---===[ Qubes Security Bulletin #41 ]===---

                             2018-06-13


  Speculative register leakage from lazy FPU context switching (XSA-267)

Summary
========

On 2018-06-13, the Xen Security Team published Xen Security Advisory
267 (CVE-2018-3665 / XSA-267) [1] with the following description:

| x86 has a hardware mechanism for lazy FPU context switching.  On a
| task switch, %cr0.ts (Task Switched) gets set, and the next
| instruction to touch floating point state raises an #NM (No Math,
| later known as Device Not Available) exception.
| 
| Traditionally, FPU state has been large in comparison to available
| bandwidth (and therefore slow to switch) and not used as frequently as
| cpu tasks tend to switch.  This mechanism allows the OS to only switch
| FPU when necessary, which in turn increases performance.
| 
| Some CPUs however speculate past an #NM exception, allowing register
| content to be leaked by a side-channel.
| 
| An attacker can read x87/MMX/SSE/AVX/AVX-512 register state belonging
| to another vCPU previously scheduled on the same processor.  This can
| be state belonging a different guest, or state belonging to a
| different thread inside the same guest.

This is yet another CPU hardware bug related to speculative execution.

Patching
=========

To resolve this issue, the Xen Project has provided patches disabling
lazy FPU context switching on affected systems.

The specific packages that resolve the problems discussed in this
bulletin are as follows:

  For Qubes 3.2:
  - Xen packages, version 4.6.6-42

  For Qubes 4.0:
  - Xen packages, version 4.8.3-9

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.

Credits
========

See the original Xen Security Advisory.

References
===========

[1] https://xenbits.xen.org/xsa/advisory-267.html

--
The Qubes Security Team
https://www.qubes-os.org/security/