[XSA-435] x86/Intel: Gather Data Sampling

ISSUE DESCRIPTION

A researcher has discovered Gather Data Sampling, a transient execution
side-channel whereby the AVX GATHER instructions can forward the content
of stale vector registers to dependent instructions.

The physical register file is a structure competitively shared between
sibling threads. Therefore an attacker can infer data from the sibling
thread, or from a more privileged context.

For more details, see:
https://www.intel.com/content/www/us/en/developer/articles/technical/software-security-guidance/technical-documentation/gather-data-sampling.html

IMPACT

An attacker can infer data from different contexts on the same core.
Examples of such data includes key material, cipher and plaintext from
the AES-NI instructions, or the contents of REP-MOVS instructions,
commonly used to implement memcpy().

VULNERABLE SYSTEMS

Systems running all versions of Xen are affected.

See the Intel documentation for a list of affected processors.

CPUs from other hardware vendors are not believed to be affected.