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: <a href=“https://www.intel.com/content/www/us/en/developer/articles/technical/software-security-guidance/technical-documentation/gather-data-sampling.html”>https://www.intel.com/content/www/us/en/developer/articles/technical/software-security-guidance/technical-documentation/gather-data-sampling.html</a>

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.