack VT-x support.
(By contrast, our original plan to backport PVH would have worked only
when the hardware supported VT-x or equivalent technology.)

Please note that this mitigation is expected to have a noticeable
performance impact. While there will be an option to disable the
mitigation (and thereby avoid the performance impact), doing so will
return the system to a vulnerable state.

The specific packages that contain the XPTI patches for Qubes 3.2 are
as follows:

  - Xen packages, version 4.6.6-36

The packages are to be installed in dom0 via the Qubes VM Manager or via
the qubes-dom0-update command as follows:

  For updates from the stable repository (not immediately available):
  $ sudo qubes-dom0-update

  For updates from the security-testing repository:
  $ sudo qubes-dom0-update --enablerepo=qubes-dom0-security-testing

A system restart will be required afterwards.

These packages will migrate from the security-testing repository to the
current (stable) repository over the next two weeks after being tested
by the community.

If you use Anti Evil Maid, you will need to reseal your secret
passphrase to new PCR values, as PCR18+19 will change due to the new Xen
binaries.

[...]

Here is an overview of the VM modes that correspond to each Qubes OS
version:

VM type \ Qubes OS version         | 3.2 | 4.0-rc1-3 | 4.0-rc4 |
---------------------------------- | --- | --------- | ------- |
Default VMs without PCI devices    | PV  |    HVM    |   PVH   |
Default VMs with PCI devices       | PV  |    HVM    |   HVM   |
Stub domains - Default VMs w/o PCI | N/A |    PV     |   N/A   |
Stub domains - Default VMs w/ PCI  | N/A |    PV     |   PV    |
Stub domains - HVMs                | PV  |    PV     |   PV    |