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. When Xen needs to emulate such an instruction, to efficiently handle the emulation, the memory address and register operand are recalculated internally to Xen. In this process, the high bits of an intermediate expression were discarded, leading to both the memory location and the register operand being wrong. The wrong memory location would have only a guest local effect (either access to an unintended location, or a fault delivered to the guest), whereas the wrong register value could lead to either a host crash or an unintended host memory access.
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.
All Xen versions are affected. The vulnerability is only exposed to x86 guests running in 64-bit mode. On Xen 4.6 and earlier the vulnerability is exposed to all guest user processes, including unprivileged processes, in such guests. On Xen 4.7 and later, the vulnerability is exposed only to guest user processes granted a degree of privilege (such as direct hardware access) by the guest administrator; or, to all user processes when the when the VM has been explicitly configured with a non-default cpu vendor string (in xm/xl, this would be done with a `cpuid=' domain config option). The vulnerability is not exposed to 32-bit PV guests. ARM systems are not vulnerable.