docs: security: Confidential computing intro and threat model for x86 virtualization

docs: security: Confidential computing intro and threat model for x86 virtualization

Kernel developers working on confidential computing for virtualized
environments in x86 operate under a set of assumptions regarding the Linux
kernel threat model that differs from the traditional view. Historically,
the Linux threat model acknowledges attackers residing in userspace, as
well as a limited set of external attackers that are able to interact with
the kernel through networking or limited HW-specific exposed interfaces
(e.g. USB, thunderbolt). The goal of this document is to explain additional
attack vectors that arise in the virtualized confidential computing space.

Reviewed-by: Larry Dewey <larry.dewey@amd.com>
Reviewed-by: David Kaplan <david.kaplan@amd.com>
Co-developed-by: Elena Reshetova <elena.reshetova@intel.com>
Signed-off-by: Elena Reshetova <elena.reshetova@intel.com>
Signed-off-by: Carlos Bilbao <carlos.bilbao@amd.com>
Message-ID: <98804f27-c2e7-74d6-d671-1eda927e19fe@amd.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>