FreeBSD: VID-56f0f11e-ba4d-11e6-ae1b-002590263bf5 (CVE-2016-9383): xen-kernel -- x86 64-bit bit test instruction emulation broken

The Xen Project reports: The x86 instructions BT, BTC, BTR, and BTS, when used with a destination memory operand and a source register rather than an immediate operand, access a memory location offset from that specified by the memory operand as specified by the high bits of the register source. A malicious guest can modify arbitrary memory, allowing for arbitrary code execution (and therefore privilege escalation affecting the whole host), a crash of the host (leading to a DoS), or information leaks. The vulnerability is sometimes exploitable by unprivileged guest user processes.