There are two closely-related bugs. When Xen emulates instructions which generate software interrupts it needs to perform a privilege check involving an IDT lookup. This check is sometimes erroneously conducted as if the IDT had the format for a 32-bit guest, when in fact it is in the 64-bit format. Xen will then read the wrong part of the IDT and interpret it in an unintended manner. (CVE-2016-9377) When Xen emulates instructions which generate software interrupts, and chooses to deliver the software interrupt, it may try to use the method intended for injecting exceptions. This is incorrect, and results in a guest crash. (CVE-2016-9378) These instructions are not ususally handled by the emulator. Exploiting the bug requires ability to force use of the emulator.
An unprivileged guest user program may be able to crash the guest.
Xen versions 4.5 and newer are vulnerable. Older versions are not vulnerable. The vulnerability is only exposed on AMD hardware lacking the NRip feature. AMD hardware with the NRip feature, and all Intel hardware, is not vulnerable. Xen prints information about CPU features on boot. If you see this: (XEN) SVM: Supported advanced features: ... (XEN) - Next-RIP Saved on #VMEXIT then you are not vulnerable because you have an AMD CPU with NRip. If you see this: (XEN) VMX: Supported advanced features: then you are not vulnerable because you have an Intel CPU. The vulnerability is only exposed on HVM guests. ARM systems are NOT vulnerable.