x86 pv guest kernel DoS via SYSENTER

ISSUE DESCRIPTION

The SYSENTER instruction leaves various state sanitization activities to software. One of Xen’s sanitization paths injects a #GP fault, and incorrectly delivers it twice to the guest.
This causes the guest kernel to observe a kernel-privilege #GP fault (typically fatal) rather than a user-privilege #GP fault (usually converted into SIGSEGV/etc).

IMPACT

Malicious or buggy userspace can crash the guest kernel, resulting in a VM Denial of Service.

VULNERABLE SYSTEMS

All versions of Xen from 3.2 onwards are vulnerable.
Only x86 systems are vulnerable. ARM platforms are not vulnerable.
Only x86 systems which support the SYSENTER instruction in 64bit mode are vulnerable. This is believed to be Intel, Centaur and Shanghai CPUs. AMD and Hygon CPUs are not believed to be vulnerable.
Only x86 PV guests can exploit the vulnerability. x86 PVH / HVM guests cannot exploit the vulnerability.