A vulnerability allowing malicious geust to crash the host was found. Early versions of Xen on ARM did not support "multicall" functionality (the ability to perform multiple operations via a single hypercall) and therefore stubbed out the functionality needed to support preemption of multicalls in a manner which crashed the host. When multicall support was subsequently added these stubs were not replaced with the correct functionality and therefore exposed to guests a code path which crashes the host. Any guest can issue a preemptable hypercall via the multicall interface to exploit this vulnerability. Both 32- and 64-bit ARM systems are vulnerable from Xen 4.4 onward.

Mitigation:

On systems where the guest kernel is controlled by the host rather than guest administrator, running only kernels which do not make use of multicall functionality will prevent untrusted guest users from exploiting this issue. However untrusted guest administrators can still trigger it unless further steps are taken to prevent them from loading code into the kernel (e.g. by disabling loadable modules etc) or from using other mechanisms which allow them to run code at kernel privilege.