---===[ Qubes Security Bulletin #53 ]===---

                             2019-11-13


   	TSX Asynchronous Abort speculative side channel (XSA-305)

Summary
========

On 2019-11-12, the Xen Security Team published Xen Security Advisory
305 (CVE-2019-11135 / XSA-305) [1] with the following description:

| This is very closely related to the Microarchitectural Data Sampling
| vulnerabilities from May 2019.
| 
| Please see https://xenbits.xen.org/xsa/advisory-297.html for details
| about MDS.
| 
| A new way to sample data from microarchitectural structures has been
| identified.  A TSX Asynchronous Abort is a state which occurs between a
| transaction definitely aborting (usually for reasons outside of the
| pipeline's control e.g. receiving an interrupt), and architectural state
| being rolled back to start of the transaction.
| 
| During this period, speculative execution may be able to infer the value
| of data in the microarchitectural structures.
| 
| For more details, see:
|   https://software.intel.com/security-software-guidance/insights/deep-dive-intel-transactional-synchronization-extensions-intel-tsx-asynchronous-abort
| 
| An attacker, which could include a malicious untrusted user process on a
| trusted guest, or an untrusted guest, can sample the content of
| recently-used memory operands and IO Port writes.
| 
| This can include data from:
| 
|  * A previously executing context (process, or guest, or
|    hypervisor/toolstack) at the same privilege level.
|  * A higher privilege context (kernel, hypervisor, SMM) which
|    interrupted the attacker's execution.
| 
| Vulnerable data is that on the same physical core as the attacker.  This
| includes, when hyper-threading is enabled, adjacent threads.
| 
| An attacker cannot use this vulnerability to target specific data.  An
| attack would likely require sampling over a period of time and the
| application of statistical methods to reconstruct interesting data.

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

Only Intel processors which support TSX and have hardware mitigation
against MDS are affected (see the XSA and the Intel advisory linked
above for details and a list of affected processor families).

Note: There was no embargo period for this XSA.

Patching
=========

The Xen Project has provided patches that mitigate this issue. A CPU
microcode update is required to take advantage of them. Note that
microcode updates may not be available for older CPUs. (See the Intel
advisory linked above for details.)

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

  For Qubes 4.0:
  - Xen packages, version 4.8.5-12
  - microcode_ctl 2.1-29.qubes1

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-305.html

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