x86/AMD: Debug Mask handling

ISSUE DESCRIPTION

AMD CPUs since ~2014 have extensions to normal x86 debugging functionality. Xen supports guests using these extensions.
Unfortunately there are errors in Xen’s handling of the guest state, leading to denials of service.

1. CVE-2023-34327 - An HVM vCPU can end up operating in the context of a previous vCPUs debug mask state.
2. CVE-2023-34328 - A PV vCPU can place a breakpoint over the live GDT. This allows the PV vCPU to exploit XSA-156 / CVE-2015-8104 and lock up the CPU entirely.

IMPACT

For CVE-2023-34327, any guest (PV or HVM) using Debug Masks normally for it’s own purposes can cause incorrect behaviour in an unrelated HVM vCPU, most likely resulting in a guest crash.
For CVE-2023-34328, a buggy or malicious PV guest kernel can lock up the host.

VULNERABLE SYSTEMS

Only AMD/Hygon hardware supporting the DBEXT feature are vulnerable. This is believed to be the Steamroller microarchitecture and later.
For CVE-2023-34327, Xen versions 4.5 and later are vulnerable.
For CVE-2023-34328, Xen version between 4.5 and 4.13 are vulnerable. The issue is benign in Xen 4.14 and later owing to an unrelated change.